Gibson GA18T blowing fuses

[acorkos]

Is the power transformer shorted? this amp is blowing fuses and has some funny voltage readings on the rectifier socket.

the schematic shows 280vac on pins 4 and 6, 5vac on pin 8, 305vdc on pin 2.

when i measure the voltages, i get 313vac on pins 4 and 6, 290vac on pins 2 and 8, and < 1vdc on pin 2 and 8.

the 470 ohm screen resistor (on pin 8 of the rectifier and on the screens of the power tubes) smokes when in the circuit.

thanks for your help!

[Enzo]

Pull the rectifier tube, does it still blow the fuse? I bet not. I suspect the transformer is fine. I also suspect the cap after the 470 ohm resistor is shorted or your rectifier tube is shorted. Or both.

WHen that amp was made, the mains voltage was more like 110VAC to 115VAC. Now most of us have about 120-125VAC coming out of the wall socket. SO all the secondary voltage will also be about 10% higher. That explains the 313VAC instead of 280VAC. Besides, those voltages are approximate.

The 5VAC is BETWEEN pins 2 and 8 on the rectifier, NOT with respect to ground.

Leave the 470 ohm resistor out, now what happens at pin 2 of the rectifier? You should get DC voltage ther of at least a couple hundred volts. The first filter cap is probably also shot. If it and the rectifier are OK, then there should be at least 300VDC there, and almost no AC. If there is a lot of AC on pin 2. then either the rectifier is shorted or the filter cap is completely gone.

[acorkos]

I pulled the rectifier tube...fuse blows

I pulled off the OT CT wire...fuse blows

I removed the first and second filter caps....fuse blows

I removed the 470R....fuse blows

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I pulled all the wires off the rectifier socket except for the PT secondaries....fuse blows after 4-5 seconds when the rectifier tube is in the socket. Fuse holds with the rectifier tube out. Fuse blows with a known good rectifier tube.

There is no DC on pin 2

the rectifier tube gets warm, but there is no filament glow

[pdf64]

Check that there's not a problem at the mains switch. There's hardly any clearance between the solder tab terminal and (grounded) metal body case. A stray strand of wire there gave me this same problem on mine. Peter.

[Enzo]

OK, with the rectifier tube out, all the caps and resistors become irrelevant.

At this point, disconnect all the power transformer secondary wires, tape them off and leave them sticking in the air. Now if it still blows a fuse, then either the power transformer itself is bad or there is a mains wiring short like pdf suggested.

[acorkos]

if anyone's interested, it turned out to be a bad power transformer.