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I think all the problems are on V4 which is the send/return tube... pin 2 is far too noisy to measure voltage readings and pin 8 is hidden under a capacitor so I can’t test it. Pin 1 strangely gives me 0 volts. And the only valve where I’m getting normal buzz when testing is the PI (V5)

Is V4 definitely a 12A_7 type? It’s strange because it can’t normally have voltage on the cathode (pin 3) without voltage on the anode (pin 1) too. With the amp off, measure the resistance of pins 3 and 2 to ground. I’d also check what’s feeding pin 7 of V4 and why it’s at 38v.

156k pin 2 and 1.5k pin 3

Double check the voltage on the 100k resistor on pin 1- measure both sides of it.

Looks like that’s a serial fx loop, so try a patch lead from Send to Return and see if that gets a signal going. Could just be bad contacts on the jacks.

I’m drawing up a schematic for that tube. I replaced some parts on the board and a doubt came to mind, if I inverted the send/return wires by mistake, would that cause the issues I’m having?

Nah, it would just swap the function of the jacks around. From your photo, the right jack would be the send and the left (in the corner) jack would be return. I bet V4B is a cathode follower, which would explain the voltage on pin 6 and 7.

There is continuity on the pin1 area, on the other side of the circuit. But where I use the test lead when powering up, there is none so I’m guessing that it’s just the top side of the board that has no contact with that pin

Ok put everything back together and now getting 252v on pin 1 and buzz, pin 2 is still too noisy to get any measurements. And 75v on pin 8.

Also 470k resistor on pin 2 is only giving me 150k

jimmy74 wrote: ↑Sat Mar 13, 2021 12:55 pm Also 470k resistor on pin 2 is only giving me 150k

That’s because the 470k is in parallel with the resistor to the fx loop plus the ones to ground on the send side.

Have you tried plugging your guitar straight into the return jack? Or running a patch lead from send to return? All the voltages seem ok on V4 so I still think the signal through the jack switches may be to blame.

Yep getting signal through the return jack but not through the input, not even with the patch cable in the send/return

Do you have a spare 12ax7 you could try in V4? Or, does probing pin 7 make a loud buzz at least? Otherwise, the problem may actually be upstream of V4.