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hum in ac-30 build


 
4/7/2005 10:16 AM
mod
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hum in ac-30 build
ok, i have just rebuilt my 1st build an ac-30 built off a hoffman board kit, it's got 2 chanels "normal" and "top boost" it sounds friggin awesome, except i am getting a bit of hum on both channels, the hum increases as you turn the volume up, when you play you can't notice it, but it's something that i would like to fix, it would seem the hum is being induced b4 the vol controls, now i have checked and quadruple checked the wiring and everything seems cool, i'm using switchcraft style jacks(tied to the ground buss), and i also have a seperate chassis ground, could there be a ground loop here? what's the best way to check? also do you guys use marshall style jacks and isolate from the chassis?
 
4/7/2005 11:27 AM
loudthud
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Hum can enter the signal path at many points. Does the hum go away completely when the volume controls are at minimum ?  
 
Do you have an elevated heater supply ? That's where the center tap of the heater winding is connected to a positive voltage of 10V to 50V or so.  
 
Try this: Disconnect the grid of the first tube from the input jack right at the tube and connect it directly to the ground of it's cathode resistor. That should kill any hum coming from the input jack's ground.
 
 
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4/7/2005 11:54 AM
mod
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"Hum can enter the signal path at many points. Does the hum go away completely when the volume controls are at minimum ? "  
 
no, there is always some hum  
 
"Do you have an elevated heater supply ? That's where the center tap of the heater winding is connected to a positive voltage of 10V to 50V or so."  
 
yes, i beleieve so, have to recheck when i get home though, 98% sure yes.  
 
"Try this: Disconnect the grid of the first tube from the input jack right at the tube and connect it directly to the ground of it's cathode resistor. That should kill any hum coming from the input jack's ground."  
 
i will try this friday night, tonight i will be going to see melvin sparks, thanks for your help!!!
 
4/12/2005 2:00 PM
Paulg
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The best advice I can share re-AC-30's is: Keep the grid wires to the EL84's short. I had terrible noise and oscillation problems until I re-located the board and put the resistor right on the tube socket. good luck!
 
4/12/2005 11:08 PM
anonymous
keep output tubes grid wires short. Place grids resistors directly on tube sockets. Use dc heater supply for preamp tubes. And the most important thing...use a galactic ground. Sort of a star gorund, but with distributed capacitance.
 
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