VT25, VT25A Push Pull rejuvenation Part 3

...SOLD...

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 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

Close up view, construction is very similar to Western Electric VT25.

5R4GYS RTC Philips

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

10 / VT25 Visseaux bright light.
These tubes are among the best 10 I have heard, just outperformed by Visseaux A710 big plate triode. These have oxide coated cathode and concentrate the very best of the VT25 and the VT25A in a single tube. From my own experience they better the famous PX4. Years ago I sold a bunch of these (Osram brand) to get a few A710's...Unfortunately these need B4 sockets and cannot be used in this amplifier.

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…so 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).

Last minute update...

I recently bought some Hytron VT62/801A as I was curious about these tubes sound (never had the opportunity to hear 801A's before). What a surprise! Immediately appealing with a rich, truthful and accurate restitution.
Detailed to the extreme with a very convincing low end (quite different of the 10Y/VT25 I have, the bass extension is similar to VT25A's) they are very very musical and a first choice on any kind of music. A great find.

also picked up these ones, excellent too...they make the light sing

801A amplifier in my music setup

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

VT25, VT25A Push Pull rejuvenation Part 2

...SOLD...

On the bench for surgery.

These major changes just leave transformers, choke and the 4 pin sockets on the chassis.
I also kept the hum and push pull balance potentiometers but removed the electrolytic (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).



Load, bias and supply

Unlike SE amplifiers, push pull load cannot be directly drawn fron Ip/Up characteristics. We must use a "composite" load to reflect the behavior of tubes in phase opposition. When current increases in one tube it decreases in the other and the resulting current in one half the transformer primary for any fixed grid voltage is I = Iv₁ - Iv₂.
At the quiescent operating point E_bb (usually the max permissible anode voltage ) the current is I = 0 and the "composite" load must pass through the point E_bb x I = 0 and reach the Ec = 0 line at a designated point for maximum efficiency.
Complete theory explained in Radiotron Handbook 4th Edition pages 573/578.
To make it short, the theoretical maximum output occurs when load line intersect the   Ec = 0 at 0.6 Ebb. The current at this point is I_Pmax.



The push pull load for two tubes is    R_L = 1.6 E_bb / I_Pmax   , here 16K plate to plate.

Maximum theoretical power           Po_max =  I_Pmax² x R_L / 8

I choose a biasing point between class A and class B (Ec = 0) mainly because I had some nice resistors for the purpose and did not need full power from these tubes.

Supply

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 (E140 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 / 95 ohm, 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

High End Munich 2017

Had the opportunity to get a professional pass for the MOC and I spent 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 turntable mat 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 sounding electronics with ELROG tubes

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

Kondo setup

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

Merci Philippe et Laurent.

VT25, VT25A Push Pull rejuvenation Part 1

...SOLD...

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 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 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.

A 5µF coupling cap will insure wide frequency response with a -6dB cut off @ 6Hz for a 5 Kohm AC load but like for a parafeed load the coupling cap is dependent of the transformer primary inductance and is calculated large enough to avoid resonance at an audible frequency. In this case, considering a 50H primary inductance for the NC14, the resonance frequency f₀ = 1 / 2π √LC is about 10Hz. Lowering this value can give some bass boost to the circuit. Interesting for those who feel the VT25 family of tubes lacks of solid bass extension.



To be continued