T-Dogg
Well-known member
So I'm trying to diagnose and fix this amp, which was given to me in non-working condition a while back. I'm also half trying to understand the basics of tube amp circuitry, and have a couple o' general questions on the schematic, available here:
http://users.aol.com/portaflex/schems/g-15.gif
First off, when the amp is powered up and switched out of standby, there is a very loud, constant hum. Pretty low frequency. Adjusting the volume or tone controls does nothing to change the output or sound of the hum. The input jack does not appear to be shorted. Based on what I've read, this points to a couple of possibilities --
Bad smoothing caps
Bad power tubes
Bad tranny... power, I guess? possibly output?
Does this seem a reasonable assessment? Because the power tubes are tough to find nowadays (7591's I think) I figured I'd test the caps first... (BTW, one of the few things I do know is to discharge them first )Then I guess the tranny's, although I'd have to read up on how to do this.
I'm gonna start on it tonight, if anyone has any input I'd love to hear it. Unfortunately, I don't own a scope (nor would I know how to use one at this point!), so my testing is limited to a digital multimeter that can also check capacitance. BTW, should I take the caps out of circuit to test them?
Also, if anyone got to check out that schematic, I have a random question about it. I recognized the two smoothing caps as those following the rectifier stage, and think I understand what they do at a really high level(take much of the ripple outta the DC right?)... But what is the purpose of the cap coming off of the primary going to ground? Is this some sort of smoothing cap too?
http://users.aol.com/portaflex/schems/g-15.gif
First off, when the amp is powered up and switched out of standby, there is a very loud, constant hum. Pretty low frequency. Adjusting the volume or tone controls does nothing to change the output or sound of the hum. The input jack does not appear to be shorted. Based on what I've read, this points to a couple of possibilities --
Bad smoothing caps
Bad power tubes
Bad tranny... power, I guess? possibly output?
Does this seem a reasonable assessment? Because the power tubes are tough to find nowadays (7591's I think) I figured I'd test the caps first... (BTW, one of the few things I do know is to discharge them first )Then I guess the tranny's, although I'd have to read up on how to do this.
I'm gonna start on it tonight, if anyone has any input I'd love to hear it. Unfortunately, I don't own a scope (nor would I know how to use one at this point!), so my testing is limited to a digital multimeter that can also check capacitance. BTW, should I take the caps out of circuit to test them?
Also, if anyone got to check out that schematic, I have a random question about it. I recognized the two smoothing caps as those following the rectifier stage, and think I understand what they do at a really high level(take much of the ripple outta the DC right?)... But what is the purpose of the cap coming off of the primary going to ground? Is this some sort of smoothing cap too?