GoodSoundClub - Romy the Cat's Audio Site

In the Forum: Melquiades Amplifier
In the Thread: Single-stage Melquiades vs. DHT amps

Post Subject: Alternative filament suppliesPosted by floobydust on: 5/5/2009

A typical switching supply does not evenly load the input (sinewave) power source and can create some dirt on the AC line (Paul's "backwash" statement??). Depending on many variables, the amount of contamination the switching supply could contribute might translate to "the cure is worse than the cause". Custom Japanese vacuum-tube gear creator Tossie has used some switching supplies for filaments before... one such amplifier was using a 4E27 output tube.

http://www2u.biglobe.ne.jp/~tossie/FJ-4E27-J.html

While I can't vouch for any performance limitations, audible artifacts, etc., it's not my choice for a DC supply in audio gear. I've also not seen any switching supplies in ADCs, DACs or other traditional digital audio gear (CD/DVD players, DAT, etc.) considered to be of high-quality. These components still use linear regulated supplies, usually multiples to isolate digital and analogue (circuit) sections.

I would have a difficult time agreeing that a 60Hz (or 50Hz for other countries) could contribute a positive affect on the sound, but that's simply my opinion based on my earlier post. Taking the filament supply frequency up beyond the audible range would eliminate the audible hum but would not eliminate the intermodulation or (Romy's) doppler-effect. It's quite possible that the effect itself would be inaudible as well at sufficiently high frequencies, but I would still take the approach of testing multiple tubes to get a close match (gain, power output, distortion and filament hum balance level) first and then test for the same performance using a high-frequency supply.

While I certainly prefer an AC filament supply on DHTs, certain DHTs are simply unsuitable for such an application... two which come to mind are the original single-plate 2A3 and the Western Electric 300B. Both have center-tapped (or dual-mirrored) filament structures and hum balance to any reasonable amount is virtually impossible. Others which are usually unsuitable are transmitting tubes which use a helical thoriated-tungsten filament, like the 3C24, 808, 4E27 and many others.

Regards, KM

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