a Direct Heating Triode blog

In the DIY world of triodes amplifier lovers, most people know the best of the large power tubes like the 211/VT4C or the 845. These tubes are easy to source, still manufactured by Chinese, Czech or German. It's even possible to find some NOS ones from GE, RCA…
There are plenty of circuits to try for the hobbyist and parts like output transformers are easy to find.
This topic will make you discover some of the best European triodes and especially french ones. For audio use everyone, almost everyone, knows the tubes from Mullard, Philips or Mazda, but very few have had the opportunity to get on Neotron, Visseaux or SFR.

These manufacturers made tubes mostly for administrations and are rarely present on the tubes vendors stalls still active today. In fact, most of these tubes were bought by the Japanese in the 80's and are now selling at indecent prices. Understandable when you see manufacturing quality that may turn you pale. Philips MC1/60 or SFR E60M where built to a very very high standard even for the tube industry of that time.

Let me introduce our competitors.

Philips MC1/60

This Dutch power triode was specifically developed for audio, rated at 75W plate dissipation,
very close in specification to the 211/VT4C, but easier to use due to its lower trans-conductance.
The MC1/60 have a second advantage with its 4 V oxide coated filament rated at 3,3A (to compare with the 10 V/ 3,25A of the 211). We will talk latter on an other advantage of this oxide coated cathode in term of music sweetness. Sweetness and naturalness shared with the very best of the 4 V triodes like the PX4 or the AD1.

SFR E60M

French military triode made by Société Française Radioélectrique in the 40's. Incredibly well made with cost no object materials, twice the weight of its Philips relative, these tubes can bear the 3X75B military code. Very close in specification to the MC1/60 which is very helpful due to the lack of information concerning the SFR company products.
Like the Philips it's a 4 V / 3,3 A oxide coated cathode, anode power rating 75W.

MAZDA TM100

French military triode made by Mazda in the 30'/40',
also can bear the 3X75B military code.
Very different construction with glass rods to support filament springs.
Despite the old fashion and less impressive construction it is probably the best sounding triode of this group, and probably one of the best sounding ever.
Oxide coating cathode, 4 V / 3,3 A, power dissipation 75W (better not to go beyond 60W).
All these triodes can be switched as they have the same filament ratings, use the same socket and are very close in specifications.

Top view shows the very different construction.

The 6 watt monster amp

My first intention when I decided to build an amp around these tubes was to get a reasonable output power with the best possible sound. You will say this is a very personal and very subjective goal but during the past 20 years I built a lot of amp, preamp but only kept 3 ones after intensive listening test with golden ears friends (10 / VT25 SE and VT25 / VT25A AB1 PP amps, 6J5 / NP206 Tango transformer coupled line preamplifier). These materials serve as references to my future constructions.

My requirements are small in term of power. I mainly listen to Klipsch La Scala speakers upgraded with JBL 2470 drivers and Alk filters and at 105 dB I can shake hell with just 1 watt.
So I decided not to push my tubes too much and make them sing at 5/6 W output.
Looking at the curves I found a satisfying operating point at 600V / 80 mA with a 3.5K load,it may be weird at first sight but at that point internal resistance is mainly 1.6K (S=8mA/V and μ=12.5). This gives the maximum output with a 3.5K load (and I had the transformers on the shelf for a long time, it helped a bit to make a decision).

The driver was a more difficult choice. I needed very low distortion, large RMS output voltage, low output impedance, large bandwidth. Not easy to combine. After several tests with the most commonly used drivers and tubes I stuck to a 6SN7 SRPP feeding a transformer. In that way I had a well defined load for the SRPP, a very good bandwidth despite a coupling cap (no current through the transformer) and the possibility of less final distortion using a 10K/40K step-up transformer that double the driver voltage. Moreover I could use a negative voltage bias and the possibility to set the current depending upon tubes to be used.

The 6SN7 choice was not a choice. This tube is well known for its linearity, its sound qualities and I have plenty on hand.
Long time ago I have modelized the SRPP based on a very interesting thread by Merlin Blencowe from AudioXpress ( The optimized SRPP ) and I just have to fill some fields in a Excel file to have my operating points, resistor values, gain and output impedance.

Now some math considerations to see the different AC voltages involved in this project.

Power output should be 6W/8 ohm or 6.9 Vrms.
Output transformer have an impedance ratio of 2.28 10-3, a voltage ratio 4.78 10-2 (-26,4dB).
It means for the power tube a plate voltage of 144 Vrms or 408 V peak to peak to get my 6.9Vrms.
Looking at the MC1/60 curves this can be done with 48 V peak to peak on the grid (17Vrms).
Gain of power triode is 8.5 (+18.6dB), confirmed by calculation where

G = µ x Rload/Rload + ρ.

Now to get 17 Vrms for MC1/60 complete anode swing I just need 8.5 Vrms (24 V peak to peak) out of my 6SN7 SRPP due to a 10K/40K interstage transformer used in that circuit. Easily achieved, under 300V anode supply the SRPP circuit give a gain of 13.4 (+22.5dB),means 0,63Vrms at input (1.80 peak to peak) and very low distortion.

The amplifier overall gain is :
+22.5dB (6SN7) + 6dB (Transformer) + 18.6dB (MC1/60) – 26.4dB (Transformer) = +20.7dB

To be continued......
full amplifier circuit, construction details, listening test, tubes to be used, mods and tweaks.

Back on the air with amplifier circuit

Voltage requirements.

We need 600V for the final stage, 300V for driver, negative bias voltage, heater voltage.

High voltage is not very high, I can take advantage of vacuum tube rectifier (helps for soft start) and polypropylene capacitor for main supply. I prefer the sound of these caps over the usual electrolytics.

For driver stage I choose choke input filter with bleeder resistor. Choke input is less disturbing for the power transformer and ripple is very smooth.

Main power transformer windings.

1- 500_0_500 V will give me the proper 600V high voltage with GZ34 rectifier.
2- 350_0_350 V will feed the SRPP with 30H choke input filter and ultra fast soft recovery diodes.
3- 50V for bias.
4- 7.5V for MC1/60 to provide 4V choke input filtered too.
5- 6.3V for 6SN7.
6- 5V for GZ34.

On the bench

Sub chassis ease construction a lot.........

Amp finished

Tamura TN351 interstage transformer with it's 1:2 voltage ratio helps a lot to drive the final tube easily with minimum distortion from the driver.
The little black switch next to the 6SN7 socket is for a local NFB loop. I can choose between 0 / 3.5dB feedback. Personnaly I prefer no feedback.
Screw speaker terminal, grey paint and AEI silver plated socket give a retro fashion look to the amplifier.

The second black switch next to AC connector give me the choice to earth or not the amplifier,helps to get the lowest noise floor.

Screwdriver potentiometer and large panel meter for easy current setting.

The mighty E60M tube @ 600V / 75mA

Listening test

Amplifier is dead silent on my Klipsch, I have to put my ear against tweeter to hear a slight hisss.

First impressions .... punchy, very clear and detailed without being tiring. Beautiful and delicate mids and highs (as fine as the 10/VT25 amp !! I can make comparison on mono recordings VT25 amp on one side, MC1/60 on the other), voices are natural, very articulated and living like (Madeleine Peyroux_Careless Love).
Bass are impressive, this is the first time I can physically feel bass my Klipsch (Bach_ Cello suites, Pierre Fournier). Excellent soundstage with lot of depth, very airy and detailed,I ear the very, very slight shift of the bottle neck on the guitar, the slight fingers taps on strings ( Pedro Soler_Sombras or Selmer 607 Gypsy guitar CD) never heard before! It's good, very good in fact even on very complex music ( Verdi_Messa da Requiem_Giulini).
I also listened to E60M and I have great difficulty in distinguishing the tubes. More body may be? It depends a lot upon input tube. Best with MC1 / 60 are the VT231 Ken Rad (sooooo punchy in the low end), Hytron VT231 or Brimar 6SN7 GTY with E60M or TM100.

