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In Reply to: RE: Dynakitparts.com: who makes their transformers? posted by LowRedMoon on October 07, 2016 at 09:28:36
"What output tube set are you suggesting for use with that large 3.3K transformer?"
A matched pair of "reissue" Gold Lion KT88s from AA sponsor Jim McShane in each channel. The magnetic headroom needed to dodge GNFB induced core saturation is present. Even with adequate magnetic headroom, I still add a cap. between I/P jack and grid to ground resistor that creates a high pass pole whose F3 falls between 16 and 18 Hz.
Filtering infrasonic noise at the I/P combined with magnetic headroom deals with LF error correction signal induced O/P "iron" core saturation. High gm in the small signal complement provides resistance against slew limiting induced by a HF error correction signal.
Eli D.
Follow Ups:
Very good suggestions as usual, Eli. Thank you. My initial idea was to build an El Cheapo type using the transformers and 7868 tubes from a highly problematic Sherwood S5000. I still haven't given the El Cheapo a chance, mainly because a pair of triode wired 6V6es isn't enough power for my use, though 7868s as triodes might do it. That amp still draws my curiosity. I've loved the few low power PP triode amps I've built.
I have a new work shop set up (after a long move) and am eager to build again.
The local technical institute has closed and is having a sale. I may find some test gear there.
Adam
Adam,
Sherwood S5000-II O/P "iron" does not have UL taps. You certainly can make a pentode mode "El Cheapo Grande", which will yield 30+ honest WPC. Make certain that you regulate 7868 g2 B+.
The TungSol data sheet for the electrically equivalent 7591 provides all sorts of good operating condition info. Employ combination bias to work around the "fixed" bias grid to ground resistance limit. "Standing" each pair of O/P tubes on a 100 Ω/470 μF. self bias network provides a convenient test point for setting "idle" current. Please notice that only 1 bias set trim pot. is needed in each channel.
If the UOS 7868s you have test strong, consider selling them and buying current production EH 7591s from Jim McShane. Cross check the prices for current production 7868s and 7591s and you'll understand why I make the suggestion.
If you need a new power trafo, AnTek definitely has affordable candidates.
Eli D.
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