Sudden loss of volume in Vibrolux

[macula56]

Halfway through a gig Saturday my amp started cutting out on me. after a few minutes the volume level dropped to about maybe 1/4 of it's normal level and it was distorted. Both channels were affected. Took the chassis out when I got home to start checking voltages and look for anything out of the ordinary but as usual it was now working just like it was supposed to. I have never had an amp do this so I was wondering if any of you have experienced this. Since both channels were affected I am guessing it's something in the power section but since it is now working fine I can't really verify that. Any suggestions as to what might cause such a drastic drop in volume and make it distort like that? Thanks, JMac.

[hesamadman]

That's a common reaction of failing tubes. When was the last time they were replaced?

[macula56]

All of the tubes, both pre and power are good.

[sluckey]

Maybe a loose connection  such as input, speaker jacks or tube socket. Maybe a bad solder connection or ground connection. See if you can make it act up by tapping around with a chopstick and wiggling wires.

[macula56]

I tried all of that Steve. Can not make it do it again.

[sluckey]

Can't fix it if it ain't broke. I hate those kind of problems. Now it's just gonna hide until you let your guard down. 

[macula56]

Yes indeed. Maybe I will just rebuild the power section just in case.

[SILVERGUN]

I had a similar failure at a show once myself.
It was the result of a failing filament winding in the PT.
The amp would be fine until the PT would heat up and a winding would intermittently open.
 
I should have known by the flickering of the power indicator lamp on the front panel because it was tied into the filament winding, and when the amp would act up that bulb would flicker.
 
But I got the same failure...extremely low output and aweful distortion was the indication that the filaments were failing.
I switched amps with the opening act and lived to tell about it.
 
This was what initially sparked my curiosity and began my amp repair hobby.
Luckily it was an easy find for me at the time.

[hesamadman]

Not questioning you at all. I'm only suggesting because I had this same issue but how do you know tubes are ok?

I had this issue in a kustom 72 coupe and my 2204. Cut out mid gig. Took it home. Worked fine. Then cut out mid practice again. Tubes checked out ok but once swapped, no more issue.

[macula56]

@hesamadman     
I am pretty sure  they are good because I used them the next night in another amp and played for 4 hours with no problems.

@SILVERGUN       
I completely forgot about the pilot light incident from a couple of weeks ago. I was playing fairly loud and I hit a chord and the pilot light went on and off quickly and the fuse blew. I replaced the fuse and took voltages readings but nothing seemed off. The amp worked fine up until Saturday. I think I have another PT I can sub in to check that theory. Thanks. 

[SILVERGUN]

@SILVERGUN       
I completely forgot about the pilot light incident from a couple of weeks ago. I was playing fairly loud and I hit a chord and the pilot light went on and off quickly and the fuse blew. I replaced the fuse and took voltages readings but nothing seemed off. The amp worked fine up until Saturday. I think I have another PT I can sub in to check that theory. Thanks.

Well, I would be very happy to hear that I was of some help...sounds like we're onto something.
 
I went through some of the same troubleshooting procedures, including re-tubing, cleaning the socket and replacing the indicator bulb, and then finally it hit me.
It took playing the amp at a show to reveal the true issue because I left the power on long enough to heat it up. I was using a PC board amp and it heated up so much that it also lifted the trace off the board, so I fixed it all at once. Not sure if it was the chicken or the egg at that point, but I'm guessing that it was the heat from the PT that caused the heat at the board.
I repaired the board trace and the PT was still heating up so I switched out the PT and all was well.
That's what I get for snagging an amp off of Craigslist and thinking it was a good idea to take it right to the stage.
 
Mine never popped the fuse, and now that I think about it, I should've checked the value of that fuse to see if the previous owner had made the mistake of just increasing the fuse rating.

[Ritchie200]

Like SG said, sometimes it takes a while under power for these things to rear their ugly head.  Maybe power on your amp, go out and cut the grass, and come back to it a few hours later.  Have you checked tube socket tensioning?  Grab and oven mit after its been on a few hours and move the tubes around in the sockets.  Any sputters and noise and that might be it.  Vibrations from a big power chord with some volume may be causing intermittent contact.  A few big bumps in current draw might be enough to pop the fuse.  Tubes can also become intermittent after being on for a while.

Jim

[macula56]

I believe I have narrowed it down to the reverb circuit. I now have full volume but it's distorting at 4 on the dial where before that was a nice warm clean tone. If I bypass the reverb circuit the tone is warm and clean but a little less volume. When I put the reverb circuit back in it's distorting more than it should. Reverb is not my forte so I don't really know what to look for as far as what would cause distortion. Any suggestions? I checked the tube sockets and sprayed them with deoxit. And I am using known good tubes.

[willhugh]

I had the same problem with an 82 Fender Champ II. Luckily, I had it connected to a scope when it happened again. I saw that half of the sine wave cut out. It turned out to be a loose tube socket that lost connection when it got hot. I replaced the sockets and retainers, problem solved.

[2deaf]

If I bypass the reverb circuit the tone is warm and clean but a little less volume. When I put the reverb circuit back in it's distorting more than it should.

How did you bypass the reverb circuit?

[shooter]

not sure if it's *correct*, but if it was mine, I'd try it like pic

[2deaf]

That would leave the recovery tube with no grid reference to ground.  If it did have a reference to ground, there would be a significant boost when the 3.3M is eliminated from the voltage divider.  Maybe he just pulled the reverb driver tube and turned the reverb control clear down and called it bypassed.

[shooter]

turned the reverb control clear down and called it bypassed

that too!, I was thinking you wanted to eliminate the *parts* from the circuit. 
which could be done by leaving the 220k in for the grid reference - I missed that
But leaving the *2nd 1/2 7025* in, makes normal and vibrato channels outta phase at the mix point to the PI

don't really know what to look for as far as what would cause distortion

about the same as a *normal* circuit, old carbon R's, leaky caps, bad-iffy connections, including inside the tank.

[macula56]

I removed the reverb tubes and ran a jumper from the .022 on the plate of the vibrato channel to it's corresponding 220K mix resistor. Sounded just like the normal channel. All of the resistors in the reverb section are old so I'm gonna replace them and see what that gives me.