Amazing to see after so many years that the 4 V tubes have always been killers compared to the other ones (2.5, 6.3, 7.5V …..). I have been playing with plenty of these classics (2A3, 45, 50,811,211, 845 and so on) and found each time they were inferior to the 4 V ones *. Is it a matter of cathode coating, is 4 V the best voltage for regular emission ? I have no answer, just listening facts that aficionados of the PX4, PX25 and AD1 perfectly know.

* Just one exception, the Visseaux A710, a 10 with BIG plate (as big as a 50) using a 7.5 V oxide coated filament. It's to my knowledge the only 10 of that kind and the only one that sounds like a PX4.

Have to wait for some friends to come and make a very serious listening.
At that time this amp is the very best I have ever built.

Some CD's for this test

The 10 is a very old triode and probably one of the first power tube. It's linearity and sound qualities brought this tube a favourite among many DIY's. Dozens of amp and preamp using this tube and it's relatives (10Y, 801A, VT25) can be found on the net. The only and main drawback is it's flea power.
Long time ago I decided to build an amp that could drive speakers with 95/98 dB efficiency and the logical solution was push pull operation.
In that way I began to modelize an AB1 operation push pull to reach 2 goals, higher output power (about 4/5 watt) and better amplitude distortion figure ( Push pull operation helps to compensate characteristics curvature). For that purpose I had on hand a pair of Tango FX50/16G, one of the finest transformers of Mr Hirata company.

Sylvania VT25 in "modern" ST shape

To get the very best of these tubes a good driver with high output voltage and low distortion is mandatory. Furthermore the perfect way to split signal in two is by interstage transformer. The main difficulty is to find one that accepts some primary current with large bandwidth, I finally selected the Tango NC14 for its very good reputation and its current handling capability.

To provide enough voltage a two stage driver was necessary. After several tests with different dual triodes my choice remained on the 5687 tube.
However due to driver limitations and NC14 step down voltage ratio (1/0,7) I can't go very deep in AB. Operating point is defined by plate voltage ( 415V ) and cathode resistor (3,3 Kohm) giving a steady plate current of 12 mA @ Vg -38V. The load (8 Kohm) of each tube give me the maximum plate voltage swing, here 275V peak.

5687 operating points have to be carefully chosen to give a large voltage with minimum distortion. It is especially critical for the 2nd stage as anode current must remain in the 15/18mA to keep good bandwidth in low frequencies from the NC14. To drive the VT25 full power we need to swing grid ± 38V peak, it means ± 55 V anode peak for the 5687 due to transformer voltage ratio.

Amplifier circuit

Power supply circuit
Two independent 7.5V supplies for VT25/VT25A tubes (just one shown on schematic)

Construction details

To check power tubes bias voltage I used a 6AF6 magic eye, although not accurate it gives an original look to the amplifier.

There is a bonus, I can use VT25A tubes without any change in amplifier circuit.

Sylvania VT25A big plate triode and oxide coated filament. Power rating seems to be the same but gives a bit more output mainly because of its lower internal resistance.

All Tango amp in white slim chassis...

and fed with good looking and good sounding tubes.

Sylvania VT25 bright light

Magic eye for fun

Philips 5R4GYS is a perfect companion for the Sylvania VT25

Sylvania VT25A soft glow...

Tests

The amp puts out a reasonable 5.8 Watt before clipping ( 6.5 Vrms/8 ohm) and THD @ 1000Hz do not exceed 2.60% , not that bad. Second and third harmonics are barely visible.

With VT25A I got 6.9 Watt and same distortion figure. Nice little amp.

Listening...

I have used this amp for years and it is an all around music maker with lots of punch and refinement. Compared to my MC1/60 I would say the VT25 PP is at its best on vocals, small jazz and classical formations, guitar and piano. It is an amp for people seeking for naturalness and accuracy in music transcription.

Some CD's and LP's that I like to listen

The EL84 is NOT a DHT, but still a fascinating tube for many triodes lovers (I belong to this group) who recognize its sonic qualities.
Great manufacturers like Dynaco, Scott, Leak, Loyez just to name a few, sold thousands of amplifiers using this cute little pentode and for many serious listeners of that time these amps were by far superior for music reproduction to their more powerfull concurrents using EL34, 6550 or KT88.
Yes this tube is very involving, it as the speed ( that usually lacks to the bigger tetrodes and pentodes) the naturalness and ease so many people do like in the very best triodes. It reminds me the 10, the E130 or the RS242, it makes music living like and it's power limitation is it's only drawback. Remember that in the 60's there were not so many speakers with 97/99dB sensitivity to perfectly match a 10 watt amplifier.

In the past 20 years I made half a dozen amplifiers using this tube, always with great pleasure, and that's why I decided to make a post about it.

Also because I always keep an EL84 amp to drive my beloved Wharfedale Super 12 RS/DD. Magnificent full range speakers especially on open baffle.

The amp I will talk about is not a design of my own, it belongs to Harman Kardon. I just made some modifications for safer use and better sound reproduction (I built one exactly like the original for comparison).
Why the HK amplifier ? I found during all these passed years making electronic stuff that simplicity is usually the best way to fidelity and the HK20 design is really simple and very smart.

It operates with a single 6CG7 as driver and phase splitter, two EL84 and a rectifier.
You can't do more simple and when you add the qualities of the 6SN7 family tubes you are right on the way for a good sounding amp.

HK 20 original schematic

As you can see, very simple with some tricks to prevent the use of a decoupling cap in the first stage. V1a bias is taken through R10 in the cathode of the phase inverter V1b. The current of the two sections flows in this resistor, the result is a smaller value that it would have been if directly grounded the usual way. This helps to lower the effect of degenerative feedback thus giving a higher gain to the first stage. Elegant, efficient and easy to calculate if you want to change the tube (I tried the E80CC instead but prefered the 6CG7 after a long listening comparison). For a perfect cathodyne balance a single R11 resistor in parallel with R8_R9_R10 will give the exact value of R7.

To calculate R11 with (R8 + R9 + R10) = Req use formula R11=R7xRreq/Req-R7

My own HK20

I was not comfortable with EL84 operating voltages.
370V on anode and 350V on screen make the tube work very hot and very hard. I am not sure the usual EL84 will last very long under such a fire.
Even the professional and super strong Mazda 7320 reach their limits, furthermore when bread boarding an amp for tests I had some hum that I could not cancel. So I decided to modify the whole amplifier supply and to pay great attention to time constants, I also preferred the EL84 UL to the pentode mode, I reduced NFB to get decent THD (1,5% @ 10W, mainly third) and a more mellow sound. I suppressed R1 potentiometer (always a soucre of trouble) and replaced it by a voltage divider to match the output of my line preamp.
Now with such a supply and mods the amp is dead silent, punchy, open with a wide soundstage.
Very detailed heights, lush mids, rock solid bass make this amp an all purpose companion.

Power supply revisited.

Inside this little giant, AB resistors and Aerovox V161 caps give a vintage look without compromising sound qualities.
Reliable Siemens capacitors to smooth out the ripple, way better than the usual electrolytics (MKV and MP J/S are my preferred) and Hashimoto OPT.

Philips Holland killer tubes, Mazda GZ32 and shielded 6CG7

The extra socket was for a buffer stage intended to be used with a 600 ohm symmetrical input transformer. I finally did not populate.

