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Parasitic Hell


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3/17/2003 1:16 AM
clyde Parasitic Hell
I recently completed a 5F6A clone, very nice sounding amp other than a nasty growl which rides on  
top of the low E string, sometimes buzzing like  
arcing or a blown speaker. Removing the 50pf suppression cap on the phase inverter improves this  
condition(!), turning the bass pot down improves it,  
turning the presence down improves it. I've tried  
suppression caps everywhere, swapped in a different  
OT, tubes, PS caps, speakers, went from .1 to.047  
on power tube input caps, put grid stoppers on the power tubes, totally rewired the amp, I think it's time to get serious! Any suggestions? TIA. Clyde.
 
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3/17/2003 4:16 AM
Chuck
I've had similar issues on a few amps and I've managed to fix them all...I have an eye tick now, but I fixed em. ;) ;) ;) ;)  
 
Usually I find that it's a single grid lead that doesn't like where it is. Once I found that two componants didn't like sharing space.  
 
A trick that has worked for me (it's very dangerous and therefore I can't reccomend it) is to pinch leads between my thumb and forefinger and move them around while I'm at it. The capacitance from your fingers on the lead seems to work very well for finding these things. you must do it while creating the problem by BLANGING the strings. I've read posts from builders who know better than to stick their fingers into a live amp. They make a lead with an alligator clip on each end and clip one end to the chassis and the other to various leads to test. This doesn't work that well in my experience and I still use my fingers. Since you'll only be working with grid leads you should be alright as long as your decoupling caps are decoupling. Be carefull and good luck.  
 
Chuck
 
3/17/2003 10:02 AM
Dai Hirokawa

Did you follow the original grounding scheme? I've played with that circuit but with a Marshall (so it would be a sort of JTM45 variant), and I solved an apparent parasitic one time (fuzziness as notes die out) by redoing the grounding slightly. I had a bus connecting the input jacks and pot grounds, etc. like usual (except it was not connected to the backs of the pots), and I had the PI filter cap grounded at the end of this line, thinking since it was in order it would be okay, but the fuzz went away when I moved the ground for that filter cap to the same place as the preamp filter. My guess was that I had currents flowing where they shouldn't be in the ground line. Dunno if that's what is the matter with your Bassman, but it's an idea, anyway.  
 
Dai
 
3/17/2003 5:41 PM
clyde
Dai,  
I have a ground buss, jacks, 1st cathode cap, and  
so on down the line, and the PI is also down the  
line. I'll try and move it, see if it helps. I  
can't believe Leo could pull it off with his apparently primitive grounding scheme. What do I  
know??????????? Thanks you guys.
 
3/18/2003 11:59 AM
Dai Hirokawa

Actually check out Mike S.'s post in the grounding thread over in the general discussion section. Maybe in the old Fenders, the grounding was thought out more than seems to be.  
 
Dai
 
3/18/2003 7:22 PM
clyde
Dai,  
That's pretty interesting. I tried your suggestions, moved the grounds around, no difference, also pinched the grid wires with my  
fingers, no difference. What really puzzles me is  
when I remove the supression cap on the PI plates,  
the noise diminishes, almost like the cap causes  
some positive feedback. My voltages are pretty  
close, only 160 on the CF instead of 180, plate  
voltages slightly over 400 which should be OK.  
Makes me crazy!
 

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