Fender Twin Reverb Problem

[DrResponsible]

Hello,

I am having an issue with a Fender Twin Reverb guitar amp.  I was practicing with my band last Tuesday, and halfway through our set it started to malfunction.

It still turns on and produces sound, but it comes out much quieter than before and shortly afterward it sort of "fizzles" out.  It also only affected one channel, the one with vibrato, (the other was already having issues before I used the amp, it seemed to work but was very quiet).  Does anybody know what may be causing this?  My first though was one of the tubes went out, but I'm not sure.  I've been getting into electronics recently and would like to try and repair this myself if it wouldn't be too complicated, but I'm not too sure where to start; so far I've only worked with guitar pedals.

Any help would be appreciated.

Thanks!

[alerich]

A few preliminary things you can do. Check all of the tubes to make sure the heaters are lit. Power down the amp. Make sure the reverb send and return cables (to the tank) are connected securely. Maybe remove and replug them to make sure. Remove and reseat each tube (preamp and power). Try the amp again. If your trouble still persists chase up a known good spare 12AX7 and 12AT7 preamp tube. One at a time replace each tube with the known good spare of the proper type and try the amp after each tube is replaced. If your troubles still persist you can probably eliminate the preamp tubes as the culprit.

Also - check to make sure the speaker cable is securely plugged into the correct speaker jack and make sure the lugs are securely connected to the speakers. Try a known good extension cabinet if you have one.

[Jack1962]

Great suggestions from Alerich , if you turn the amp off and then back on does it do the same thing everytime ? I as have many other people here have seen this before , my suggestion is check your Output Transformer

[Ed_Chambley]

If checking the tubes and speakers does not work out.  Please post the year of the amp.  Is it a reissue, silverface, blackface?  Circuit type?  Schematic?  There are many things that could cause this, but tubes are always first.

For instance, if the bias grid resistors have drifted lower.  With the lower volume, do you hear any additional distortion or is it still clean?

Leaking bypass cap..........etc.

If not sure of the year, begin by searching using the chassis Serial Number.  If the problem is deeper than tubes or speakers the folks here can point you to specific points on the schematic to check.

[tubeswell]

When did you last change the output tubes?

[stratele52]

When did you last change the output tubes?

+1000,  first thing to do

Check if all tubes are well seated

If you put new output tubes don't forget to check bias.

Second thing to do ; check all other tubes, they don't need bias check.