Personal EL84 review

I am lucky enough to have different brands in stock to make a comparison.
I wont write a lot just give my impressions, for an extensive test read Vacuum Tube Valley issue 8

Mullard soft and round sound, not my taste.
Philips RTC the very best with this amp, very well balanced.
Philips ( Amperex ) Holland top of the line with Philips RTC.
Mazda 7320 strong and powerfull sound, can be tiring.

TFK the worst nothing else to say.
Tungsram excellent performer, lacks a bit of punch in low end, the best on voices.
Russian Reflector 6P14P/K (and only this suffix, means vibrations proof. NOT EB or ER) a real surprise, very good sounding, on par with Philips RTC.

Some great CD's to listen with

The 6J5 is a general purpose not to say an ubiquitous small signal triode.
Its linearity is well known and appreciated by many DIYer's. It is commonly said that what goes in is what comes out. It is the successor of the glorious 27 / 56 family of tubes

and has been declined in numerous shapes (G, GT, Metal) and constructions (i.e.the weird E11488) to meet industry or military requirements. The British denominations is L63.
The electrical characteristics of the 6J5 are identical to the 6SN7 with just a single triode in the glass enveloppe.
Using such a tube when seeking for high fidelity audio reproduction is a very good choice.
Complete data here

Below 6J5G / L63 GEC ST shape tube in white military box.

and fancy Marconi L63

A simple line preamp, theoretical approach.

Building a very small but very good sounding line preamp as always been challenging.
Very simple means just a few parts of the highest quality for utmost sound
reproduction. The simplest would be nothing more than an attenuator which I tried with some disappointments. No gain, lacks of dynamic and random noise. Something was wrong in my approach of simplicity and I decided to use an active element to reach my goal ( that clearly appeared to be an impedance matcher with gain). Obviously an active element in signal path means alteration and I had to decide which tube would be up to the task.
Playing with triodes for decades taught me that the ones that could be used are not so numerous. For the indirect I would go to the 27 (76, 6C5, 6J5) family of tubes or the Bi and PTT100, for the direct the RS242 and relatives, the 841 and some post tubes like Aa and PTT0. All these tubes share excellent linearity, medium µ and exceptional sonic qualities. Unfortunately most are expensive or too scarce for the average amateur I am and I decided to go for the 76/6J5. These are very cute 6,3V triodes, affordable and easy to source. A good point for future sonic character comparisons of different brands.
Having a pair of Hirata's NP206 (20K/600 ohm) on hand I started to think about the best operating point for the lowest distortion.
My first choice was for the 76 but its higher ρ did not perfectly match my transformers,

thus I went for the 6J5 (which I did not regret…).

Some drawings and calculations...

A line stage in my system must have a gain of 2/2,5 (+ 6/7dB) to accommodates my amps sensitivity. Loading a 6J5 with 20K will roughly give a gain of 14.5 (+ 23 dB) with a fully decoupled cathode and at operating points Va 240V_250V / Vg -8V / Ia 8mA.
With a 20 Kohm load distortion appears to be very small at usual grid input voltage. Most CD players have a 0,5 to 1V rms output asymmetrical mode, means a maximum of +/- 1,4 Vpeak on grid. Right in the linear region.
The transformer voltage ratio is 1.73 10^-1 (-15,3 dB) multiplied by 14,5 it gives me an overall gain of 2.50. Just what I need!

Line stage drawing. Very, very simple, the components choice will determine the qualities of the preamp.

Construction.

As I said previously parts must be of very high quality.

First of it, the main attenuator. Might better not consider the usual potentiometer but prefer a stepped attenuator. Just 2 resistors in the signal path making a precise voltage divider.
Good rotary switch and quality resistors give better tracking and balance than the usual plastic or carbon pot.

One exception the ALPS RK40 "Black Beauty". Having both on hand choice was not easy and I picked up the stepped attenuator randomly.

The tubes. I will tell you in a next post, when preamp will be completely finished, my impress upon the different 6J5 / L63 I have on hand.

The transformer. Here a Tango, but any good transformer with 20K primary handling 15 to 20 mA will be fine. Some good ones from Hashimoto like HL20K-6 (I love Japanese trannies).

Resistors and capacitors. Plenty of choice, I used Takman metal film resistors with some vintage Sic Safco low ESR professional caps.

Tips. Short leads, star grounding plus some good oil caps just next to the transformers. Especially important to keep a good transient response when PSU is on a separate chassis. In that case I always split in two the last decoupling capacitor. Can really see the difference with an FFT analyzer.
Some feedback can help in the very low end, and just for once I prefer this preamp with a small amount (2/3dB). Better sound focus and tighter bass.

Next step power supply considerations, complete preamp in its new suit plus some listening tests ....

High voltage requirements

Needless to say that a good supply, free of ripple, is essential in SE stage that have very bad CMRR. This generally dictate a large capacitor bank at the expense of transient response due to great time recovery.
As I prefer a punchy response it needs a good calculation of the filtering cells to keep ripple at least 10^-5 (- 80 dB) below last stage voltage. Means 240*10^-5 or 2,4 mV rms while keeping the smallest possible time constant.
Using a input choke of large value greatly helps reducing the following capacitors value. Furthermore behind the choke the ripple is very smooth and quite sinusoidal, generally 2 cells are enough to have an almost perfect continuous voltage.

Main supply

To calculate the ripple behind the choke we have to know the ripple factor η = 1,2/L*C (L in Henries and C in µF) where C is the first filtering cap.
An estimation of this cap can be done if we consider a time constant not more than 15 millisecond. The choke having a DC resistance of 800 ohm, C will be 15 10^-3/800 = 18,7 µF, the closest usual value is 16µF.
Then η = 0,00125 and the ripple on the first cap will be ηV = 0,00125*265 = 0,33Vpp or 0,117 Vrms. Network time constant is 13 millisecond.
A second cell with 1,2 Kohm and 16µF will leave a small 25mVpp / 8.8mVrms ripple with a 19 millisecond TC.
A last cell with 2,4 Kohm and 10 µF will leave 1,6 mVpp / 0,56mVrms and a 24 millisecond TC.
These 0,56mV AC represents a -113dB attenuation below the 240V feeding the 6J5, way better than the -80dB expected, while keeping a very fast recovery power supply. I did some tests with pulse trains and supply is rock solid.

* Time constant is a very important factor in audio, it determines the capacity of an RC system to respond to an AC signal modification and is essential in the ability to follow the sound envelope or ADSR. It is too often neglected when calculating a power supply.
In the case of AC circuits, the RC time constant tells you which signals the circuit passes through and which signals the circuit filters out. An RC circuit acts as a high pass filter which passes high frequency signals and blocks low frequency signals.
When we apply an AC current to a capacitor we are basically sticking some charge onto the capacitor and then taking it off. If we do this at a low rate, smaller than the time constant, then the capacitor has time to discharge. But if we put the charge there rapidly, at a speed much greater than the time constant, the capacitor doesn't have time to discharge. In this way, we can pass high frequency signals through the capacitor.
Remember :
-1 A given capacitor charges 99,9 % at 5 RC.
-2 As a rule of thumb, TC should increase from the first to the last decoupling cell.

No more technical considerations...

Construction

Like for the line preamp just a few parts but of high quality.
An RCore transformer for low overall magnetic loss, high efficiency (just bettered by the toroidal) and slim design to fit my chassis.

A rectifier, good capacitors (German F&T dual 16µf and a couple of Siemens 8µF MP/JS + 2µF Sprague PIO to make my 10µF last decoupling cap), a few resistors, a filtering choke and quality sockets.

Add some hardware and plenty of time....and you are done.

In colour with Neotron 6J5G

For chassis I used 2U professional grade steel ones, 3 layers blue grey paint, very well suited for tubes with lots of openings for optimal air flow. They provide good shielding and are super rugged. I just add some screen printed front and rear plates for better appearance and way better WAF.

