VT-25 Preamps in Lockdown

One positive thing I can get out of this COVID-19 lockdown is that people are building more audio equipment. Here is a great story from Sridhar Ganti who has built a few DHT preamps over the years.

VT-25 with SMPS filament supply

Sridhar strived to get the VT-25 preamp working with SMPS supply for the heaters. As everyone knows, SMPS are challenging to implement due to their wide-band ripple noise.  They are ok to implement in an output stage where the step-down effect of the output transformer and the SNR is significant to make this noise very low at the output . However, at a preamp level is another story. The signal levels are tenfold or more lower so the noise injected at the cathode is crucial. Think about the DHT as a differential amplifier. Is not just what gets at the grid, it’s also about the cathode. What the triode amplifies is the signal difference between the grid and the cathode.

I tried and failed with the VT-25. I couldn’t get rid of the midband noise. I added a series choke and it wasn’t good enough. In the end I gave up as I needed to add such a filtering stage which wasn’t worth the trouble.

On the other hand, Sridhar went through this. Despite my discourage to avoid the SMPS, he went through the experimental journey and succeeded.

Sridhar’s version of the VT-25 DHT Preamp

Here is Sridhar’s write-up on his preamp:

The 6 diodes give a drop of approx 7.8V. The 801’s are shock mounted on a sub-chassis using 6.32 sandwich mounts from McMaster Carr, link given here : https://www.mcmaster.com/catalog/126/1548. This really helped controlling the micro-phonics. I also mounted the power transformer on small rubber grommets to further isolate power transformer vibrations.
I used Meanwell SMPS supplies with the filtering circuit provided my friend and mentor John Levrault. Getting the preamp right was a little bit of a journey, I had messed up the filament connections, on one tube I connected the +ve to Pin1 and -ve to Pin4, and on the other tube it was reversed. The results was, when I powered up, there was power supply white noise. Since I had not used meanwells in the linestage before, I suspected that to be the culprit. Needless to to say after 1 week of tinkering around, John pointed out my mistake, and when I connected the pins with the right polarity as given in the 801A datasheet, the preamp powered up correctly, and it was dead quite. The initial Meanwell I had went up to max 14V, and with the 7.8V drop across the Sic, it was cutting it fine to get the 7.5V across the filaments. While trying to fix the initial hum problem, I ordered a pair of 24V meanwells, I used those with about 6.2 Ohms of dropping resistors to get the voltage to 15.5V The dropping resistors provide further filtration anyways.
Now coming to the sound, but before that, a little word on my old preamp — it was a 10Y, using LCRC-Coleman filament supply and cathode biased and had a 4:1 step down transformer from Dave Slagle — Intact Audio. So it was a very good benchmark. However, the gyrator load with duelund caps and meanwells filament supply took the sound to another level – more blacker background, more details, the three dimensionality with the music improved in all directions. This is undoubtedly the best linestage I have ever built.
Attached are some pictures of the line stage, its internals with the external filament supply, and picture of my system. The amps are Class A2 805 monoblocks using 211 as the driver with a source follower driving the grid of 805.
Ale – thank you for the experiments you share with the community, and also being very patient in working with novices like myself :-). Owe you a nice dinner next time I am in your part of the world.
Cheers
Sridhar
Here is his circuit implementation:
And a view of the inside:

The VT-25 preamp driving the 211/805 SE mono blocks

A VT-25 from South Korea

Equally, Chul has implemented at record speed his version of the VT-25 preamp. After completing an amazing 01a preamp before:

Author: Ale Moglia

"A mistake is always forgivable, rarely excusable and always unacceptable. " (Robert Fripp)

3 thoughts on “VT-25 Preamps in Lockdown”

  1. Well done Sridhar and Chul. An awesome effort by both of you. And yes, it’s great to see some action with the 10Y/801 DHT front. After giving up 5 yrs back, just today I rescued my 10Y from the back shed. When I fired it up, it was like it had only been on yesterday.

    Just like Sridhar, I have Coleman regs with a beautiful supply and Tango line out trannies. For the B+ of 215V, 80 rect. into LCLC and biased at 17mA (max for Tangos). Beguiling timbre and tone, but no bass and no speed nor drive, hence why it was in the shed.

    …But a few weeks back, I was alerted to Ale’s gyrator circuit. My heart skipped a beat (for this was always an “unfinished” project) and after researching the idea for a couple of days, ordered a pair of boards. Ale advised yeterday that they have begun their long journey to the Antipides right now.

    I have decided to go SS with the B+ supply too – wish me luck! I am a slow worker, but will be sure to post my results when I have them completed.

    I have to say it is very encouraging to see others out there having a crack at DHT preamps and offering up their efforts for all to enjoy and use. Thanks lads.

    Cheers, Red.

    Reply

  2. It’s a pleasure to write here. I tried 10Y pre some years ago on a test board setup and felt immediately that this tube is something different. At the test board, I could try 01A, 26, 71A, 12A, and 10Y, although not in their optimal conditions. The 10Y gave more grown-up, bigger, and in upper-class level.

