I wanted to share a nice review provided by Lionel Borcelot on his journey in building the VT-25/10Y/801a preamp as shared previously. I think this is a good example on what to expect and how to adapt/reuse existing components at hand to get the most out of this design. Bravo Lionel, excellent job.
“This is the captain’s log in building a prototype 801A/VT62 DAC pre-amp. This was a tandem build with Martial Eisenkremer ( Audio Tube Service) who, as claim to fame, has been requested time and again by Keith Richards to service his amps and guitar before the RS go on stage at Stade de France. I must mention here that this effort has only occurred as a result of Ale Moglia, Rod Coleman and Thomas Mayer’s pioneering work in DHT research and the very kind attention and help that they provided both in communicating directly and via their blogs which details, step by step, the stages of development together with the schematics. An audio-philanthropic enterprise, no less.
Most of the components in this build are off the workshop shelf. This does not necessarily imply that they are shoddy, just that they had some previous incarnation in some expensive albeit outdated gear. Nevertheless, next build will employ something a bit more luxurious like Lundahls. The original filament transformers were totally inadequate as they seemed incapable of providing sufficient juice. So the next best thing on the shelf was a 100VA toric with 2x 24V secondaries unwound to 16V. Raw DC supply is quite standard, rectified by diodes and provides approx 22V to the Rod Coleman regulators. Not too much care needs to be taken there, the regulators are gyrators and they do a magnificent job. It is very important to read the manual and follow Rod’s step by step set up. Next step will be to implement separate transformers.
I have chosen not to use the heavy metal brigade (LCLC) for the B+, but rather the Bartola gyrator which had done, on previous projects, such a great job on the 26, 45 and 4P1L pre-amps. So B+ stage one is a simple and standard 5R4WGB rectifier circuit, which diode, incidentally, I prefer to all others due to its unfailingly crisp and precise albeit very musical sound. It belies, its appearance, potato masher, and its cheap price. Hiroyasu Kondo used it in his 211 Ongaku.In this scheme, its casual a walk in the park for this rectifier.A simple but effective L (20 mH), C (22uf mundorf) filter, the gyrator is there to do the rest. And it does it magnificently, on top of which you can dial in a very precise B+. All voltages and current are as per Ale Moglia’s schematics (gyrator version), 200V B+, 20mA on the plate.
For the initial firing up and 100 hours burn-in of the tubes the filament is tuned to the datasheet values of 7.5V and 1.25A. This is important in order to give NOS tubes a chance to grow up to their fighting best. I have employed the filament bias route with SIC diodes rather than the clunky and very hot resistors path, it seems, for the curious, to be the elegant way to go, both in terms of space, heat and certainly sound. Very, very clean, crisp, wide and deep soundstage. For those who only know cathode bias this has the distance between Paris and Wisconsin. In these conditions and in spite of the somewhat cheap build, the background is as black as it can get, I mean totally noiseless even with full volume into the speakers. It is a pretty good indicator of a reasonable construction, particularly when the B+ supply is on board, which I do not recommend for a 26 build.
I would like to add here that there a few components that, it is my belief, should not be skimped upon. The grid to ground 47K resistors (if you are not using a volume control there) are silver non-magnetic tantalums, you can really hear them. They are magnificently expensive, another choice is the Charcroft, Z foils. Good quality teflon sockets (HiFI Collective) do not go amiss. Ht wiring is ofc copper silver plated (from Audiophonics). All other connections are made with 0,8 pure silver wire either in teflon sleeving for the filament and coton sleeve for the signal. Last but not least most of the components are mounted in suspension with silent blocks.
I have used a heavy duty (12 mm sides and 5mm top and bottom plates) aluminium case which I do not recommend as there is no ventilation per se. And as you will discover some of the components create a great deal of heat inside the case.. So, I bore several large holes in the bottom plate and installed 5 low noise fans, 2 in adduction, 3 in extraction. They are quite silent and have their own power supply. This allowed me to decrease internal temperature from 51° to 31° max with a minimal delta T to room temperature of 7°. I suppose its ok in winter but with 38° in summer we shall have to switch to the 26 pre-amp.
How does this sound? To my ears like what I had never heard before. The tube is commanding, it can pull out the most minute pieces of information from the dark recesses of the recording, but does not highlight it, it makes it appear where it belongs. The placement is uncanny, everything in its own place, the stage is both wide and deep, everything appears sonically where it ought to be. On good recordings the reality is freaky, prodigious…One did get accustomed to the 26 which shares a great deal with this tube, but there is more of everything with this one. If more is better as your creed this one is for you.
In conclusion, a word about myself and my abominable addiction to music. I am an architect and designer with a 20 year heavy duty addiction to DIY machines, simply because I could never afford the lovely boutique machines that made many pricey promises, but entirely unsure whether they were kept. In 2018 I won a very sought after price awarded by the Liliane Bettencourt Foundation in Paris for the design of an innovative speaker. A 360° degree emitting dipole, 100Hz to 18Khz. Please stay tuned… »
Lionel Bourcelot
Very good review of this legendary Valve. I don’t think you can beat the use of Ales Boards, I hve built Amps using these vs Iron & I prefer the boards!
I took one look at this latest scheme and realized that I had 95% of the parts and that It was time to do it. I’ve been collecting parts for a few years starting with the 801’s. The potato mashers are my favorite too since Denis Had put me onto them 20 years ago. It’s true they make for a crisp and clear – and relaxed presentation. Together- is magic!
The single chassis is challenging. The b+ chokes are coupling to rods raw supplies, I’ll try a few things but will most likely move the heater raw supplies to a separate box.
Heat management is also a major challenge- the diodes will get a larger sink connected to the case so I can put the top on.
The sound is really fantastic, and it will go great with with my SET45’s and open baffle lowthers once I get the noise floor down to where it can be.
Thanks Alè you’re a genius and you and Rod and Thomas are doing really innovative stuff together. David
Hi David,
Great to hear you managed to build it and you’re happy with it. It looks really good on that picture. As I always say, attempting a single chassis build on a DHT preamp is a major challenge. Doable, but tricky. Anyhow, you can try bolting the SiC diodes to the top plate to ease with the heat dissipation?
Enjoy that fantastic stage, you won’t be disappointed.
The 45 is one of my favourite valves. I have a hybrid (aka Super Triode) experiment going on to get 8W out of the 45. I made the PCBs and just need to do a few more tweaks before I get this to listen. If works well would make the 45 sound to get back into my system again!
Cheers
Ale