Listening report

The 6J5 and its relatives are too numerous for an extensive test. I will give my impress upon the most interesting ones I have in stock. Anyone wanting to build this simple BUT "great performer on any kind of music" preamp will easily find a wide range of readily available tubes.
All the tubes listed below are up to the task, some are more involving or more waaahou !! on a specific material, but a very few simply made me listen to the music instead of searching the preamp qualities and / or shortcomings.
Most need a few burning hours to deliver there full colour.

6J5V Radio Technique
Best of all. Just say evident whatever the music. Delightful triode, vivid, natural with lots of air, detailed to the extreme but never tiring. This is the 6J5 "just sit and listen". These PTT tubes were intended for very long life service, each tube came with a numbered biographical sheet to keep records for 15 years!

6J5G Neotron
Would be my second best tube. A bit more brilliant with slight grain on voices, still stays very detailed. Due to incestuous relationships between tube makers Neotron's were possibely made by Visseaux, I have some tubes bearing both Visseaux and NT labels. So who made what ?

L63 / CV1067 GEC Straight Bulb
Nice sound, would say polite, lacks the details of the RT's or Neotron's, no grain but a smaller sound stage that make music more intimate.
By comparison the L63 GEC ST (see part 1) have a bigger and wider sound but are surprisingly very slow tubes that give the feeling of wrong tempo.

6J5WGT Raytheon
I did not like much this tube despite some remarkable ability on voices. Muddy sound with grain, tight and punchy bass but not as accurate as the other ones. Huge sound stage, lush presentation BUT if you like Billy, Ella and Louis or country singers with hoarse voices, this tube gonna make you cry.

Last of the batch 7193 NU
I had this tube for decades and it is the kind of weird thing you don't want to exhibit. It is a mistake, and I regret not having foreseen the holes for the anode / grid wires. This tube is a killer by many aspects, it has the CV1067 GEC qualities with a much more authoritative presentation and a wide but realistic sound stage.

I was willing to test the 6P5G that bear an excellent reputation, unfortunately impossible to source a NOS pair at a decent price.

Like in every tube based device the rectifier plays an important role in final restitution.
Here, due to the very few parts used its impact is more than evident and the choice made will determine the preamp character. I tried all the ones that could fit, directly or indirectly heated. On the dozen I listened to, the very best for neutral but vivid presentation are Mullard GZ30/CV2748, Mazda GZ32 and STC/ Brimar 5Z4G, with a slight margin for the GZ30 in terms of bass resolution and voices transparency.

Back on the air...
Normally there should not be a part 3, but I recently found a few Visseaux 6J5G and put them to good use in my line preamp.
While listening to CD's and LP's I know very well I was so surprised by the overall qualities that I called my wife for a neutral and objective appreciation. It did not take a long time to get the certitude that what we were listening at was the most realistic of all we have heard till today. These tubes are among the very best, not to say the best, of all the 6J5 I plugged in my preamp. More important, when used with the MC1/60 amplifier the combo brings music reproduction to a truthful level hard to beat.

Visseaux was a French manufacturer established near Lyon that made high quality tubes (each being individually tested) till the 50's before becoming an ITT subsidiary.

Construction is typically Visseaux with copper alloy fasteners, grey mica and black anodes.

Amazingly the tubes I found bear the Radio Technique logo, which is very surprising RT being a Philips subsidiary. Could it means Philips bought tubes from Visseaux and relabeled them ?
This would have been very unusual because the tube history told us the contrary, Philips sold hundreds of thousands of tubes that where relabeled by famous companies such as Siemens.

A snapshot of my main sytem.

6J5 line along with a tetrode based LCR phono preamp ( I will post one day when power supply will be definitely set) and its Entré ET100 (Soltear Electronic Japan fed with special Tamura's I suspect to be TKS83 relatives) step up transformer companion.
Garrard 301 armed with JVC 7082 on a very heavy plinth.
DL103, AT33 Mono and the incredible Entré EC30 to go with this arm.
MC1/60 amplifier.
Modified Klipsch La Scala ( Wooden horns + JBL 2470's, AlK Xover ).

For this report we listened and appreciated

A while ago I found some peculiar tubes that I bought for a decent price.
These E140 tubes, like the E60M, where manufactured by SFR and it was a reason for me to buy some.

SFR tubes are among the best constructed I ever saw in my life. Needless to say I knew nothing about these triodes and at that time I was not sure I could use them for an audio project.
They remained a while on a shelf till I decided to find some useful data's. Searching the web brought me very little information, not a surprise (all SFR production was intended for military use and records or data's are really unobtainable). But little was enough to catch my interest. I found that this tube was similar to a Philips transmitting triode, the TC04/10, for which I found data sheets.

I spent a lot of time plotting points to make a decent Excel Ia_Va/Vg graph that I could compare to the Philips data's.
The tubes are the same (almost) and at first sight not ideal candidates for audio use. High µ, high ρ, 10W power handling. I was a bit frustrated.
However these kind of transmitting triodes reminded me the mail I had with the late Nobu Shishido (I bought his book but did not understand a single japanese writing and he has been very helpful in translating some parts). In the last card I received from him he explained me how to use a line out transformer to drive tubes in A2 mode with low impedance and grid current.

At that time I was just wondering about the benefits using tubes with grid current while avoiding large NFB amount to temper HF ringing and keep good overall bandwidth. Moreover in the 90' I still could find plenty of good tubes I could use A1. It was simply not my way thinking HiFi.
I was wrong.

I have today a more open mind and I decided to give a try to Nobu's approach. It was challenging as I never used tubes that way. I was in "Terra incognita". I took some time to read more about A2 modulation and decided to start this project whatever the result.
A careful look at the E140 _ TC04/10 curves shows up the very good linearity with or without current grid.

A few maths later I realized that this project was feasible and I ordered parts to build an amplifier.
In this peculiar amp grid current do not flow all the time (unlike Shishido's amp) and I cannot take advantage of permanent current cancellation.
The E140 is biased such a way that the amp works A2 for a small portion of grid swing, then switch to A2/A1.
Many would think that this abrupt change will impart some sonic alteration.
Believe me it's not the case. I have breadboarded one unit and made a try with my cumbersome work, this amp sings very, very well even with the cheap parts I had on hand!
Encouraged by the results I am now waiting for high quality Hashimoto irons.

A small batch of triodes is a good idea when initiating such a project...

And to sort tubes to get matched pairs whenever possible is a good one too.

Next step, driver requirements. Stay tuned.

Class A2 amplifiers need a more elaborated driver than the usual A1.

Technical approach

To source the E140 grid properly it imposes a low impedance driver stage. It is the heart of this project. I could have used a simple cathode follower directly coupled to the final triode, but to have a steady E140 operating point I had to use a well regulated high voltage supply, which I did not want. A medium power triode used as voltage amplifier loaded by a step down transformer was the Shishido's way. I bought two Tamura A-8713 (20K/600) line out trans. Not too expensive they accept 20mA unbalance primary current with very decent bandwidth.
Though providing a quite low impedance path to the E140, the IT voltage ratio (1,73 10^-1 or -15,3dB) commands a tube capable of a very large anode swing with low distortion. Not so many candidates. Keeping in mind the necessity of a current not exceeding 10mA trough IT (it roughly corresponds to twice the E140 grid current at maximum positive swing) the 12B4A is the tube to go. At 20K load its linearity is excellent and is able to deliver 350Vpp with low distortion, giving a driving voltage behind IT up to 60Vpp.
I do not need that much.

Back to SFR E140 triode

after a careful study of the E140 characteristics the best operating point for full power is

Vak 240V, Ia 40mA, Vg +10V, Rload 5K.

