Bieworm wrote: ↑
Thu 04/21/22 4:14 pm
I would try this:
Change the 350 ohm resistor between C216 B and C to 1k5 5W
Add a 1k 3W resistor between C216 C and each screen
Change the cathode resistor to 330 ohm 5W
Deleting the futile current draw of the unused tube doesn't affect anything IMHO, except a little more gain in the preamp.. maybe
I checked this current amp against another in my shop, same exact circuit. Same mods (short of preamp bypass caps). The wiring is correct, there was not an error as I wondered, there are two variations of how wiring gets from C216c to the power tubes: Cap wired to power tube 2 screen, then wire to power tube 1 screen OR cap wired to power tube 1 screen, then wire to power tube 2 screen. Either case has the wire from R240 (350r) going to power tube 1 screen.
Also, I swapped the caps, tubes, and fuse from current amp to the other one, tested voltages, and I get the exact same scenario - ~330 on screens and ~320 on plates. That must just be how these amps are, though maybe not ideal as Josh pointed out. They certainly lasted through decades of use by the military, and I have several friends playing them without issue both in stock and modded formats.
Is it R240 that is between C216B and C216C? That what it looks like. I can change that to 1k5 5w, will just need to order that part.
When you say to add a 1k 3w resistor between C216C and EACH screen, do you mean 1 resistor and connect pins, or separate resistors for each screen and therefore separate wires from the cap to each screen? I'm trying to visualize it, as my 18w diagram for my ef86 build just uses 1 resistor from cap, then wires screens together. I don't fully understand the concept thus my confusion - I'm trying to increase resistance between C216C and the screens right?
Then it looks like if the above is correct for R240 swapped to 1k5, then that leaves R228 changed to 330r right? Or do you mean change R241, the 220r 8w resistor to a 330r 5w?
If you mean R241, I'll have to think through how I implement that and where I put it. R241 is a long ohmite that doubles as a standoff to hold the turret board in place. I could leave it in place for its mechanical function and add the cathode resistor elsewhere, like down with the 2200 uf cap in the new terminal I put in. Alternatively could I put another resistor in series with it to get it up to 330r or close? I have about 50 10w 50r laying around from a Hammond organ. Probably easier to do option A.