Tube amp troubleshooting assistance

[cedarskies]

So my Fender PA 135 suddenly stopped making sound mid show. And now it won't make sound. Well, that is not entirely correct, because it will produce a quiet, volume independent 60hz hum.

I have swapped cabs. I have swapped inputs. I have swapped cables. No dice.

I have consulted that geofx tube amp debugging page (http://www.geofex.com/ampdbug/ampdebug.htm):

A good way to divide and conquer is to turn the volume control(s). (...) if the volume control does not affect the hum, the cause is somewhere after the volume control.

If the speakers have only a modest hiss or hum coming from them, that indicates that the power amplifier section, speakers and speaker wires are OK, and that the fault is probably in the lower-signal sections of the amplifier.

Preamp tubes bad
Preamp power bad
Input cord or connector is open or has dirty contacts; this can include the effects loop jacks, if present.
Input jack dirty or corroded
Open volume or tone control
Open, shorted, or failing resistor or coupling capacitor
Faulty signal wiring

That's a lot of things that could be wrong. I have a basic understanding of electronics and soldering skills. I know how to discharge capacitors and what things should scare me. But I am pretty bad at troubleshooting and don't know anything about tubes.

Can I get some assistance in how to figure out what's up?

[Gone Fission]

Easy level troubleshoot: Do all the preamp tubes glow orange when powered on? One could have gone south. I had a phase inverter tube go once, the one closest to the power tubes, which is good for a scare, since no output. (Swapped it with the "normal" channel tube, the one furthest away and for inputs I rarely used, and kept going.)

[imJonWain]

^ I agree it's hopefully that simple, tubes die, it happens. try and report back?

[cedarskies]

so i took the preamp covers off and indeed one of the preamp tube closest to the power tubes was not lighting up. so i bought a new one and replaced it. and sound! but, it is really quiet. like, crank it all the way up and it reaches a reasonable bedroom practice volume quiet.

i think next thing to try is swapping my new tube and the other tube that is the same kind around and see what happens.

[imJonWain]

Looking at a schematic you have 4 preamp tubes, 1 reverb and 1 phase inverter. I'd assume 4 preamp tubes are 12ax7 and the other 2 are the 12at7. When was the last time you replaced them?
http://www.prowessamplifiers.com/schema ... pa135.html

[cedarskies]

i have never replaced them and i've had the amp for two years.

the 12AT7 is the one that was not lighting that i replaced and then was quiet, and then swapped with the other 12AT7 that was in there (looks like that goes to the reverb, which has never worked anyway...)

and after doing that tube swap it's loud and functional again! yay! thanks everyone!

[imJonWain]

Sweet! you may wanna spring and just replace all the tubes at once for reference if you've been giging 2 years on it or at least the preamps for time being but I don't follow my own advice anyway haha. Also for the reverb if the tube is good you can probably replace a broken Photoresistor on the board and it'll work again.

[Gone Fission]

The optocoupler would be for trem, which that amp doesn't have. The two most common failure points for the reverb circuit are gonna be the tube and the spring pan. You can test that by turning the reverb all the way down, the master volume way down, and patching an rca cable straight from reverb send to reverb return. Play something in (looper or similar is handy here) and slowly turn up the reverb. If the sound gets a lot louder, it's because your feeding a hot loud output back into the amp and producing a lot of volume and drive. If this works, something between your send and return was bad. If you're lucky, it's just a cable or two that's bad. If the reverb pan is bad, it might be a good idea to replace the cables, too.

BTW, the reverb booster trick isn't the greatest idea to leave permanently set up. Too much to really deal with and may stress the reverb drive transformer. As another vote against, Fender did a more thorough implementation of this idea on the silver face amps with pull boost and people fucking hate the boost and will often neuter that function.

[cedarskies]

so the reverb actually did work for a while. but only for the first week i had it. because then i cascaded all the channels into each other with the reverb on all of them and apparently you can't do that and the power transformer died. when i got the amp back from the tech the reverb didn't work either but he had it for many months and i was just happy to have it back. i suppose it's probably likely the reverb transformer went out as well?

i'm not sure i understand what your method is. there's no send and return on this amp. the reverb has an input and output, but if those are wired together won't that just be a loop with no input? (or output?)

[imJonWain]

oops! my bad!

[Gone Fission]

i'm not sure i understand what your method is. there's no send and return on this amp. the reverb has an input and output, but if those are wired together won't that just be a loop with no input? (or output?)

The reverb send to the pan is the output of a little low-wattage power amp. The output from the pan is a lot weaker than the input because there is a lot of loss of energy through the pan, so there is a makeup preamp stage for the return signal. It's a cheap and easy hack mod, known for ages, to run an RCA cable from input to output as a poor man's overdrive, eliminating the losses in the tank. Jumping the reverb send to the reverb return happens to also be a handy way of testing whether there is a dead short between those two points. Most likely to fail is the pan, but either cable is also possible, so try the direct jump with each of them in turn.

The transducers that take the electric signal to drive the springs and then convert the mechanical energy back to electricity at the other end, they tend to be the failure point from what I gather. I've heard of people doing micro surgery on the connections to get everything happy, but if the tank is a dud, the easier way for most of us is to buy a new pan.