It permits an anode swing of 330 Vpp with a grid swing of 40 Vpp.
The output transformer have a 4 10^-2/-28dB voltage ratio, thus 330 Vpp or 117 Vrms will give 4,68 Vrms/8 ohm or 2,7W. With a dummy load the amp puts out 3,8W before clipping.
Enough for any sensitive speaker.

12B4A point of view

To get 40V pp on E140 grid the 12B4A triode must have an anode swing of
40 x 5,78 (ITvoltage ratio in that way) = 230 Vpp
The operating point will be set at

Ia 10/11mA with Rload 20K, Rk 3.3K, Vak 225/235V, V+ 260/270V

On the characteristics below we can see that this tube is up to the task.

First stage

this was the trickiest choice I had to make for this amplifier.
I needed a high voltage capability tube mainly because most of the A2 circuits I studied used some NFB and that I kept in mind the use of a local feedback loop between power stage and driver to:
1- get a better damping factor.
2- cancel amplitude distortion when tubes reach their extremes.
3- keep a good overall bandwidth.

To make a long story short I finally choose the E80CC among half a dozen contenders
( 5687, E182CC, 12BH7A, 6CG7...).
The E80CC is renowned for its sonic qualities and very low distortion. Many professional audio devices used this tube primarily intended for computer use.

The 12B4A needs 40 Vpp (14,1 Vrms) on grid to deliver 230 Vpp. Even with a 6dB feedback loop between the last stages the E80CC will swing the 12B4A with low distortion. Below a 150K loaded tube with a 32 Vrms (90 Vpp) output @ 1.4 Vrms (4 Vpp) input shows the high gain capabilities.

In facts DC load differs from AC. AC's one takes the next stage grid resistor Rg₂ in account. In that case if I make Rg₂ = 220K the AC load is about 90K. As seen below it does not change a lot the gain capability but slightly increases distortion (Philips data sheets give an output voltage of 20 Vrms (56 Vpp) @ 3,4% distortion under 250V/100K, we should be very close).
The main problem with high Rg will be Miller's effect. I will point out the incidence on 12B4A bandwidth in the next article.

Just two makers for this very fine audio tube, Philips and Tungsram.

Next episode: complete schematic, power supply and more...

Tubes selection made, now it's time to mix up all these good things.

Before publishing amplifier schematic let's have a look to the driver limitations.

High frequency: 12B4A & Miller's effect

The input capacitance of the tube C, in conjunction with the source impedance Rg of the stage, forms a simple low pass RC filter with an upper -3dB cutoff frequency equal to:

f = 1/(2π x Rg x C) where C = C_gk + C_ga x (Gain_12B4A +1)

The -3dB point with a 220K Rg resistor will be: f = 1/(2π x 220K x 39pF) = 18.8kHz
As I can neither reduce the load (90K _ See part 2) of the previous stage without lowering the E80CC gain nor increase Rg, some local NFB will help to keep a good overall bandwidth.
A 4 to 6 dB loop fine tuned with an FFT analyzer will give the desired bandwidth and distortion figure, definitely set by listening test.

Low frequency: inverted interstage transformer

Tamura IT is used with inverted primary / secondary coils.
In that way E140 grid current in the secondary cancels a portion of 12B4A current flowing in primary coil.
Unlike all Shishido's amplifiers working A2 all the time, this one can swing the E140 A1 where no grid current appears. This is why I have carefully chosen a 12B4A operating point where anode current do not exceed 10mA.
With such a primary current the transformer low frequency response is very good.

Amplifier schematic

Power supplies

This amplifier requires both high and low voltage.
Low voltage is required to bias E140 grid through interstage transformer. Needs to be very steady and adjustable, a LM317 is the simplest way to achieve good regulation and very little ripple. It requires just a few parts and can be easily implemented using PCB's.

High voltage supply is a bit more complicated than usually mainly because the power stage high voltage (240V) is lower than the ones needed for previous stages (270/260V). It imposes to split the supply in two just behind the first choke keeping in mind the necessity of well calculated cells to insure good transient response. In that way I had to consider both ripple attenuation and time constant factor.

I choose choke input filter for low, almost perfect sine wave ripple and good regulation. Moreover it is less stressing for the power transformer than capacitor input filter.
Unlike capacitor input filter the ripple is not a function of current. It just depends upon voltage, choke and capacitor.
It can be calculated from formula:

V_ripple = η V_HV where η = 1,2/LC (L in Henries and C in µF).

In that case it will be 0,00048 x 260 = 0,125 Vcc or 0,044 Vrms and represent a 60 dB attenuation of the ripple behind rectifier (44 Vrms). Very efficient!

Last post will talk about parts selection, amp construction and tests....However there is so little people interested in this blog topics that it will probably be the last one.

Anyway I thanks those who took time to read my publications.

In every audio equipment there is a gain or volume control to set music reproduction to an enjoyable level . This control is usually devoted to a simple potentiometer mostly for cost reasons. On pricey materials you can find more elaborated and expensive devices like L pad attenuators but if there are just 2 or 3 resistors in signal path to minimize sonic alteration the reflected impedance constantly vary like in basic potentiometers.

In professional gears, like mixing consoles, pots are very seldom used and signal attenuation is provided by more elaborated units. T pad and bridged T pad are the most widely used.
If they essentially remain voltage dividers these particulars setups are intended to keep a constant impedance whatever the attenuation. Very important when used in 600 ohm lines they can be of real benefits to prevent bandwidth alteration in amateur constructions.

Such attenuators are by far the best way to control signal. Unfortunately good NOS units by Daven, Tech Lab or Langevin are scarce and expensive but it is possible to the average DIY'er to build its own one with the great advantage of choosing the impedance to meet particular loads (I have on hand some excellent transformers with uncommon 900 ohm loading impedance).

They are quite easy to build, you just need a good soldering iron, some quality pliers, a bunch of resistors and plenty of time. I personally use Dale RN60D, Dale RLR7C or Philips/Vishay MRS25 (excellent for the task, easy to find and affordable). I also prefer the bridged T attenuator over the more classic T pad because it is fully symmetric.

pic taken from allaboutcircuits.com

Bridged T calculator greatly eases resistors calculation.

A good rotary switch is essential. Must be of the shorting type (make before brake) and have at least 20 contacts. Some old Siemens are excellent but hard to source.
Fortunately very high quality equivalents have been made in the former East Germany by RFT and are easy to source. They have 24 contacts, silver or palladium on copper. With such a switch you can build a 0 to -40 dB & off / 2 dB by step attenuator.

Step by step construction

First edit a file with impedance load, desired attenuation and resistors values.
For a 600 ohm one with a 2dB step the following values are:

RFT rotary switches are the best available choice for the price. Cold war time material, they where intended to work under adverse conditions. Very well constructed they are easy to disassemble and reorganize to suit our purpose.

1- Fully disassemble switch (except rotary mechanism) to access the phenolic wafers.
The silvered ones need some gentle cleaning with a piece of fabric (Never ever use chemicals !)

2- With a 3 wafers unit, set 2 wafers back to back for the R1 series resistors using the small spacers.
These will be populated later. I choose to make the common loop first because it is the trickiest part to do.

3- Make a common (Ground) loop from a silvered copper wire and insert a short fiberglass (or any other insulation material) sleeve to prevent any unwanted contact

4- Common loop hold in place and soldered on contact 1, this is the off position

5- Shunt resistors R2 bent and cut to proper length prior to solder. It helps a lot to solder first the resistor on the opposite contact to the common loop. It makes this one stiffer.

6- Common loop fully populated

7- Now it is very easy to feed the R1 series resistors wafers

All R1 resistors soldered...

