Gibson Ranger GA20-T Rebuild

[TomCarlos]

Yes Chuck....

We did all the above- different guitars, cables, whackaroo on the case, etc. Since it is a problem with both channels, that is why I thought it would be from the Phase Inverter and beyond. But I will check the heaters because if there is a drop there, then that would be affecting all tubes.

[soundmasterg]

I had a sort of similar issue with a project amp half a decade back....it would drop in volume for awhile, then recover and go back to full volume, and then drop again, and then back to full again....it ended up being an oscillation from a trem tube interfering with a gain stage that was too close. Not saying that is the same problem in your amp though, but perhaps there is an oscillation going on somewhere? The one in this amp I described I could not see the oscillation on the scope and I have a dual trace Tek 100 MHz scope. My brother's amp doesn't have the problem your friend's has so the layout isn't a problem with these amps.

I think you are right in starting with phase inverter and beyond, and also heaters....anything that is common to both channels essentially.

Greg

[TomCarlos]

Thanks...

I removed the tubes from Channel 2 (gain and trem tubes) and tested using Channel 1 only. Then I removed the tube from Channel 1 and tested using Channel 2- same resilts. So it helps me nail down the area to test. I found a video that talks about using a #20 gauge twisted pair for the heaters. This amp probably uses #22 and the wires are just laying there on the chassis, non-twisted. So for cleanup purposes, that is something else I will address.

I'm taking a short break and gave the amp back to my bud. I'll get back on this one in a couple weeks. I want to solve this problem!!

Tom

[goodguy]

Replacing the large filter caps on my Gibson Lancer GA35-RVT... the ones in there are brown paper (CDF 6673 CAP = RED wire #1 20MFD 450v, Red Wire #2 20MFD 450V, blk wire common/neg) and have 3 wires. What do I replacement with as most caps have 2 leads?

[Jazz P Bass]

If you can find a dual 20uf/ 450 can cap, that will do.

What is inside is two separate capacitors.
Red+ & Black- for each cap.
The Black leads just happen to be tied together.

[TomCarlos]

I used separate caps... You can get something like an F&T 22uf, 500V for less than $5.

[Enzo]

I'd just use individual caps, but AES does sell made uop dual ones for just such a purpose. Electrically it doesn't matter.

[johnnieboy2]

GA-20t amp question...

Hope I can get a response to an older posting here re: Gibson 1958 GA-20t...

I'm working on one of these and am experiencing a lot of interaction from the depth control on channel 1 volume. I know the tremolo is used on channel 2 only, but is it normal for this to occur. It really cuts the volume almost entirely off. Just curious.

[TomCarlos]

I want to make sure I understand the problem.... You are plugged into Channel 1, playing along, but you then turn the Depth control (other channel) and you loose volume on Channel 1? Very odd indeed. Have you tried removing the 5879 and 6SQ7 tubes to see if you have the same problem?

By the way... are we talking about original parts in this amp? No odd-ball modifications?

And what about playing through Channel 2? If you adjust the Depth Control, do you loose volume on that channel?

[johnnieboy2]

Yes, just as you said, but channel 2 has no issues itself. Just channel one volume is the issue.