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6bq5 triode/pentode/UL mode switch?

gadget73

gadget73

junk junkie
Subscriber
I just plunked down the cash for a set of Edcor transformers for my Magnavox 9300. These have UL taps, and I was considering making use of them via a switch so I could change the operating mode of the output tubes. Thinking about this further, I didn't see much reason why I couldn't switch the tubes from triode to pentode to UL mode with a 3 way switch. Before I go digging into this, could someone confirm how exactly this would get wired?

I'm assuming the moving contact of the switch goes to G2 via a 100 or so ohm resistor
one pole of the switch to the UL lead on the transformer (UL mode)
one pole to the plate (triode mode)
one pole to B+ (pentode mode)

is this correct, or did I miss something ?
 
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A lot of amps have a switch from triode to UL, as the operating conditions for both modes are very similar. Pentode mode often is different enough to make switching to one of the other modes not as trivial. I think the power tube's cathode resistors might need a different value, maybe different feedback too.

I think your description of the wiring is correct. You could also put a resistor between the UL tap and the switch pole if the schematic called for it.
 
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