8- Reassemble the shunt resistors wafer, add the two Z0 impedance matching resistors and you are done. Expect 8/10 hours for a complete unit. Now you have a nice attenuator ready to use

Any question, feel free to write

This project was initiated by a request of a line preamp with selectable input sensitivity to match sources with different voltage outputs. This preamp should also have a low output impedance to drive long connecting cables without any loss.
With such goals in mind it became evident that a simple 6J5 preamp would not fit these requirements.

1- The use of two gain control stacked together would greatly impact music transcription.
2- The gain of this preamp would have been too low to accommodate an input voltage divider up to -6dB.

A high gain stage with a voltage divider at input and a volume attenuator at output (which is the best way to control the gain from my humble opinion) was the way to go.

For that purpose there are a lot of good circuits with good linearity and gain, cascode, totem pole, push pull. Push pull circuit, seldom used in preamplifier, is very interesting because it cancels all even harmonics. With such a circuit you can expect very low distortion using triodes but a phase splitter (transformer or tube) add complexity to the circuit, increases final cost and is hard to implement in a normal chassis.

One circuit shares this low distortion, high gain capability without the necessity of a splitter stage, the SRPP.

I have been working with for a long time and I know how it works (well), its limitations and drawbacks (very few when correctly understood and used).
Some would argue that it have a special sonic signature, especially on voices, with a kind of emphasis that could be appealing but do not reflects reality.
Believe me, when correctly used and adjusted (a FFT analyzer is of great help) it can be very neutral without being clinical or cold with huge dynamic and stunning resolution and accuracy on complex music.

SRPP (very) short history and use

Originally intended for video use (1943) it was widely used to drive low impedance capacitive loads with high gain and wide bandwidth.

SRPP as it should be used.

As indicated by its name, the SRPP is a push pull even if not evident at first glance.
I won't bother you with any mathematical demonstration because it is not the main topic and there are very good publications that help to understand this circuit, Audio Xpress The Optimized SRPP and Tube Cad SRPP deconstructed.
Never forget it works like a push pull only when loaded and gain, distortion, bandwidth greatly depend upon this load. Moreover its linearity is a direct relation with the flowing current which itself depends upon load.
The good thing is if you have a well defined load that won't vary with the amplifier connected behind, you can fine tune the circuit to get the lowest distortion playing with only 3 resistors. Reason why I choose to load my SRPP with transformer.

Finding a good one was easy because I had some Tamura on hand.
The TKS20 used in this preamp is a 600/10K line input transformer but can be reversed used as there is no current flowing through primary.
Tamura are among the best transformers I had to work with. Unfortunately most are discontinued and prices skyrocket on the second hand market.

Previously I said that the best way to control gain was the use of an attenuator behind the transformer. I was lucky enough to have some Daven 600 ohm dual T pad, perfect for the purpose.
These pieces of engineering are unbelievably well made with plain silver contacts and sorted resistors for perfect tracking. They were built to last a lifetime. The only disadvantage is a 6dB insertion loss that have to be taken in account when calculating overall gain.
These attenuators are very hard to find today, especially NOS, and it is interesting to build its own T or bridged T attenuator. See Pots & Attenuators tutorial.

Tube choice

To reach my goals I needed a tube that gives a minimum overall gain of 4 (attenuator will divide it by 2). This means a SRPP gain of 16,30 (transformer voltage ratio seen by tubes is 4,08) and a tube with a µ of 20/30, a ρ of 7/12 K to correctly match the 10K dynamic (AC) load.
Double triodes like 12BH7A,13D3,E80CC would be good candidates. In this study it is stated that V1 tubes are identicals.
I finally used the E80CC because I had excellent results in the E140 amplifier and it is easy to source compared to the British 13D3. I made some simulations with the 12BH7A but gain was too low.

More to come...

SRPP calculation

The heart of the SRPP is the 3 resistors used in this circuit (the upper one is generally omitted but it is bad practice if you want a perfectly balanced push pull). Easy to calculate they just depend upon tube parameters and AC load.
With a fully decoupled cathode all have the same value which is :

                                               R = Rload + ρ / µ -1,5

Current through tubes can be determined by the following formula

                                        Io = 1/2 [ Vht / 2 ρ + R ( µ +1,5 ) ] Ohm's law...

Gain calculation is way more complicated and is an extrapolation of the simple triode gain mathematical relation

                  G = µ Rload [ ρ + R ( µ +1 ) ] / ( ρ + R )² + Rload [ 2 ρ + R ( µ +1 ) ]

An Excel file makes a very handy tool to calculate all these parameters. Just has to be filled with tubes characteristics and AC load.

I also use it to determine the DC load RaV1. Drawing this load line on tube curves helps a lot to check linearity and allows to precisely set the working point (NB: This load to not take in consideration voltage drop in the 2 upper resistors, it is necessary to add it to anode voltage found on curves to have the exact Vak value).
This load is Io formula's denominator

                                                 RaV1 = 2 ρ + R ( µ +1,5 )

It is almost like a 47K loaded single tube and data sheet shows that we can expect very low distortion. In the case of a classic anode follower we have 4.1% mainly second order at 50Vrms!. In this preamp we deal with smaller voltages and the even harmonics will be greatly canceled by the SRPP configuration. Furthermore the 1.3K value has been selected using a HP 3561A dynamic analyzer to get the best distortion figure.
The SRPP operation will be very close to the Philips values, with a 57K load we have a 3.07 mA current for a 4V cathode voltage and a calculated gain of 18,11.

Line preamp schematic

Nothing special except an unusual input voltage divider. Gives some headroom on high level sources. Three positions, 0 -3 -6 dB permit a very fine gain control in association with the T pad. I used the excellent GRAYHILL 44 series rotary switches for both input/output selector and input attenuator. The quality is outstanding.
Like in the 6J5 line preamplifier just a few parts. Resistors are Holco H2, Philips (not Vishay) MRS25 and coupling capacitors ITT PMT/2R.

These are to my humble opinion (and to my friends ears...) some of the best caps for the price along with AEROVOX V161 and ERO MKC 1860 / KP1832. They give a very neutral and accurate restitution without spending hundreds on exotic parts for a somewhat slight improvement. I made tests with well-known pure copper or silver film and foil ones. There was an improvement in transparency or finesse on certain parts of the musical message but each time to the detriment of the overall tonal balance .....went back to the ITT's.

Looking inside...

Can't be a more compact wiring

Next step: power supply, fully assembled unit and tests

Power supply

Unlike the 6J5 line preamp I used a CLC filtered supply. I usually prefer the LC for its very smooth ripple behind choke but listening tests demonstrated better dynamic with SRPP. A good calculation of ripple rejection and time constant helps to achieve a fast recovery supply with a minimum of cells. To minimize intermodulation the supply is splited in two after the second capacitor

Ripple calculation

On C₁ the ripple is calculated by formula V_C1~ = 10 I/C where I is the current through circuit (in mA, here 7mA) and C the filtering capacitor (in µF).
V_C1~ about 1.9 V_RMS with a time constant of 7 millisecond.

On C₂ the ripple is calculated by the voltage divider formula Vout = Vin Z₂/Z₂+Z₁ where Z_L1 is 37.6K @ 100 hz and Z_C2 50 ohm at the same frequency.
V_C2~ is about 2.5 mV_RMS with a time constant of 24 millisecond. Thanks to the 60H choke that smooths out the ripple with great efficiency. A 20H one would have been enough but I had this one on hand.

The same applies on C₃ with R = 2.7K and Z_C3 133 ohm. It leaves a 0.12mV_RMS ripple which represent a -130dB attenuation at a time constant of 33 millisecond.

Parts

Good sounding Philips/RTC EZ81 rectifier along with old Tango Hirata choke and some Siemens MP/JS and F&T capacitors are the parts of choice for this power supply.

