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After the success of my last projec...

1/31/2005 7:15 AM
Chris Beck
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After the success of my last projec...
After the success of my last project (http://www.geocities.com/n9zes@sbcglobal.net/champ.html), I moved on to my latest project, a guitar amp for my uncle.  
 
(http://www.geocities.com/n9zes@sbcglobal.net/dsc02229a.jpg)  
 
(http://www.geocities.com/n9zes@sbcglobal.net/dsc02231b.jpg)  
 
The last items (the transformers from Electra-Print) arrived Friday, so I could wrap it up this past weekend.  
 
The schematic is based around the '51 Supro (http://www.decsi.com/~sroberts/Valco_Supro/Valco_Supro.htm) and the early 50's GA-20 Gibson (https://schematicheavencom.secure.powweb.com/gibsonamps/ga20.pdf) . It is a 6SJ7GT front end driving a 6SL7GT phase splitter which drives a pair of cathode biased 6V6's. Power supply is 5Y3 into 8uF--->10H--->30uF--->24K--->20uF. Plate and screens for the output are from the 30uF cap and the front end and phase splitter are from the 20uF decoupling stage.  
 
Added to the design is a hi-lo input like the tweed Fender amps, and Ultralinear/Pentode switching for the output stage.  
 
My problem, again, is hum. Lots of it. So far, aside from mass replacement of stuff, I have done this:  
 
0.5) Amp is quiet with volume control off. As soon as the volume control comes up, it hums.  
1) Isolated the 6SJ7. If I pull it out, the amp is dead quiet. Only the faintest (5mV) of ripple on the speaker terminals.  
1.5) Tied grid of 6SJ7 directly to ground. No change.  
1.6) Added extra filtering to power supply. No change.  
2) Power supply ripple at the last filter stage is zero. Only maybe 50mV ripple at the output stage filter cap.  
3) I measure 150mV hum voltage at the plate (after the coupling cap) on the 6SJ7. It's 120Hz, and really spiky/nasty looking on the scope.  
4) Multiply that times 40 for phase inverter gain, and you get like 4 volts noise going into the 6V6's. Yeah, it's a LOUD hum.  
5) 6.3V filaments have virtual center tap to ground from a pair of 50 ohm resistors.  
 
My Champ clone uses almost exactly the same front end circuit (just no phase inverter), and with the volume control all the way up, I get a faint hum and tube hiss. I looked at the wiring in there (which was just tossed on some terminal strips) and I can't see anything I missed on the new amp. The new amp is wired up on a turret board all nice and neat.  
 
All power supply voltages (filaments, etc) measure fine. OP transfomer measures fine. Grounds all have continuity. I even tried splitting up and regrounding with no luck at all. Somehow I'm getting a noise voltage into the 6SJ7 directly it seems.  
 
I swear that when I first had it on the bench, after correcting a few wiring errors, I had no hum at the output. It just happend 'all of a sudden'. So, maybe I have a bad component around the 6SJ7? I replaced the plate load resistor, but nothing else so far.  
 
So, plan of action is to replace the parts around the 6SJ7 and see if that helps. If not, I'm going to have to strip the guts out and start over. I can't imagine it's a bad power or output transfomer causing the hum? It's probably something really stupid, but I'll be darned if I can't see it, and I've been over it a dozen times.  
 
Help!!!!  
 
Chris