Stop channel interaction on modded bassman

[Kev01]

 Hello , I rebuilt the bass channel of a silverface bassman aa371.  I give it a Marshall voice and used the untapped half of the tube . I also installed a post phase inverter master vol . All the mods turned out great , but I have a lot of channel interaction between the stock and modded channel.  I wanted to be able to A/B switch the channels with one amp , but when I get the gain up on the hot channel there is a lot of bleed over into the other . These mods I did are documented and also freshened up the caps . I'm not an engineer I build guitars so I'm not sure where to start at to stop or reduce the interaction be it mixing resistors or what . Any help greatly appreciated,  thanks in advance Kev

[sluckey]

My crystal ball is in the shop for a good buffing.

[Platefire]

Those mods you say are documented, does that mean you have a schematic???

[bmccowan]

A schematic, layout, and hi-rez pics are very useful for troubleshooting your amp. Don't wait to be asked. JUST DO IT!

[PRR]

My crystal ball is in the shop for a good buffing.

That won't buff out.

It's worn out. Needs rings and valves. Maybe a new turboencabulator.

[Esquirefreak]

My crystal ball is in the shop for a good buffing.

That won't buff out.

It's worn out. Needs rings and valves. Maybe a new turboencabulator.

Don't forget the needle valve and float adjustment before startup. Haha.


As for OP - It sounds like the channels are sharing K resistor/bypass cap. In that case try and split those (give each triode its dedicated cathode C/R). At least that's where I'd start. But without a schematic it's really a guessing matter.

/Max

[JPK]

Please document the mods in this thread. You have some of the smartest amp guys on the planet here (not talking about me).

[Kev01]

I'm having some internet issues so I did try to upload . It's an aa371 bassman and I'm wondering if upping mixing resistors might help without messing with the impedance.  Sorry I'm doing this from my phone and not trying to sound pitiful I'm loosing sight in left eye due to a brain tumor so the last month has just been fun .

[sluckey]

It used to be an AA371. Only you know what it is today.

[Kev01]

 I converted the bass side to a Marshall tone stack and and tapped into the unused half of a preamp tube , other than that everything else is still aa371 other than converting it to a real bias control . No mods to feedback or power section . When I get the gain up I have more interaction on the clean channel than I want . I'm sorry I can't get a schematic up right now .

[sluckey]

You said all that previously. But that short description does not provide enough clues to even guess. Hence my crystal ball comment earlier. We're all willing to help but you just haven't provided enough information.

[Kev01]

Again I'm sorry , I will get them as soon as I can . I actually made my living repairing guitars and started on tube amps of my own to educate myself because the nearest place for any type of good work is 4 plus hours away . I have been doing the easy stuff , filter caps ,bias and bad caps . My close friend was an incredible amp guy ,but passed away a few years ago , so I kinda feel lost without my Yoda .

[bmccowan]

Well I'm no Yoda, but I am sorry about the eye issue Kev. Agree that without a schematic advice is difficult/worthless. So I will only suggest where to look. In looking at that standard schematic the mixing resistors feed into that tube that has the unused stage. So the changes you made, undoubtedly change that - so, first place to look. Also, that amp has unequal mixing resistors it seems - 220K and 470K - up the other to 470K? I think common wisdom (not Yoda wisdom) is to not go higher than 470K. As I said, that may be worthless, but easy enough to change back.

[sluckey]

that amp has unequal mixing resistors it seems - 220K and 470K

And the mixer is hot, ie, plate voltage on the mixing resistors.

[Kev01]

This it guys

[sluckey]

There's a much better schematic in Hoffman's Schematic Library. We all know what the original looks like. But yours has been heavily modified. Only you know what you have now.

[Kev01]

This it guys

Sorry the schematic is no longer available and i would be here 10 hours trying to draw it out . my issue with schematics is the straight line can double with my vision now and the sucks big time so been using layouts more and that is taking time . this is a CBS lets fix what wasn't broken amp and the lead dress was a spaghetti monster and i got that sorted out first . i apologize for my ignorance  and feel your pain with me as i get the same with guitars all the time .

[Kev01]

 What would be the best fix for the different mixing resistors and is that my issue ? This poor amp was abused when I got it and brought everything to factory  before the mods. Like I said im trying to get amp smart after 40 years of repairing and building guitars. They say ignorance is bliss , but no its not

[sluckey]

Sorry the schematic is no longer available and i would be here 10 hours trying to draw it out .

Or, we could all try to guess what you have. Probably take more than 10 hours though.

Oh wait, my crystal ball is back and it looks brilliant. And it says...

   Assistance is futile!

[bmccowan]

They say ignorance is bliss , but no its not

so true.
Only two ways I know of to get at "your issue." Have the Yoda's here (Sluckey, PRR, etc) look at your as built schematic. Or trial and error. My choice would be the former, even if it does take hours to draw up. Trial and error could take one hour or 100.
Not blaming you, but the information is getting more complicated with a spaghetti mess in the mix. Its a fairly complex amp that has been made more complex. No need for Fenders to be spaghetti messes - that's Gibson's job. You are not likely to do this, but I would remove all that is not standard and then rebuild to one of the more liked Bassman circuits. And I'd likely have Doug make up a turret board for the circuit you choose. At the conclusion you will have both an amp and a schematic.

[Kev01]

I was hoping it was a poor design by CBS mixing the channels and would be and easy fix I have all the wiring sorted out and I'm actually pretty anal about wiring , but my expertise is in passive guitar wiring .

[Kev01]

It was the mixing resistors . I bumped the other to match and much much better .

[bmccowan]

I'm glad it's better - best of luck with all.