Tube choice and tests

Not a long or tedious quest for different brands of tubes to be tested, just two makers for this fine tube; Philips (can bear Valvo, TFK, Siemens and so on names) and Tungsram. I attentively auditioned these two competitors and the result is disconcerting. In this setup the Philips that bears a reputation of excellence is not the winner of the test. The musical rendering appears fuzzy despite some great qualities in terms of tone and speed. It gives a blurry image that leaves the observer perplexed. On the other hand, the Tungsram brings music to a level of clarity and enjoyment seldom heard. It has incredible precision and gives a density to the sound that makes it perfect on any kind of material. Very detailed, not to say clinical sound but on the good side of absolute neutrality. You like it or not but it never leaves indifferent and if you are after audio perfection you get very close to it. Furthermore this tube gives an almost holographic image of the sound stage. Great, great tube !
As expected distortion is very low. I get 0.9% @ 1KHz and 2V_RMS output, mainly second order. Noise floor is also very low and hard to measure on my FFT analyzer, thanks to the very good CMRR (or PSRR, it's the same) of this circuit. In facts we have the same power supply ripple rejection qualities than in parafeed setup and some will consider the SRPP a parafeed cousin with an active load in place of the cumbersome anode choke.

SRPP line preamp completed

...in its vintage blue gray and black look. This one is now étude numéro 3, the E140 amplifier was étude numéro 2 and 6J5 line preampétude numéro 1

same connecting organization than the 6J5 line preamp. Outputs are doubled by Lemo 0 coax outlets and this feature will certainly disapear in future works. The little male plug is a nightmare to solder considering the 0.6 mm central pin !

Last minute update...

I made some transformer connection mod. Although I did not notice any sound change, its intellectually more interesting not to have the C₁ electrolytic decoupling cap in the signal path.

some CD's and LP's I appreciated a lot, and there are many others ...

This amp have been my music companion for more than two decades and the only one I kept among the few I built until I came across the Philips MC1/60 triode.
It had all the qualities expected from the 10 family of triodes and I especially enjoyed the VT25 Visseaux in this configuration for its rich texture and refined sound.

Both amps remained for a while in my system but listening after listening it became evident that the big triode SE was better in terms of speed and impactness. The VT25 amp finally went to the attic as I have no room for two amps.

I never totally gave up the idea of a better VT25 amplifier but was not in the necessity to ask myself "what makes the difference between these two amps ?" Moreover reflection time being always beneficial to find solutions to a given problem, even unconsciously, I let things slowly growing.

During the past year the few all electronic devices I built made me realize the essential contribution of the PSU in the final result, and naturally came to me this question : Is my VT25 amp lack of punch a supply issue ?
So, I put it back on the bench for major modifications including revisited supply, better implementation, different bias point....and new driver.

In my very first prototype the bias point was quite in the middle of the load line and the amp worked almost A1, current flowing all the time at 22mA. This is not the best for triodes push pull operation, neither for power efficiency (even if I am not really concerned with 104dB speakers) nor distortion and I decided to move AB₁ with a setting current in the 8/10mA range. The biasing resistor was just increased to a 3,5 Kohm value. On the new amp the resistors are 3K, 1% sorted, for a 10/12mA flowing current. For some years now I use the vintage Sprague Koolohm non inductive resistors or Kiwame carbon in tubes cathodes with good results.

I also totally reconsidered my work which was not the most pertinent in term of good implementation: long path from the decoupling caps to the active devices, potentiometer to set input level (excellent for unwanted hum and noise), multiple wiring points (the best for ground loops), poor filament filtering and bad supply time constant. Despite these negative points the amp was performing very decently and I can expect some clear improvements with the right modifications.

Prior to change the power supply and the amp being completely stripped, I modified the driver on one amp. I wanted to take advantage of the very good results of the SRPP stage in the MC1/60 amp and performed some blind test to see (...hear) which driver was the best. The 5687 transformer loaded SRPP is the winner, no discussion. Excellent image and tempo, deeper sound stage, extended low end (no current through IT primary means wider bandwidth). Better linearity and lower distortion are this setup assets when coupled to the Tango NC14. Furthermore the good PSRR of this circuit will help to simplify HV supply. A good point and first step to an improved amplifier.

5687 µ &ρ

5687 SRPP resistor, gain and current calculation

5687 DC load line. Tube will work in a more linear region of characteristics than the previous stage

One minor drawback is a more demanding driver stage. I need to provide a 2V_rms input voltage when 0,9 where sufficient with the former one to swing the amp full power. However I can switch to the E182CC / 7044 for higher gain if necessary.

Good 5687 and E182CC chosen from my stock for this test

Amplifier revisited schematic.
A 5µF coupling cap will insure good bass extension with a -6dB cutoff @ 6Hz

To be continued

Had the opportunity to get a professional pass for the MOC and I spend 3 days of pure enjoyment listening to some of the best audio gears an amateur could dream for.

Also had the chance and pleasure to meet Thomas Mayer and listen to his great sounding electronics. This usually happens once in a lifetime and it was a very rewarding.

Had a talk with Martin Brenner of Vinylista who brings interesting solutions for better LP's transcription. I was especially interested in the new Tenuto bronze sub-platter for my 301.

Took some pictures of what was the most relevant at this fair for the modest amateur and DIYer's I am.

Silbatone huge speakers system

GIP Speakers

Beautiful Western Electric reproductions.
Mr Koji Kikawa gave me a general products catalogue and I was surprised to see the number of speakers you can get from them.

Thomas Mayer great electronics with ELROG tubes

I personally appreciated the vinyl preamplifier coupled to a Garrard 301 on Vinylista plinth and Thomas Schick tonearm.

And during my roaming I found a well recorded vinyl by Audio Note

Merci Philippe et Laurent.

On the bench for surgery.

This major changes just leave transformers, choke and sockets on the chassis.
I also kept the hum and push pull balance potentiometers but removed the electrolytics (replaced by Philips 021 low ESR) and made a new ground line from solid 1mm² (17 AWG) silvered OFC wire.

Amp being almost completely stripped first thing to do was to change the 5687's socket.
Originally I used a phenolic Chinch one but after several tube tests the contacts became loose and the way I built my amp gave me no possibility for an easy replacement.
It is important to select good sockets to prevent any unwanted noise. When troubleshooting an old tube equipment, this is the first thing to check.

Today I use excellent Russian military ceramic ones with heavily silvered and super tight contacts. So tight that I can lift the amp when removing the tube! Not to be confused with cheap Russian's manufacture with thin clad metal frequently found on the web. These are the best I have on hand with old Schurter silver or gold plated ones (West German made in the 60's).

Supply choice

I had to make a decision, C or L input filter ?
Setting the amp for AB₁ operation will favor choke input filter as there is some current variation depending on how deep I modulate the amplifier. In this way the choke acts like a constant current device, reason why it was widely used in class B amplifiers.

On the other hand I have noticed better dynamic with SRPP when using a CLC filter.
I finally opted for the choke input filter mainly because its inductance helps the current to change very little during the AC cycle thus providing an almost perfect DC to the whole circuit (E130 amplifier). Moreover this kind of filter is less stressing for the transformer and the rectifier and permits the use of a quite large capacitor behind choke without sacrificing the network time constant. In that way I can calculate a well filtered supply with just two cells (push pull configuration have a very good PSRR). Minimalism and efficiency.

It reminds the great electronics of the past with just two small capacitors in the main supply and no hum at all. These guys knew their job.

One important point when using a choke at input is a «starting current» through circuit. Below this minimum current the choke acts like a resistor only.
The minimum amount of Henries depends upon the total resistance in series with rectifier R_s and the internal resistance of the circuit R_i, which is the voltage to current ratio behind choke (Ohm's law).

Lmin ≥ ( R_s + R_i ) / 6πf where f = supply frequency

Usually R_s is small compared to R_i and can be neglected so we can use a simplified formula.

