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In Reply to: RE: DC coupled 2A3 amp with two rectifier tubes.... posted by fvale on April 13, 2009 at 04:57:14
Hi fvale,
Dual rectifiers, "Yes", but totally different than what you are thinking !
I have been very involved with the DC SE two stage topology for over two years now, and discussed it at length with experienced SE DC amp designers. Here are my present design opinions:
(1) Do NOT use any delays in a DC amp. Allow the entire amp to come up, together, all at once. In practice, this is the most reliable and best way to do SE DC start ups, and to have the longest tube life and reliability. Years ago, Carey had separate / delayed start ups, NO MORE !!
(2) For 25 years, I always felt that totally separate supplies were the best design approach. In SE DC amps, this aspect gets "trumped" by the need to have both stages come up together!! The easiest way to insure this, is to have ONE single power transformer, and one rectifier system, feeding both stages.
The power transformer needs to be "special" if you want the amp (any tube amp) to be great. I feel it must be very over rated, so as to have absolute stability, so mucic's demands on the supply does not pull it down, in voltage and peak current delivery, in any appreciable way (inherent voltage regulation, a NATURAL voltage regulation, starting IN the power transformer.) Hey, hey, its "KISS" voltage regulation !!
This means, your two watt 2A3 amp needs to be run from about a 700 or 800 watt power supply, for the BEST obtainable sonic result! ONE power transformer like this will easily and greatly outperform two "typical" separate power transformers folks apply, to the front end and final's stages. One power transformer and rectification system will also allow both DC stages to reliably start up together and in time with each other.
Use a thermistor, in one leg of the power transformer's primary, in all applications, typically a CL-80 (47 Ohms cold).
(3) Use two rectifiers, to complement the low Z of the power transformer, and the demands of reproducing music, by placing two ALIKE rectifier tubes in parallel with this single high powered power transformer.
Paralleling the dual plates of high Z tube rectifiers essentially lowers the Z of the rectification function, and in the vast majority of amplifier applications, particularly with 2A3s, it will be perceived by listeners as sonically superior to use of a single dual rectifier. ( I've "instigated" several DIYers to build this, over the last two months, with generally good results.)
(4) In a properly executed DHT amp, particularly using 2A3s, avoid the use of rectifiers with a cathode, such as a GZ-34. Use DHRs (directly heated rectifiers, NO cathodes) because these DHR types will preserve nuancing and layering, and timing of musical events in a manner superior to all cathode type rectifiers. So if you want to hear the nuances "as best as you can obtain them", use TWO DHRs !
I personally, for safety reasons, no longer use mercury vapor DHRs, and so, for me, choices might become two 5R4GBs, two 5U4GBs (most easily to source), or two 5V3A dual rectifier DHRs, operating as a single diode, with the plates in parallel.
(5) All of my above comments, are applicable only if one is trying to build the best amp possible. It assumes proper wiring inside the SE amplifier. Many SE amps, in MY opinion, are at C- to F- in this regard.
My SE listening tests in January 2009, lead me to conclude that internal wiring of SE amps is possibly the most overlooked performance avenue left today. If you don't obtain the optimum transfer efficiency, along with the corresponding widest bandwidth, your SE amp is under performing.
SE amplifiers are a "can of worms" and to build a great amp, a LOT of aspects must be properly addressed, with only the right (best) solutions, all applied at once, as a total design.
Again, these five points outlined are my current thoughts and experiences, your experiences will surely vary from mine, and that is perfectly OK with me. I respect everyone's DIY efforts, no matter how humble it may be.
Its the good battle we all face. Thanks.
Jeff Medwin
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