JTM45/5F6-A pre amp into a '68 Bassman

[aclempoppi]

Hi, just thought I'd chime in about reversed leads in the output section, I've done this same thing when replacing both tube sockets. My experience was a low pitch positive feedback.Man, was I bummed out, thought I did such a clean job. But an older PA builder clued me into my mistake. Yeah, learning from failure is invaluable! I have a question : where and how are the NFB, PI, and V3's associated components grounded? I can't tell from pictures if it's a mechanical or soldered connection. Don't mean to steal your thread, Hired Hand, but I was wondering if Solderstain could elaborate on the "backward OT" situation in this SF. I didn't have any issue when I R&R the OT in my '68. Peace Art

[solderstain]

Solderstain is correct, but most Fender amps don't share an input as this appears to do. If the stages are both being fed by a common input and one is in phase with the input, it's possible it could be feeding back thru the other stage.

ampdoc1

Somehow I must have missed where the OP said he was using a common input - can you point that out to me? It seemed to me that he was simply reconfiguring one of the channels. I didn't see where he'd boiled it down to one input... Thanks!

[solderstain]

Don't mean to steal your thread, Hired Hand, but I was wondering if Solderstain could elaborate on the "backward OT" situation in this SF. I didn't have any issue when I R&R the OT in my '68. Peace Art

I'll try not to run this too far from the OP's topic, but it seems I already have...

Print out an AB165 schemo, and an AA864 schemo, and lay them side-by-side and trace the power section out. Notice where the NFB connects in the AB165. When you reconfigure the power stage of an AB165 so that it's consistent with the much-better AA864 design, you have to swap the the NFB to the other side of the PI. In order for the NFB to be in the correct phase, you have to reverse the wiring of the secondary as well - i.e., swap the green and black wires. I did all of that to my '66 and it's a huge improvement.

Cheers.

[aclempoppi]

Thanks Solderstain for the info. I looked at the schemos, and see what you mean. I'll pull the chassis and get back in a new topic. Art

[C Moore]

OK, in the photo below you can see (I Hope) where the two channels enter the PI. There are two 220k resistors that tie into one .0022 cap. That cap feeds the grids of the PI.
There is a yellow wire and a orange wire that feed each of those 220k resistors. The orange wire comes the treble pot of the 5F6-A channel. The treble pot is fed from the cathode of the 12AX7 in that circuit.
The yellow wire is coming from the plate of the 12AX7 of the AA864 circuit. Then both of those 220k hit the .0022 coupling cap and then on to the PI.
I hope I have explained that clearly. As I have said before, I have just a very basic understanding of electronics, and a lot of this stuff is pretty new to me. I really do appreciate all the great help I get from TAG.
Thank You

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