Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Geoffb on November 29, 2018, 03:35:43 pm
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Hi Folks.
My friend a asked me the fix his Gibson Bass Head and has also supplied a circuit diagram for a GA-100. Although the tube layout looks the same as the circuit diagram ( see attachment ), the valve in V1 position is NOT a 6EU7 as shown on the circuit, a genuine 6EU7 to too large to fit in the valve base and the datasheet of a 6EU7 does not match the voltage reading I'm getting ( see attachments of amp and valve ). These are:
Pin 1 = 258v (suspect Anode)
Pin 2 =35.4v (suspect cathode)
Pin 3 = 72v ( suspect Cathode)
Pin 3&4 = 0v ( joined Heater)
Pin 5 =0v
Pin 6 = 334v (suspect cathode)
Pin 7 = 0v ( audio input from jack sockets)
Pin 8 =2v ( suspect Cathode)
Pin 9 = (heater)
The above pin out is similar to an ECC83
The fault is reported a hum on the output and checking V1 is as far as I've got..
Can anyone identify this valve, or indeed if this amp is a GA-100?
Regards
Geoff.
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V1 on almost every Gibs amp I fixed got replaced, socket included, I used 12AU7A. from surfing looks like a GA100, schematics do show the eu7. Good luck, they are a different beast but I like 'em hands down to ANY fender :icon_biggrin:
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The tube in your pic is a 7 pin tube. Maybe it is a 6AV6 or 6AT6?
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If the tube in the photo comes from the amp, it may be one of the two V2 - V3 6BD6 tubes
Franco