Fender Bassman AB165 issues

[RainEnsemble]

Hello, guys. I'm new here obviously. I got a Fender Blackface Bassman AB165 (1967 year). It's normal channel was thin and overly bright for my strat, and bass channel wasnt really usefull.

So I called a tech, and he kinda repaired the amp - changed some caps and did Andy Ruhl mod.
It now sounds a bit better, usefull bass channel, but normal channel still lacks mids and got way too much treble even with volume on 3, treble on 0, bass on 8.

I tried doing some mods by myself: uncut 220K 6l6 resistors (just uncut them, haven't replaced wiring without them), changed normal channel's from 250pf cap to 330pf. Also tried some other things, but reversed it back. Oh and it distorts now too early - volume on 3 = some not so pleasant overdrive when strong picking, and I don't really want this.

I don't know if tech did the job done correctly (there was 2 amp guys in a town, when the other was busy), and I don't know if someone changed something in amp before me.

So the main question is: What could give such a result in a thin (especially on normal channel) and too bright sound of an amp, and how to fix it?
I could provide a photo if needed.
Thanks for help and attention!

 

[JonSnell Electronic]

Personally, I would put it back as per the schematic. Many hundreds were sold and they never had a thin Marshall sound.
I would, after returning it to original, check the 6L6GC bias current. You are looking for about 40mA per valve for the smooth gutsy sound. Cooler, say 20mA will give you a thin distorted sound, too much say 70mA will shorten the life of the valves and run very hot.
It is a very simple design and usually very reliable if the 6L6GCs are in good condition.
Hope this helps.

 

[Burnedfingers]

Personally, I would put it back as per the schematic. Many hundreds were sold and they never had a thin Marshall sound.
I would, after returning it to original, check the 6L6GC bias current. You are looking for about 40mA per valve for the smooth gutsy sound. Cooler, say 20mA will give you a thin distorted sound, too much say 70mA will shorten the life of the valves and run very hot.
It is a very simple design and usually very reliable if the 6L6GCs are in good condition.
Hope this helps.

I would take this advise JonSnell Electronic has given as the best possible advice and way to proceed.

 

[RainEnsemble]

Personally, I would put it back as per the schematic. Many hundreds were sold and they never had a thin Marshall sound.
I would, after returning it to original, check the 6L6GC bias current. You are looking for about 40mA per valve for the smooth gutsy sound. Cooler, say 20mA will give you a thin distorted sound, too much say 70mA will shorten the life of the valves and run very hot.
It is a very simple design and usually very reliable if the 6L6GCs are in good condition.
Hope this helps.

Thanks for your reply! I will call the other tech and we'll take it back to basic circuit first, and THEN maybe try some mods (bias + mid pot). Also I gonna leave a feedback after job is done for other folks like me.

 

[dotneck335]

Here's the "Andy Ruhl mod":
AA864 Bassman vs AB165 Bassman
Hmmmmm..........????

 

[RainEnsemble]

Here's the "Andy Ruhl mod":
AA864 Bassman vs AB165 Bassman
Hmmmmm..........????

Yep

Okay, I lead my amp to aa864 (instead of ab165, because I found photos of an actual aa864, so it was easy) circuit, now it sounds way better, but I need to change bypass caps that could be bad (or not workin at all)