tubeswell wrote: ↑Mon Feb 05, 2018 11:41 pm
For getting guitar tone out of it, I'm all for keeping stock Fender values and lowering the B+ to around 340-350ish.
If you're going to put in zeners, mosfets or resistor, it's no longer stock. But I'm totally on your side in recreating the plate voltage of the originals -- but what was it?
I'm talking about getting the plate voltage down to where the 6V6 will idle at 12W on the plate with a 500R cathode resistor (which was the stock resistor in the original circuit). But closer to 400V might be where a '50s amp ends up when you plug a PT designed for 110V mains into a 125V wall voltage.
tubeswell wrote: ↑Wed Feb 07, 2018 9:06 am
If you decide to try zenering down the HT winding with a string of axial-mount zeners (of 9-15V each) between the HT winding CT and ground, see the attached schematic. This works perfectly well for amps with cathode biased output stages. Mount each zener with appropriate clearance from each other/hot components. Each zener will see about 50mA (tops) in a 5F1, so for 15V zeners that's 15V x 0.005A = 0.075W. 1W zeners would be fine - 5W ones even better.
Looking into this, thanks so much for the schematic. Have you applied in this already in some other amps?
Mod Edit: User deleted post and modified this content. Removing their spam replacement. The rest of the thread contains some useful information from forum members.
Last edited by modman on Sun Mar 04, 2018 9:22 pm, edited 1 time in total.
I didn't say it needs to. You can run the tube as hot as you like if you don't mind how long it will last. As to the '500R', 500R vs 470R is the same difference. Older Champ schematics show 500R. Later on 470R became the norm.
340-350V is closer to a centre bias Class A load line (with a the 5F1 OT) than 400V when using a 14W tube. 400V would require a 19W tube to get centre bias class A with an 8k reflected load. 340V enables centre bias Class A with a 14W tube into an 8k load. For a 12W tube running into an 8k load, you need 310V on the plate to run in centre bias Class A. If your reflected load is higher (say 10k), the plate voltages will also need to be a little higher for the same type of operation.