Diagnosing Bad Tube in Guitar Amp

I'd attack it in this order specifically (largely what others have said)...

1) Check to see if it's a preamp or power amp problem. Can you drive another amps power amp with the Preamp Out jack, or drive your Power Amp (Power Amp in) with another amp's Preamp Output. That will at least narrow it down to preamp or poweramp

2) Try new tubes. Get a new 12AX7, or 6L6, (depending on which section you decide to attack), swap one preamp tube at a time (or a pair of power tubes) with a new one(s) to see of that helps. If the power tubes do the trick, don't forget to check the bias. You don't want 'em running too hot and these amps do have a bias adjustment.

3) Check solder joints on all tube sockets, and the input jack and pots as well. Heat them up and resolder if they look even remotely suspect.

A very large percentage of the time, one of these things will fix it. As a previous poster said, buy a "matched" pair of power tubes if you replace them. We'll save the bias discussion for later if you need to go there.

A bad cap in the right place can cause your problem I'm sure, but "generally" speaking, in my admittedly limited experience, I've had sound quality problems (motorboating, hum, ticks, strange oscillations, etc) when one of those big filter caps starts going. Could be a cap, but I'd check the basics first.

As a side note, I fixed one of these recently that had an open plate resistor on 1/2 of the phase inverter. It had some output but sounded terribly distorted no matter what the settings. I doubt this is your problem, since you seem to have very little output.

Hope this helps.

gUiTaR sPaZ
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Joined Aug 6, 2016
I realize this thread is OLD but I learned quite a bit about the underpinnings of Fender tube amps just recently. I have a hot rod deville, made in the 90s. It has the same distorted, low output as described above in the original post. Fender uses Flameproof resistors in the output stage off pin 4 of the 6L6 tubes to protect the output transformer and the power tubes. In many of these amps using 6L6 tubes, the Fp resistors are 470 ohm, 1%, 1Watt. They are meant to open like a fuse when there is an overload. Why not a fuse in series with a resistor? Check these first before you go retubing, recapping, resoldering and all of that other good stuff that's been suggested. They burn, they open and you would never think to check them because they just look so benign sitting there at the output.
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Joined Feb 4, 2005
Yup....also, there have been some issues with the PCB - thin traces and so on. I wish it was PTP... Being near the hot 6L6's, and with mechanical jarring (gigging, normal use...) traces can get buggy, sometimes they lift. Anyone reporting this problem, I look around the sockets to be sure THEY have good solder joints, then I check the resistors.

I bias mine at 70mV, and have had no real trouble with it in the 10 yrs. I've had it. Gigged frequently, too.