My Fender Twin Amp Not Working - get your guesses in

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Let's play a game.

An externally hosted image should be here but it was not working when we last tested it.

I was practicing tonight for a gig tomorrow at a local festival. About an hour into the practice (not running very hard and not playing constantly), my Fender Pro Tube Twin Amp stopped working. I don't have time (or really the knowledge) to troubleshoot and fix it myself, so I'm taking it in tomorrow morning in hopes that my local tech can get it working. Let me tell you about it and tell you what I've checked so far. Let's see who can guess the problem.

Seriously, the real problem is that I will have to use a solid state MG series Marshall instead of my tube amp. I will try to make it out alive. :apathic:

For a while now, the amp has had an intermittent hum that isn't loud, but annoying at times. The hum volume was consistent regardless of the position of any volume/gain/tone/reverb/etc knob on the amp. Nothing made the volume of the hum change. The hum would show up when the amp was cold or hot. It would go a way whenever it felt like it.

When the amp stopped working, the hum was there, but no signal from my guitar could be heard. After a little troubleshooting, the hum is no longer there. I can not detect any sound at all coming from the speakers at this point.

I pulled the three fuses (main fuse and two under the balance & bias plate) and checked for continuity, but all three were good. I swapped every tube in the amp with old tubes I have that were still working just fine when I swapped in the TAD power tubes and JAN Phillips preamp tubes I now have.

I pulled the chassis out of the enclosure and visually inspected things... lots of capacitors and resistors in there. There was no obvious damage. I screwed everything back together.

The power tubes are about 4 years old and the preamp tubes are about 2 years old. I play somewhere between 6 and 8 hours a week. I drive the amp hard enough to sometimes need the full power setting, but I might be able to get away with the 1/4 power setting most of the time if I tried. I definitely don't run hot enough to overdrive the clean channel. I use both clean and gain channels. I use the vibrato occasionally. I never use the reverb. I use a few pedals from time to time to either get a different distortion sound or boost the level for lead work. Those are in line on the input. I have a multi-effects pedal I run through the effects loop, but rarely use it for anything other than a tuner.

That's about all I can tell you that might matter. So get your guesses in and see if you can call the busted part(s).

Here's a schematic of the amp if anyone wants to see it.
Why guess ? Take a multimeter or an oscilloscope and measure along the signal path, working from the input onwards. First determine operating conditions for all the tubes, then trace the input signal (instead of strumming your guitar or using a signal generator you can use some other signal source, say powerline voltage stepped down with transformer at least 200-times assuming ~115V mains voltage, or computer/CD player etc. playing at very low volume - say at 1 or 2 out of 10).
Given your fault description, it was obvious that it could not be the speaker so I hope the techies there are better than their non-tech colleagues!

Diagnosing faults remotely is not easy. Guessing would only be useful if it were known that there is a common fault like this on that particular amp. Looking at resistors/capacitors can sometimes find a fault, but usually not.

You didn't tell us what troubleshooting you did to get rid of the hum, after the music faded. That might give a clue. Of course, you could have introduced an extra fault!
Arnulf: Because you can't play a guessing game without guessing.

DF96: The non-tech guys answer the phones, run the register, and sell guitars. Well, one of them is the owner, so he usually just talks about fishing. The hum went away shortly after all the amp stopped working. It wasn't actually fixed. It may or may not have anything to do with the other issue.
The hum was pretty low. It might have been 60 cycle. It wasn't consistent at all and kind of sounded like it was boiling in and out at times.