Lmin ≥ R_i / 940 for 50Hz and Lmin ≥ R_i / 1130 for 60Hz

In this case, with an estimated current of 30mA (VT25A 2x10mA + 5687 10mA) @ 440V, the minimum would be ~14666/940 or 15,6 Henries. My choke is 10H, to work properly I need to pump 15mA more and the simplest way is a bleeder resistor.
The resistor value will be 440V/15mA or 29,3K, closest standard value 30K and 6,7W dissipation. Need to use a 30W one and it will be hot, thus have to be cleverly located away of any heat sensitive part like electrolytic capacitor.

Two filtering cells command a quite large smoothing capacitor. For example a 200µF C₁ capacitor will provide a very low impedance path to the 100/120Hz AC while keeping a time constant below 20 milliseconds for fast recovery and good transients. The ripple on C₁ will be 2,64 10^-1 Vpp or 9,3 10^-2 Vrms (6J5 line preamp for calculation). A second cell with only 8/12µF will floor the ripple to negligible value to properly feed the SRPP.

30W bleeder resistor

Push pull balancing and hum potentiometers. Shunt resistor straight from choke to the point where all grounds will return.

PSU schematic

more to come

Wiring and components

I started wiring the SRPP driver first.

It is the trickiest part to build as I favor compact, direct to socket implementation whenever possible. I also like to use soldering lugs like SATO's when I have enough room (MC1/60 driver). In my youth I was very impressed by Tektronix wiring method using ceramic soldering tags.
The third resistor (upper anode) is missing, it will be directly soldered on the last decoupling cap when filtering bank will be held in place in chassis.

Star grounding, point to point wiring, short leads, no shielded wire (whenever possible) are the basics of a trouble free construction. I actually make most of my supply and ground connections with solid and find it to work well. Had the chance to find a roll 0,5mm² heavy silvered OFC wire for that purpose.
Allen Bradley, Corning, Koolohm resistors, ERO MKC 1860, Philips 021, F&T electrolytic and SEL polypropylene decoupling capacitors are the reliable parts of this amp.
Like in all my works I use sub chassis for the HV and filaments filtering caps and it greatly eases construction.

From outside looking inside....clean and clear.

Third SRPP resistor from 5687's upper anode straight to the decoupling capacitor. Very short connection.

Listening

To say it straight, this modified amp sounds really great. LC supply and SRPP driver made a HUGE improvement on sound qualities! In addition to the fact that it is now dead silent this amp drives my La Scala effortless, mainly because of the low damping factor (mandatory on these speakers) and music is rich, full bodied, detailed, involving... Not enough words to say how I am delighted whatever the material. This amp has the kick and speed of the big DHT SE amp with a wider and deeper, almost 3 dimensional sound stage. Its extremely low noise level helps to catch micro details and brings music reproduction to a level of clarity and realism seldom heard.

Obviously the tubes combo changes the amp character and the best to my ears are GB 5687 Sylvania, VT25A Philco (a rare find, even scarcer than the very sought after WE, that need some burn in time to develop their rich texture) and 5R4GYS Philips.

5R4GY rectifier is the tube of choice with 530V AC and the Philips/RTC deserve their reputation of naturalness and accuracy, they are way better than the RCA's I used first. An other choice could have been the EY500A, they need a 6.3V filament supply and provide a HV soft start which have some advantage with oxide coated cathode triodes (unnecessary with thoriated tungsten tubes).

VT25A Philco

5R4GYS RTC Philips

Prefer the VT25 (Sylvania, Visseaux or Neotron) for strings music (excellent on guitar) or as a mid/upper band triode in an active system . They lack, to my opinion, the bass extension the VT25A have but are unbeatable on voices.

10 / VT25 Visseaux with that unique tungsten light

I gave a try to the E182CC. This tube is a good contender in place of the 5687 to get more gain from the driver (not really an issue in facts). Sound is a bit more thinner and it does not extend in the bottom end like the Sylvania Gold Brand's but stays very natural, articulated and vivid.

As I said this amp is very involving…sooo good that when I stopped listening it was three in the morning! Needless to say the MC1/60 went to the attic (for the moment).

Just a few CD's and LP's for this report

A rare find initiated this project. A pair of new and mint audio transformers by LIE Belin came into my hands at a moment I was thinking how I could put to good use some tubes I have in stock for a long time.

A short history about that French company will help to understand why I was so amazed by this discovery. During the 50's LIE was a manufacturer of top of the line professional audio equipment and turntables. The company was bought by Mr. Belin a French engineer inventor of the Belinograph the FAX ancestor. Under is clever direction the new company LIE-Belin sold cost no object equipment and parts to the former RTF (French Radio Television). These parts are now very sought after by French DIY'ers for their outstanding qualities especially turntables, but not only.

Belin turntables make the best of professional gears, like EMT's, look like toys. Their top of the line weighed about 250lbs and was intended to be used in the most severe conditions 24 hours a day. None of them had an issue. If you can source such player on the second-hand market you will understand what ʺ last forever ʺ means. Here a nice example of a TD32 from HIFI VINTAGE.

Back to my transformers. The ones I found are 10K/100 Ω split, capable of handling 60mA with a frequency response of 30/45 KHz ± 0,5 dB. Renowned for their transparency and accuracy, these beefy units weight about 4lbs and are as big as an output transformer. For some they are the best transformers ever and unfortunately some scrap merchants did not hesitate to dismantle complete working units to grab these gems. What a shame!

SN312 mixing console using LIE Belin transformers. Pictures from Audiovintage Forum

Tube choice

With such treasure in hands I was in need of a more than decent tube to go with, but just a very few can accommodate the steep voltage ratio (0,1) while providing a usable output voltage.
High trans-conductance and high µ triodes or pentodes (triode wired) are certainly the best choice. I stock EC806S, EC8010, 5842, D3a and C3g that can be good contenders. Drawing load lines revealed that the first ones did not have enough headroom, most biasing in the -1,5 to -1,8 V range. Means with a usual 2V rms DAC output, tubes will hardly clip even is not using the preamp full throttle. The best suitable was the C3g, triode wired (full data-sheet here).
NB any 10K/150Ω transformer from Hashimoto or Lundahl will certainly perform very well.

I am quite reluctant to remove the aluminium shield just for a “better” appearance. For those who want to know what's inside there is a pretty nice picture from Bartola Valves

Ip/Up curves show a very linear tube and the µ and ρ (40 and 2,3KΩ) will give a gain of about 32 or 3,2 on transformer secondary. In the case it would be necessary to flattens overall bandwidth I will provide a place for a switch to select a feedback loop (3 to 4dB).
The slightly higher gain than the E80CC SRPP will help me to drive with ease the rather low sensitivity VT25/VT25A push pull amplifier that I use today in my system.

Preamp schematic

more to come and happy new year......

MC1/60, E60M, TM100 SE amplifier - Part 1

MC1/60, E60M, TM100 SE amplifier - Part 2

VT25, VT25A Push Pull amplifier Part 1

VT25, VT25A Push Pull amplifier Part 2

EL84 Push Pull amplifier, the little giant

6J5 line preamplifier Part 1

6J5 line preamplifier Part 2

6J5 line preamplifier Part 3

E140 SE A2 amplifier part 1

E140 SE A2 amplifier part 2

E140 SE A2 amplifier part 3

Pots and attenuators

E80CC SRPP line preamplifier Part 1

E80CC SRPP line preamplifier Part 2

E80CC SRPP line preamplifier Part 3

VT25/VT25A push pull rejuvenation part 1

High End Munich 2017

VT25, VT25A Push Pull rejuvenation part 2

VT25, VT25A Push Pull rejuvenation part 3

C3g Line preamp part 1