Fender Blues Deville preamp problem - very low volume

[moss0031]

Hello all, I've used this site as a resource before, but never posted. Thanks in advance for whatever help you can offer! I work on synthesizers, mostly, and don't have a lot of amp experience.

Here's the patient:

Fender Blues Deville 212 (mid-90s tube amp)

Symptoms:

Very, very low volume. On at the highest volume (or drive) settings is it even possible to hear the instrument plugged in. This occurs on both the clean and drive channels.

Plugging an instrument into the "Power amp in" results in normal, loud performance. Plugging the "preamp out" into another amplifier results in no sound. Hence, I think I am correct to believe that the problem is in the preamp. I don't have extra set of 12AX7 tubes to test with, but I have exchanged the three preamp tubes around in the amp with no appreciable difference. All tubes in the amp glow when turned on.

The other odd thing, the red indicator light (next to the power switch) doesn't work either, but this may just be a bad bulb, as I do read about ~6 volts between the two contacts under the bulb.

Any ideas where to start in the preamp? Let me know if I can supply any other useful information. Thanks!

[Enzo]

Go down the row of small tubes and check the voltages. All the plates - pins 1,6 - should have some B+ voltage on them. All the cathodes should have some voltage. a couple volts for a plain gain stage, and substantial volts for a cathode follower or similar stage.

If the volts are OK, then apply a signal and trace it through the stages.

[moss0031]

Got it, Thanks Enzo!

Thanks Enzo! That helped me find the problem, which I will post here to help add to the archive.

I tested the voltage on the plates of each tube and that led to two potential problem areas. I saw about ~250V on pin 1 of each tube, but only saw that amount on pin 6 of the third tube (closest to the power tubes). Checking the schematic, I noticed that the preamp circuit doesn't even use the second triode (pins 6,7,8) of the middle 12AX7, as they are just all connected to ground. Hence I determined that my problem was with the second triode of tube 1 (pins 6,7,8). Tracing back from pin 6, I found a resistor (R9 on the schematic) that was reading infinite resistance. Replaced that and everything worked beautifully. Also, in reference to my first post, the indicator bulb was indeed burned out. Thanks all!