high plate voltage on one side of a 12ax7

[rackmonkey]

That's what I'm saying. The actual components are not matching the schematic. The general circuit is the same but there is a number of differences.

On this stage:
Plate resistor is 100k instead of 220k
Grid resistor is 1M instead of 10M
Cathode has a 1.2k resistor and 47uf cap instead of being grounded.

Looks like someone modified it to use cathode bias instead of grid leak bias. 

 

[samgraysound]

Grid and cathode are the same voltage, so tube should be full-on like a diode, with lots of current through the plate resistor, and a low voltage at the plate.

This indicates a bad tube, but you already tried a known working tube.

You are probably not actually checking the tube pin voltages, you are checking the socket voltages, and assuming that they are the same. If you can get in there, can you get to the end of the actual pins? Measure between a pin and it's socket, looking for any voltage. Sockets do go bad, loss of spring tension, corrosion, etc..

Just a longshot.

Gene

Alas the socket has a shield mount thingy so I couldn't get at the pins.

So I replaced the socket. Now it is all new socket, all new caps and resistors for all pins. New ground wire. Put another new 12ax7 in there.

No difference . Still no voltage on Cathode, extra high plate voltage.

Could it be some sort of heater issue? It is reading 6.3vac across pins and 4 and 5, with pin 9 grounded.

Sam

 

[radardoug]

So you have 6.3v between pins 4 and 5? And 9 grounded? Is either 4 or 5 at ground? If so, you are only lighting up one side of the tube. Look at the tube, see if both sides are lit.

 

[samgraysound]

Actually, this is wired wrong for a 6.3v heater right? It should be 4 and 5 tied together with the other end on 9.

 

[samgraysound]

It was the heater wiring!

So I guess a bad mod job? What's weird is it was all period components so it must have been a bad mod from like 1962?

 

[samgraysound]

Do you all think I need to replace these melty wax and paper caps? That happened during a brief issue witha shorted wire while I was working on it.

pic:
https://drive.google.com/open?id=1haDCpvHmhZM9hFNpl308Tfl8wO_qa3Dp

 

[rackmonkey]

I would. Even if they work now it’s just a failure waiting to happen.