That schematic is not the right one. I've got a link to the correct schematic in the first post in this thread. It's the newer Pro Tube series of amp, not the old Twin Reverb.
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Without a scope and signal source it's gonna be hard to find.
The most common problem I found with ax amps was the darn phone jacks getting broken down due to wear and tear. Usually the contacts wind up getting mauled and will no longer make or break connections properly - so check your pins and insulators on the phone jacks. (I do not have much experience with ax amps - mostly PA stuff and home stereo stuff). (Unless you wanna include a buncha Military and Government radar kinda stuff).
my particular can guess that

a) get the schematic, fold it twice, put your coffee cup on it
a) test all the diodes....look for shorts and/or opens
b) test all the tubes, gas, leaks, shorts, emissions, transconductance
c) test all the capacitors with a capacitance meter
d) test all power supply resisters for correct values with a multi meter.
e) test all chokes with an inductance meter
f) test the power tranny with a voltmeter, inductance meter, and impedance meter
g) test the out put transformer with inductance meter, and impedance meter
h) look for mods that don't make sense (caps across diodes, diodes across caps, etc

now that you have exhausted your tools set, take the thing into a shop that works on tube amps. leave the schematic home, if they ask you for it, go to another shop.
Arnulf: Because you can't play a guessing game without guessing.

DF96: The non-tech guys answer the phones, run the register, and sell guitars. Well, one of them is the owner, so he usually just talks about fishing. The hum went away shortly after all the amp stopped working. It wasn't actually fixed. It may or may not have anything to do with the other issue.

Do you want help fixing this amp, or are you just trying to impress us with your wit?

Take it to the shop. Don't tell any jokes, they may charge you double.

Actually it wasn;t certain the speaker was not involved. If it hums, then yes the speaker works, but when he reported that now it makes no sound at all, that COULD be speaker related. Fender has a main speaker jack and an extension speaker jack. The main one has a grounding shunt across the jack when empty, the other jack does not. I have seen far too many times when someone tries to figure out what is wrong with his amp, he plugs the speaker into the wrong jack, thinking they are simply two parallel jacks. That leaves the shunt contact shorting across the amp output. Especially on upended chassis amps like the Hot Rod series.

If the thing hums but no signal passes, I would not just start checking all the components, it is perfectly likely it is not a failed component, but a break in the signal path.

ANyone wanting the schematic can go to Welcome to, and under support, amp schematics, scroll down the list to Twin AMp (Pro Tube) and download it.

Isolate the problem. Is it preamp or power amp or in between?

First take a spare cord and conect FX send to FX return with it. ANy help? Now plug a cord from Preamp Out to Powr Amp In. ANy help? if either of those wakes it up, then the return or power amp jack needs service.

If it is not that simple, plug the guitar into the power amp in jack. SOund come out the speaker? If yes, power amp is OK, if not it is the problem. Now move the guitar over to the FX return jack. SOund come out?

With the amp on and the reverb up midway, rock the amp to crash the reverb springs. Does that noise come out or not? The reverb is halfway between FX loop and the Pre/Power jacks.

And test the other way. PLug the guitar into the regular input, then run a cord from the preamp out jack to some other amp and speaker. How does that preamp signal sound over there? And then connect the FX send to that other amp instead, same question.

Those simple tests will tell you what section of the amp has the problem.
For you guys playing along, thanks. I've learned some useful things so far. This seems like a good place to get some knowledge that may save me a lot of trouble in the future. There are some great posts here so far.

I apologize if anyone thinks this thread is inappropriate. I'm sure you can find something to do elsewhere if you try hard enough. PM me is you need some suggestions of what you could go do. :nod:

Enzo, I didn't even think to try crashing the reverb. That's so obvious. What can I say, my normal troubleshooting is more like, is it plugged in? Is the cable good? Are you sure?
Sorry about the delay. The shop was expected to look at the amp on 10/15, but I was going out of town for a three day weekend of camping up in the hill country. On the following Monday (a week ago) I called and my amp was ready.

It turns out that a resistor coming off the power supply was smoked. I didn't get a chance to talk to the guy that fixed it, so I couldn't get a better explanation than that. The hum that I've had for a long time is gone and the amp works like new... well, not like new because the tubes I put in there sound way better than the amp did when new.

I'm so happy to have my amp back so I don't have to borrow a solid state toy anymore.
I am with C2c on this I'd look at the jack. I did some audio stuff right after I got out of electronics and those jacks can just one day give up or the wires break off. I am now a maintenace sparky (electrician) If memory serves me correctly caps tend to be temp sensitive so try hosing them with canned air to cool them.
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