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Loud hum and PT...


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1/23/2005 1:02 PM
daddydg
Loud hum and PT...
I just rewired an old PA amp into a 5B6 Bassman. Now, I've got as audible 60Hz (I believe) hum coming from the PT. I'm not sure what is causing this. Maybe someone could help? It may also be related in some way to my next issue...  
 
However, the biggest issue and possibly related to the PT hum, is a loud hum heard through the speakers. Most interesting is that it goes away when I clip a lead between any metal part of my guitar (Tele) and the chassis.  
 
I'm sure someone can diagnose this one, but it has me stumped.  
 
ps-  
Interestingly, at one point, while testing/fiddling with the amp, I disconnected the clip lead and the hum didn't come return, as it had before. I thought I had my problem solved, but when I turned it on again, the problem was back. :(  
 
 
For what it's worth, I have the filaments grounded at the 6L6s cathode through two-100 ohm resistors. This elevates them about 20vdc.
 
1/23/2005 11:53 AM
daddydg
Couple other pieces of info...
1) The hum goes away with nothing plugged into the input.  
 
2) When the guitar is plugged in, the hum increases/decreases whent he volume knob is turned.  
 
3) I have the preamp cathode grounded to the input jack (which is isolated) which has a ground wire running back to the PS cap supplying it.
 
1/23/2005 12:20 PM
Bruce /Mission Amps

One input jack or two?  
Possible grounding issue at the input jack(s) and or the jack on the guitar itself.  
Grounding the input with a switching jack implies the hum is injected into the signal path no further then the very first stage.  
Bruce
 
1/23/2005 1:51 PM
daddydg

One jack. I can't firgure out how the hum is getting into the input.  
 
I've tried other guitars and have the same problem.  
 
Also, I can't figure out what it means that the hum stops (for the most part) when I clip a lead between the guitar and the chassis.
 
1/23/2005 2:16 PM
Jack Koochin

Have you tried different guitar cords?
 
1/23/2005 4:59 PM
daddydg

I tried different cords, no difference.  
 
I'm sure it's in my wioring, but I need help figuring out what the problem is.
 
1/23/2005 6:22 PM
jeremy
Re: Loud hum and PT...
the pickups in a tele are single coils right?  
 
 
jeremy
 

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