    I built the power supply in separate chassis and used separate transformer each for left and right heater and HT. Previously I was using a plate choke load. This was good enough but sometimes I felt the character of ‘plate choke’, and this choke is picking up some hums from nearby transformers.
    So I used Gyrator, Coleman regulator, and Sic diode bias. I just put everything following Ale. :-)) I also put the LDR volume module by Tortuga Audio. I think I did almost all that I can in my capability to respect this nice tube in preamp application.

    One big merit with this 10Y pre is that I can enjoy the music in low volume. I do not know why but this is really big pleasure for me. I can enjoy a symphony and string quartet in low volume! Not to mention Bull Evans. Now I can switch between 01A and 10Y pre per my changeable mood.

    Thank you Ale for the help!

    Reply

  3. We have exchanged a few emails between Sridhar and Rod Coleman given Sridhar’s further experiments with SMPS filament heating. So I’m posting them here as I find them extremely valuable.

    As I said earlier, I also tried and gave up using the SMPS in DHT filaments. There is a good explanation for this here by Rod which is worth reading. Sridhar (and others) claim that for the output stage of a transmitting valve they work fine. The explanation to me is that the SNR is very high due to the large voltage swings involved (vgk and Vak). However, the noise and intermodulation effects are still there!

    Sridhar’s feedback:

    Ale, Rod

    thought I will share the adventures with filament supplies, might seem rambling, hope you can bear with it

    I successfully used smps supplies with a class D inductor followed by a capacitor in my 211 driving 805 monoblocks. This was for both 211 and 805

    I tried with SMPS with 10Y, since I had a 24V SMPS, I used a class D inductor – capacitor, followed by a RC to drop the needed voltage for filament bias. It was dead quite, but people who listened to it found it bright, I also felt it

    Built the 01A preamp with LCRC and my old coleman filament reg, it blew the 801A, balanced sound

    I changed the 10Y to LCRC coleman reg, it was better sounding than 01A

    Separately, I built another 10Y supply with a pair of 15V SMPS supply, as I was issues with the regs. The filter here was like the amps — just a class D inductor and 2.2mF cap, it was noisy, you would hear a ‘whooosing’ sound from the speaker, I swapped the 15V with 24V SMPS, and added a dropping RC. Now it was quite, but man, the brightness came back and the music did not hold together

    Since I only had one good working reg, I changed the other supply to LCRC-CMC, it was quite, and now I had 2 channels to compare – one with the coleman reg, and one with passive LCRC-CMC supply. Invited a friend of mine who is a musician to give it a listen with mono records. We found both of them pretty close, but the supply with LCRC coleman reg had more detail, more airiness, and more lively, where as LCRC-CMC sounded a little loss in airiness, subdued and missing the bite

    Thanks to Rod’s help, I fixed the coleman supply, and now I have both the channels running on LCRC –> coleman reg. System sounds awesome 🙂

     

    I think I will trust Ale’s judgements from now on :-), for line stages go with gyrator load, and coleman filament supplies  

    Rod – Looking for the new 10Y regs built by you 🙂

    It was nice long and satisfying journey

    And Rod’s response:

    Hello Sridhar,

    Thank you for the history! I am glad you have found a good sound.

    “Off-line” or Mains→DC converters are a difficult fit for any audio system, but if you try to apply them to DHT filament heating the problem is doubled. This is because the filament supply can only be grounded at the output (the “cathode” ground-point or cathode bias resistor). Connecting an extra ground for EMC (ac- or dc- path to ground) at the input will create audio-current loops.  Any filament supply’s input-negative must always float, and this is not how such converters are intended to be used.

    It’s a long story, but basically, for EMC-compliance reasons the converters must have an EMC-path from the switching-loop (raw mains input, rectified) to ground.

    Either:

     [1.] The output negative is grounded to safety earth, as used by some converters with 3-pin mains plugs: this will not work at all for DHT filaments, as the filament is shorted to ground, and shorts the cathode-bias resistor and/or creates a ground loop.

     [2.] The other main type floats the output, but still uses the output as EMC return — a capacitor that effectively routes primary noise to the output!

    In this case a common-mode choke can smooth out the audio-frequency noise, somewhat, but much of the broadband noise still couples into the audio system ground, and it is very difficult to eliminate. And it has to run through the filament of the DHT to get there: right in among the music signal, IOW.

     

    This noise will find a way to ground, even through the air.

    I have spent many hours in EMC labs, getting statutory approvals for things I have designed. Mains-to-DC converters can be a big problem, and I have often had to use CM filters AND differential-mode chokes plus X and Y caps to get them through at 150-500kHz, tested for conduction back into the mains supply. Below 150kHz is not tested, and the noise is often even worse there.

    When I go to Radio Rallies (UK Hamfests) I take a little truck, to wheel away the transformers. Old industrial units, with C-core trafos, or big control-panel types with screens … These are the best ways to block out mains noise, & get good sound!

    all the best,

    Rod

    Reply

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