Gibson Explorer

[bainesamps]

http://music-electronics-forum.com/s...ad.php?p=65364

Hello, started the thread linked above re: troubleshooting this Gibson amp, it is an Explorer and is nearly the same as the GA16-T schem. circuit with slight changes in cap values in the trem circuit. So theres now further clarity in ID-ing the amp and the circuit since the initial thread was posted. All help above much appreciated! Also believe now that the 12" speaker is aftermarket. Finally getting back to it now after being gone.

The final (hopefully!) problem symptoms are as follows: one 6V6 tube is hot (which I believe is ok and biased properly, with bias voltage in the proper range) and the other tube remains cold (lower volatge) and its physically cold as well. The amp is developing ragged, but near full volume now with the trem off or on.... but it is distorted with no clarity. Sound like a blown OPT, secondary lead blown on the cool side? Bad power tube socket? Either?

Have to tackle the above problem first, but the other symptom is a pop in the first quarter turn of the vol pot before any volume developes and thats followed by a fairly loud squeal for a few seconds that slowly fades out. Its similar to the sound of reversed OPT secondaries when hooked up with polarity backwards. Im wondering if all this is symptomatic of a blown OPT as well. There are other points in the rotation of the vol pot with pops and the vol is intermittant..

I subbed a smaller OPT earlier in the process, but might have had a mismatch that compounded the problems looking for the source of the trouble early on. Thanks!

[Robert M. Martinelli]

Hi,
looks you have a defective output tube; on push pulls, when a tube fails, too much current tends to flow in the "companion" tube, even if the bias voltage looks ok this doesn't mean CURRENTS are OK, are you familiar with measuring currents on power tubes? the easiest way would be the 1 Ohm resistor between cathode and GND ( one per output tube ), once placed, measuring the voltage drop in mV across it gives you the total current in mAmps flowing through that tube...bet you' re going to read zero for the "cold" tube....

The second problem could be related to the potentiometer having a discontinuity somewhere in between, when you pass over that the circuit gets opened and you usually get all sort of disturbances, so pull the pot away from the circuit and, ( an analog "needle" Ohmmeter is the best ) measure KOhms between the wiper and either outer lugs while rotating it and see if resistance is varying evenly or if you have discontinuity points in which the pots opens and the resistance jumps to infinite...

hope this helps

Best regards

Bob

[Rob Mercure]

To elaborate a little on VR's response: In addition to bad tube sockets leaky DC blocking capacitors from the PI can throw off the bias on an output tube - measuring the currrent is the definitive test. A quikc and dirty method is to pull one tube and measure the voltage on the cathode resistor then replace that tube, pull the other, and see if the voltage is pretty close - if not the tubes are drawing different current - you can then test them both in one socket to eliminate the socket/DC blocking cap, etc.

And a bad pot sounds like the other problem - VR's call again - sub in a good pot of not more than twice the value - if you don't have the correct one - and see what happens.

In general with a bad OT the amp just doesn't work - over the years on this forum there have been dozens of posters who wonder if there OT's are bad and after further work only a very few have been actually bad. In many years of service work I've only changed a couple of dozen at most - not a frequent repair.

Rob

[bainesamps]

Thx...much!

Really want to say thanks for the added direction Rob and VR, I'll start with the power section and then move to the vol pot. Robs been hanging in since pretty much the beginning with this Gibson, thx for that!

I have tried three sets of 6V6s in various combinations that are known good tubes with the same results in the same sockets and have replaced all coupling (Mallorys)and all lytic caps (Spragues and Xicons) in the amp, plus 7-8 resistors (1w carbon film) that were out of spec by 20% and gave them all the light pull test with needle nose pliers to see if any were cooked and pulled apart.

Will try retensioning the power tube sockets also and recheck all the grounds plus the suggestions and see if I cant finish this thing up.....maybe?