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Hmmm......homebrew hum


 
9/6/2004 5:42 AM
aris
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Hmmm......homebrew hum
Just finished a clone amp, ac30TB/TopHat King Royale. 4 jumperable inputs, 2 channels....channel 1 has one volume and one tone control, channel 2 has gain, treble, mid, bass, cut, and master volume controls (cut affects both channels).  
 
The transformers/choke are the ones used in the Vox AC30HW handwired amp, purchased from AES.  
 
Anyway, the amp works...sounds great, it's loud and breaks up nicely, all controls function properly...but there is a significant hum. I've used star grounding schemes in the preamp and power amp sections, I've checked and rewired the input jacks, I've moved the transformers around, and I've moved ground wires around....without much change in the hum.  
 
I checked all the resistors for shorts, and they're all ok. I'm wondering about bad capacitors...? Anyway, when I pull preamp tubes 1 and 2, no change in the hum. Pulling the last (3rd) 12AX7 eliminates the hum (and all sound). And the cut control seems to change the "tone" of the hum. Those are the only things I can do that seem to alter the hum. Any ideas as to what might be wrong and where I should look?  
 
Thanks,  
Aris
 
9/6/2004 12:42 PM
Dan
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One thought... Are ALL the jacks isolated from the chassis? Are ALL signal grounds terminated OFF the chassis??
 
9/7/2004 8:45 AM
slowblues
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Ground loops? Could be stargrounditus.  
I like to flow all ground connetions into one line from inputs down to one ground point at PT.  
Just a thought slow
 
9/8/2004 6:12 PM
aris
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I'm feeling like it is a ground loop...can you explain stargrounditis? Do you mean ALL (pre- and power-amp) grounds connected at ONE common point?  
 
Thanks,  
Aris
 
9/8/2004 9:30 PM
slowblues
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Aris if your using starground you could be seperating conections that need to run together to ground,  
As in the first valve stage cathode resister capacitor, DC power supply electolitic ground, inupt ground.  
slow.
 
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