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Negative voltage on input


 
5/31/2005 3:34 PM
Thomas
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Negative voltage on input
I just completed a JTM45/50 clone, and I've got it sounding pretty good, but still have one problem that has me completely baffled. I thought my guitar volume pot was dirty because I got a scratching noise when I turned it, but then when plugging into a different amp it goes away.  
 
So I thought I must have DC on my input. Sure enough, I have -.5 DC on the input jack, but for the life of me I can't figure out how it is getting there. I could see how this might happen with the volume pots on the amp if the coupling caps from the first stage were leaking.  
 
I removed the input wires and 68k grid resistor from pins 2 and 7, thinking my jack wiring my be screwed up, but even with nothing connected to these pins, I'm still seeing -.5 on pin 2 and pin 7.  
 
Anyone have any ideas?
 
5/31/2005 3:52 PM
Dai Hirokawa
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the board can be conductive
 
5/31/2005 4:06 PM
Thomas
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I'm using garolite, and I'm not actually using the turrents on the board for the inputs. I'm running shielded cable from the input jacks to the 68k resistor mounted directly on the pins of the tube socket.  
 
I suppose some of the other connections to V1 could be affected. I'll try removing some components and measuring resistance between turrets.  
 
Thanks,  
Thomas
 
5/31/2005 5:52 PM
Chuck
Try another tube. Once I had a build that wouldn't turn all the way down. I never measured for voltage in this case but of all the things that could have been it was the tube. As I recall I tried another tube because I did have a scratchy volume control and I didn't suspect the caps. Anyway, odd as it sounds, with the rest of the amp wired completely correct (which means an absolute shunt to ground on the pot.?.) I still had a small signal with the vol. control at zero. tried the obvious stuff and just got more confused. Tried a new tube and the problem dissapeared. Sad that the tube I replaced was an old Telefunken smooth plate :(  
 
Chuck
 
5/31/2005 6:12 PM
clint b
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I had a tube do that with me... A Blues Jr... A different tube took it from over 1 volt to like 0.009 V. I didn't try just moving the preamp tubes around. I guess it's possible to cure it that way but I have no real info.  
 
HTH,  
 
Clint
 
5/31/2005 7:36 PM
Thomas
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Thanks, I've tried several tubes in V1 and V2.  
 
I redid the cathode (since it was a little boomy for my taste), going to a split cathode (220/820R and .68/2k7) but still had the DC.  
 
But it sounded great, until it blew up :-). Well it didn't blow up, but it started arcing on one of the power tubes (I'm running JJ EL34Ls, with a 3k3 primary). It blew a fuse before I could turn it off.  
 
Looks like the arcing was between pins 3 and 4 (or maybe pin 2 was involved).  
 
Not sure if it was a bad power tube or the problem was unrelated. I guess now its time to pll the tubes and see if I have blown any iron.
 
5/31/2005 8:05 PM
Chuck
Well at the very least you have another issue. After an arch it's a good idea to replace that tube socket. There is a path of conductive carbon imbedded in the socket material itself and you can't wash it off (though I did pull it off with a ceramic socket once).  
 
 
Chuck
 
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