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In Reply to: RE: Pentode/Triode Operating Points (6BL8,6U8,6GH8,7199 etc) posted by FlaCharlie on August 30, 2016 at 11:00:38
My experience (sample of one...)
Had a Stromberg ASR120, which uses a gain stage and inverter similar to the Dyna circuit. Rewired the sockets for 6U8, etc., no component value changes. Put in two Amperex 6BL8s; they had voltages quite different from the schematic, but two 6GH8s were right on, so that's where I stopped. If I had to make it work with a specific tube, I'd probably add a 1 meg pot in series with the screen resistor (which I'd reduce...), and adjust it for 3/4 and 1/4 of the supply voltage on the triode plate and cathode.
Follow Ups:
Thanks for sharing your experience with this as well as your strategy for utilizing other possible substitute tubes. Any comment on the sonic differences, if any?
I've got several amps that use the 7199s - including the Sherwood S-5000 that I recently restored and a Stromberg Carlson 220 that I pulled from a console a while back. At this point I'm able to borrow 7199s from unused units if any of them are bad. But it's good to know which subs have worked well for others and how to optimize the circuitry.
FWIW, I lean towards the 6BL8. Among the reasons are the availability of EI made specimens, Scott used the type in their amps, and McIntosh used them in tuners' audio circuitry. For a price, you might come up with NOS TFK ECF80 stock. NOS Siemens is another good option, if you can get them.
The 6GH8, 6U8, and 6BL8 pin out identically. Frequently, direct substitution "works". Tom Bavis' suggestion for optimization bears keeping in mind.
Eli D.
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