Gibson Super GoldTone 60 troubleshooting

[professormudd]

A friend of mine gave me this amp that is currently not functioning properly. According to my friend, the amp output is very quiet on both channels even when cranked. When he got it to my house and I turned it on, one of the EL34s was arcing something fierce inside. Lightning in a bottle! I asked him about it and he has no recollection of that behavior.

I replaced the tubes with a new matched set of EL34 and was able to duplicate the issue he told me about. The output was super quiet. I could barely hear it unless it was cranked. I ran the loop out to another amps return and it sounded nice and loud. I did the reverse and plugged my other amp's send into the Gibson amp's return, and weak output again.

So far I have only really checked the power tubes, plate voltage (485ish each side), and verified the screen resistors are actually reading the values that the schematic says.

I am not sure what to check from here, but there is no obvious physical trauma. However, it does look like another person has been in there as there are a lot of little make-shift labels taped to the transformer leads as if someone had removed the board before.

Does anyone have any suggestions as to where to look to further isolate the source of this problem?

[TUBEDUDE]

Have you checked the bias voltage? What do you measure on pin5 of your power tubes? What voltage is across your cathode resistors?

[Stevem]

I will bet that the tube arcing has shorted out the output transformer.

One easy way to test such is to monitor the amps V+ voltage on pin 3 of either tube as you flat out Crank the amp as you play hard thru it.

If you see the V+ pull down more then 15 volts then for sure the output stage/ tubes are trying to drive the OT as they should , but the OT is shorted and prevents normal output to the speaker.

[professormudd]

Have you checked the bias voltage? What do you measure on pin5 of your power tubes? What voltage is across your cathode resistors?

Pin 5 tests -41.5 on both tubes.
Bias testpints are reading 37-38mv each side

[Stevem]

Your bias voltage looks good then and the outputs are idling good also . If you now play thru the amp and see that idle current go up to 80 ma or more ( per tube ) then if the OT was good you would be hearing a big level of volume out of the speaker(s)!

[johnnyreece]

Is the speaker connection good? Flyback voltage can do all kinds of nasty things.

[TUBEDUDE]

Clean the Loop or power amp breakout jacks.

[Stevem]

Its far easier to just use a cable to just jump those jacks to make that check!

[TUBEDUDE]

Unless those contacts are just as dirty.
Q'tip or folded pipe cleaner dipped in acetone (if the jacks aren't cheap plastic) or naphtha then ran thru the jack repeatedly will do it without the need for disassembly. Easy-peasy.

[professormudd]

I have fiddled with all of the jacks quite extensively looking for bad connections. They all seem nice, clean, and in good shape. I also checked the OT connections (it is quick-connected). I found a NOS OT for this amp on ebay for $30 so I just went ahead and ordered it. I will report back after I have some results to share.

[Stevem]

Why did you not do my simple test to confirm before potentially wasting 30 bucks and probably 12 bucks in shipping?

[professormudd]

Why did you not do my simple test to confirm before potentially wasting 30 bucks and probably 12 bucks in shipping?

I had tested inputting directly into the return from several good sources.

[Stevem]

Yes that’s fine, but did you check to see if the tubes where increased there current draw which will happen if they are indeed being driven, which will cause the power supply voltage to pull down a real good amount?

[professormudd]

Thought you were asking why I didn't jump the jacks. Yeah I watched the voltage drop when I played.

[Stevem]

Ah, cool ok!