[Rob DiStefano]

Originally Posted by specialty guitars

The 47K NFB resistor did more to lower the 60hz hum, than any hum reducing trick I could remember. So if you like that mod, it will help the hum that these types of amps are prone to.

Yep, the noise suppression was immediately noticed when the NFB was added.

I tried all sorts of thins to reduce noise and hum, with limited success. I isolated the preamp grounds from the power amp grounds. Didn't seem to make a difference. I moved the 100 ohm resistors that make and artificial ground tap for the heater wires, from the preamp area of the board, to the pilot light, tried grounding it at the power amp ground buss, and to the top of cathode bypass resistor for the power tube (this is an old trick that works sometimes). Not sure if any of this went any more forwards than backwards.

The thing that made the most difference in noise was shielding the top and edges of the cabinet with heavy duty aluminum foil. Amps spray a lot of noise, that tends to wreak havoc with single coils. Shielding the cab completely seals the electronics electrically, which reduces that crap quit a bit.

I do exactly that to all my amps and was thinking about that yesterday, then forgot about it. I use Reynolds heavy duty aluminum foil, spray on some 3M spray adhesive cement, fit to the cab top - in the case of the GA-5, the foil should run down the sides and into the chassis slot to complete the ground. Alum foil makes for a good noise shield.

I'll post some pics later.

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