'62 JMI era Vox AC30 rebuild help!

[Dave H]

The cathode resistor and bypass cap is connected to ground properly. I'm also having him check the inputs to the pre-amp.

Are you sure? Both plates are high which implies the common cathode circuit is open. Measure the resistance from the actual tube cathode pins to chassis. It should be 1k5.

Edit: Thanks g1.

[Jazz P Bass]

Rant: WTF does 'out of spec' really mean?

And does it even matter?

+ or - 20% is fine with me.

[jaysg]

Rant: WTF does 'out of spec' really mean?

And does it even matter?

+ or - 20% is fine with me.

Ime, they float upwards in resistance. In a Fender, a 2K cathode reading on a 1K5 is fine. I do like replacing plate resistors with RN65...but only up front.

[guitician]

Don't assume that all the wires have continuity. Use a meter on them, because they could be open and appear fine.

[TravisThatcher]

We get 1.5k from the tube cathode pin to ground. The heater voltages are correct. Definitely still stumped...

[Tom Phillips]

We get 1.5k from the tube cathode pin to ground. The heater voltages are correct. Definitely still stumped...

Yea. Really strange. A couple of additional ideas come to mind.
1) Is the tube definitely lighting up and getting warm? You verified that there is heater voltage on the socket pins but we need to be sure that it is reaching the tube heaters themselves.
2) for the 1.5k Ohm measurement to ground, did you probe from the top of the tube socket with the tube removed?

[Chuck H]

2) for the 1.5k Ohm measurement to ground, did you probe from the top of the tube socket with the tube removed?

+1
Further, was it actually measured from the cathode pin to chassis? Or was it measured across the resistor legs or some other shortened pathway? It MUST be measured from the cathode pin to the raw chassis metal. Not even to the ground bolt on the chassis.

[g1]

Also, heater voltage should be checked between the 2 heater pins, not from either pin to ground.

[TravisThatcher]

Yeah, the heater voltage was measured between the heater pins, not with respect to ground. All the pre-amp tubes have the same, correct heater voltage and all are lighting up. With the tube out of the socket we're getting 1.39k to ground on pins 3 and 8 (from the actual socket with the tube removed).

[Chuck H]

With the tube out of the socket we're getting 1.39k to ground on pins 3 and 8 (from the actual socket with the tube removed).

Read from the socket to where? The other end of the resistor? The ground lug? The chassis?

Can you give us cathode voltage readings?

[g1]

^^^^ What he said, whatever ground you checked to must be connected to other grounds & chassis.
From the layout you posted in post #8, it looks like there is a gray wire to the main ground buss?
If you checked to that buss, then there is not much left aside from the sockets themselves. You mentioned you replaced them?
Any chance they are not making contact to the tube at pins 1 & 6?
The only other thing I can think of that could stop the tube from conducting would be lack of ground reference at the grids. Those empty pins (2 & 7) shown on the layout, are they connected to those wires shown hanging from the input jacks?

[Dave H]

Read from the socket to where? The other end of the resistor? The ground lug? The chassis?

Can you give us cathode voltage readings?

This shouldn't be difficult to fix but we really need the answers to these questions.

[Chuck H]

I get the impression that Travis is doing this remotely Having his friend take the readings maybe? Making it impossible to check things at his whim.

[TravisThatcher]

Sorry - yeah it is a bit remote though I end up in front of the thing in person every week or so.

4.6ish volts on the cathode pins. Checking the resistance from those pins to chassis we get 1.4k which bleeds down to about 1.34 (I assume that's the 25/25 cap doing its thing in there?). Pins 2 and 7 I was getting the same signal from the tip of the jacks going into the tubes on the scope - as in they are connected with that shielded wire.

[g1]

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.