gibson amp wows

[bluvudu]

Have a 63 Gibson ga 77rvt, kicks breaker, dosent blow fuse. fuse size is correct. removed the 2 x 6l6 tubes, and it does fine. tubes have checked good. transformer?? or any suggestions my friends------------

[Phil_S]

Not "wows", "woes" !
When you pull the tubes, it doesn't draw much current. That might explain why the breaker holds.

I think if the transformer was bad, it shouldn't matter if tubes were in or out. Do you have a meter and are you comfortable inside the amp? Can you tell us, with the tubes pulled, what is the voltage at the center tap of the output transformer. According to the schematic it should be 462V. Expect a bit higher because today's line voltage is probably over 120 and that is based on 115 or 117VAC.

Does the amp have its original 2 prong cord and death cap? This should be converted to a 3 prong cord and the death cap clipped without delay. That old 2 prong is dangerous and if blowing fuses, I am concerned you are in harm's way.

[jjman]

I don't see a breaker on the scheme so I guess you mean the house breaker? That would be colossal current compared to what that amp would normally use. 3A fuse is called for but I would go to a 2A for now to be safer on the amp. 2A seems to be adequate for many larger amps anyway.

[martin manning]

Is it a GFCI that is going open?

[Phil_S]

I wish the OP would answer so we know for sure. I assumed it's the circuit breaker at the service panel that blows. He says the fuse doesn't. It would take >15A to kick the service panel breaker open. I guess the current draw ramps up so fast the fuse in the amplifier doesn't blow. Something about this doesn't make sense.

Those old Gibson amps may have come with a non-polarized two prong cord. If that's the case, it is possible that the amplifier fuse is on the cold/neutral rather than the hot/line. This might explain why the fuse didn't pop.

(I have two Gibsons from c. 1964 and I put away the line cords when I changed to 3-prong. I put them away so I wouldn't lose them. Unfortunately, I put them away so well, I can't find them.)

In any case, if the PT was bad, it probably wouldn't matter if the power tubes were in or out, the breaker would probably open -- maybe not soon enough, though. I wonder if the PT is not totally fried, so when it only has the light load of the preamp tubes, it still appears to work OK, but is nearly ready for a catastrophic melt down.

[brentm]

Could also be a AFCI breaker... learned the hard way on newer homes with AFCI breakers in the bedrooms. I guess they're less sensitive than GFCI breakers.....

[Firestorm]

My money's on Martin. Leaky "death cap" would trip the GFCI. We'll have to wait and see.