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5150 problem

2/3/2004 9:37 PM
5150 problem
hey guys, i've aquired a peavey 5150 with a problem(got it cheap). the problem is that it has virtually no volume and no gain on either of the channels. i dont have a tube checker and there seems to be no components bad. sos help! any leads at all no matter how much of a long shot will be appreciated! thanx! jon
2/4/2004 12:54 AM

Isolate your problem. Plug into the effects return and see if the power amp stage still work s right. Likewise, plug into the front and run the effects send to another amp to see how the preamp is doing. That will tell us if the problem is in the front end or the rear end.  
One very common problem in amps is bad loop jacks themselves. Take a spare guitar cord and plug it into the effects send, then plug the other end into the efects return. If this bypass restores the sound, then your return jack is bad.  
MAke sure all four output tubes are running. I am not referring to the heaters in the tubes, they have to be lit of course, but I mean the tubes are conducting current or not. Check at pin 3 on all four sockets to make sure B+ is there. But more likely, check all the pin 4 for B+ voltage for the screens. An open screen resistor will shut down its tube. The result is weak distorted output.  
Look for signal on all the grids.
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2/5/2004 2:10 PM

enzo, ever had on of those ghost in the machine kind of amps? this thing is driving me crazy. nothing coming out of the preamp side of the effects loop but when you plug the guitar into the return side you get sound. cracking sounds after it has been on a while and no bad components. i was thinking either no plate voltage on the preamp tubes or a bad solder joint.(didnt make it that far, gave up and went to bed at 1 am last night) heard of a common prob with these amps? oh yeah, the input jacks are working fine, checked for continuity at jack and at grid pin checked for ac mv with a guitar plugged in(my wolfgang puts out about 35 mv output) checked all cathode res and caps. im at a total loss!! help!!
2/6/2004 1:24 AM

Don't try to guess what the trouble is, just go find it. By that I mean it is pointless to try to guess that it is an open plate resistor or something and then go check them. Just apply a signal and trace it down.  
Get the schematic, then folow along. I admit when I get an amp not passing signal, I go down the line of tubes and look for B+ on each pin 1 and 6, Then I look at the cathodes on all the pins 3 and 8. Missing B+ means an open load resistor or something easily figured out. If the B+ is real high, then the tube is likely not conducting. Likewise the cathodes. I expect a volt or two there. If I get zero, then the tube is not conducting. If I get a lot, then either I have found a cathode follower or there is trouble.  
But in the absence of some DC problem like that, I apply a signal at the input - I use my bench audio generator so I don't tie up my hands with the guitar. Then with my scope I look at each stage until I find where the signal quits. No scope? Use you AC meter. A little signal turns into a lot of signal after the first stage. You can use a meter to trace it.  
You can also use signal injection. Apply a signal at each stage and see if it comes out the speaker. In this case we start at the end and work back towards the input. You appliead a signal at the effects return and got output, so you know it works back that far. Go to the stage before the loop, and apply a signal to the grid there. Just touching your meter probe to the grid ought to inject some hum. SInce the trouble is complete lack of signal, that hum is sufficient for our needs. It comes through or it doesn't. If not we have found the bad stage, and if sound does emerge, we move back a stage and try again.  
There is enough gain in the amp that an open stage may stop the signal path, but crosstalk picks up enough that a small amount sounds in the speaker.
2/6/2004 1:59 PM

thanks, i'll let u know! jon
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