'62 JMI era Vox AC30 rebuild help!

[Chuck H]

Either the cathode reading or the plate reading must be wrong. I don't see how you can have current through the tube (as evidenced by the cathode resistor), yet have no voltage drop across the plate resistor. Recheck those plate voltages, maybe the tube is working now and there is some other signal issue.

Cold solder joint?

Either making conduction intermittent or possibly breaking the signal chain at the coupling cap?

[TravisThatcher]

I'm going to hopefully get out to work on it this weekend. In the meantime my friend is going over all the solder joints to make sure they are okay. If I were to insert a signal from the tone gen after the first pre-amp valve stage, so like the 3rd pin of the normal volume pot - would I be in danger of destroying anything? I'd like to check things down stream.

[Chuck H]

You can insert signal anywhere I the signal path. Just be aware of the loads, like volume pot settings. and approximate amplifier gains based on their bias relative to the input signal. That is, don't expect a 12ax7 stage capable of a gain of 50 to amplify a 3VAC signal to 150VAC. The tube would clip long before fully amplifying a voltage over the bias level.

[Dave H]

You could inject the signal at the pot wiper (centre pin) if you set the pot to its middle position.

It has over 3mA through the cathode resistor and the current isn't from the plates (no voltage across plate resistors) so where is it from? What do you measure for cathode voltage and resistance wrt chassis when the tube is removed?

[Chuck H]

And, just in case something weird is happening, take a DC voltage reading at the grid too.

[TravisThatcher]

So I got a chance this weekend to get in front of the AC30 and have made some good progress. Turns out that the cathode resistor had a bad solder joint - it mostly measured proper resistance to ground but was a bit flakey. Resoldered it and voila - bright and normal channels have come to life. Super quiet too as far as noise goes. Now the only issue left is that the vib channel is not amplifying properly. The vibrato itself does work. When we took the amp apart the vibrato switch was not wired up as it is in the schematic or in the layout posted here. The hot from the pedal was going to the bottom of position 3 on the board, ground to position 6 and there was a jumper from 4-7 on the bottom edge (looking at it with the power amp side facing you). We removed the jumper and wired the switch as it is in the schematic. This is an electrically different connection but to me seems to have the same result of grounding out the vibrato circuit and turning that tube off. Is it advisable to return the wiring to the way we found it?

Also - some voltage measurements:
V7 pin 1 is 88v rather than the 140v on the schemo
V7 pin 6 is 179v which seems relatively within spec
V7 pin 8 is 30v
V7 P3 measures 2.2k to ground (chassis)
V7 P8 measures 24.5k to ground (chassis)

At the bottom of R39 I get 208v. Also on the vibrato volume pot you can hear a click around the mid point when you turn it. The pot itself is the correct value and on a meter measures smoothly throughout its rotation.

Thanks to everyone so far for all the help - getting really close on this thing!