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Troubleshooting help needed: Odd behavior

9/25/1999 5:27 PM
MJ Harnish
Troubleshooting help needed: Odd behavior
Ok, the short story leading up to the problem: I install a tube driven fx loop into & did a cap job on a Guild Thunderbass. After finishing it up and testing it out thoroughly I packed it up and shipped it back to the owner. It arrived DOA; no output at all. The owner ships it back to me after troubleshooting for a simple problem proved unsuccesful. Here's where the mystery lies:  
I got the amp back and sure enough it doesn't work; it does produce sound but only if you crank the both volumes (channel and master). Then what you get is a very distorted, splattery tone that has NO sustain. It will reproduce individual notes no matter how fast you play but if you let a note sustain the output dies. The really weird thing is that if I hook up my signal generator to the input the amp produces any test tone frequency perfectly with no apparent distortion.  
After a bunch of testing I know the problem lies somewhere between the input to the PI and the power tubes (the tubes are good).  
Any ideas? I'm baffled on how to proceed other than to start swapping out parts (starting with the OT and working backwards). However, this amp is a major pain to work on since you can't replace individual components without all the connections to the circuit boards (the traces are all onthe bottom side...).  
Any advice or thoughts would be greatly appreciated.
9/26/1999 12:35 PM

Last time I had this kinda thang it was the PI tube tht had gone bad.. (have you tried swapping it out yet?)  
The sig gen part is what has me confused though.. you mean you can inject say a 70mV signal into the regular input and get a nice full power sine at the speaker jacks or ?  
9/26/1999 12:56 PM
MJ Harnish

I've tried new tubes in all positions with no luck.  
Unfortunately I don't have an O-scope, so all I can use is my ears and the speakers; what I've done is tap into the signal path at various places (at preamp out, power amp in, at the master volume) and run the signal through a SS amp I've got hanging around. I get a normal guitar tone (or tone from the signal generator) up to the master volume. However, through the problem amp's speaker all I get is a buzzy, splattery note that quickly dies off. If you continue to pick it'll keep generating the signal but it has no sustain.  
The easiest analogy is it's like playing through an amp that you just turned the power off but left the standby on. That's why I was thinking it was a bad cap somewhere from the PI onward. So my next step was to hook up the signal generator and then use my SS-amp (along with a probe constructed from a chopstick and a cap ala Weber) to test the signal path beyond the master volume. However, I get a normal tone from the problem amp's speaker when I use the generator (there may be some lower level distortion but the fundamental note is there) and it doesn't die off at all. That doesn't make any sense to me...  
Another interesting twist I've discovered.... when you turn the amp's power off (standby still ON) and play the amp sustains fine; there is still distortion on the notes (but that's normal in my experience so I'm not sure if it's the same distortion as above), but there's no die off of individual notes.  
As I've said, this has got me stumped (not really much of an accomplishment given my level of expertise); tonight I'll start working backwards in the circuit replacing one component (starting with the OT) at a time...  
9/27/1999 2:56 AM
MJ Harnish

I stand corrected; the signal goes through fine but it is destorted though it lacks that splattery, non-sustaining quality to it; I also have to crank the preamp and MV to max to get any output out of the amp.  
9/27/1999 10:40 AM
Could it be just a bad speaker connection? It has happened before, you know.
9/27/1999 4:23 PM
J Fletcher
That would be my 1st guess too. Sounds a lot like a bad speaker to me. May have been damaged in transit...Jerry
9/27/1999 9:27 PM
MJ Harnish
Unfortunately no. It's a head and it's being used with my 2x10 which is working perfectly. I've fiddled with the connections on the jack with no luck. My gut says it's a cap in the PI but that's a complete guess; I guess I'm just going to have to take the whole thing apart and start swapping out caps.  
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