BF Bassman channel interaction questions

[nateflanigan]

Hello hello, usually I post over at el34world but the site seems to be having some problems so I thought I'd mosey on over here.

I'm fixing up a friends BF bassman, mostly everything is fine. He picked up some vintage RCA tubes from ebay, to make a long story short, through trial and error I figured out that some of them aren't so good. Not totally dead but low output.

Using the normal channel (with a good tube) if I have the low output tubes in the Bass channel the overall output is much lower. Not necessarily in an entirely bad way, more headroom with humbuckers, but pretty anemic with a strat.

Now, when I put some know good tubes in V1 and V2 (still using the normal channel) it lights right up. Much more overall output, sounds great with the strat, etc.

So, is that normal? I've never noticed that kind of cross channel interaction in any of the AB763 circuits I've worked on but I guess I've never had any bum tubes. Strangely, I also noticed that if I jump the normal channel over to the bass channel, even with the volume all the way down on the bass channel it still has a very noticeable effect on the tone.

[pdf64]

There's several BF Bassman models and they have rather a large differences in design, so for a non-generic answer you need to identify which model it is.
Given that, if by 'low output tube' you mean a 12AU7, that will draw more current and lower the B+ node which feeds both channel pre-amp 1st stages.
Pete

[nateflanigan]

Hey Pete, the cabinet is identified as AA165, however I've noticed some discrepancies in the circuit between v1b and v2. So I'm not sure if someone converted it to AB165 (there's some newer components). I didn't realize there are three distinct bassman schematics until after I posted.

By lower output I mean a 12ax7 that is performing poorly. Regardless, I'll firm up the details on which version it is and check voltages with good tubes vs bad tubes.

Thanks

[martin manning]

Junk the bad tubes. How are the filter caps? If they are stale that may be contributing to the cross-talk. There is also the possibility that the amp has conductive circuit board syndrome.

[nateflanigan]

Filter caps have recently been replaced, the work looks professional. It's not that I'm looking to use bad tubes, I was just really surprised at how a bad tube in the Bass channel effected the sound/performance of the normal channel. Just wondering why or how that happens.

Also strangely having the bad tubes installed did have some desirable effects.

[martin manning]

You might get a clue by measuring voltages on both channels (pins 1, 2, 3, 6, 7, 8) with two "good" tubes and also with a "bad" tube in the bass channel. That would also give you an idea as to how the bad tube is failing.

[NickC]

Hey Pete, the cabinet is identified as AA165, however I've noticed some discrepancies in the circuit between v1b and v2. So I'm not sure if someone converted it to AB165 (there's some newer components).................

Fender was notorious for changing circuits, but sticking the previous circuit tube chart in the cabinet 'til the obsolete charts were used up.

I have a 1966 BF Bassman. The sticker-chart says it is the AA165 circuit, but it was really the AB165.

[ToneMerc]

I have a 1966 BF Bassman. The sticker-chart says it is the AA165 circuit, but it was really the AB165.

My 66 BF Bassman is the same way.

TM

[pdf64]

The 65 onward bassman had that big 27k dropper to the last B+ node, which the AB763 guitar amps didn't have.
Fed the 1st stage of both channels, so the performance of V1 would affect the operating conditions of V2.
Pete

[nateflanigan]

Fender was notorious for changing circuits, but sticking the previous circuit tube chart in the cabinet 'til the obsolete charts were used up.

I was figuring it was one of those scenarios.

The 65 onward bassman had that big 27k dropper to the last B+ node, which the AB763 guitar amps didn't have.
Fed the 1st stage of both channels, so the performance of V1 would affect the operating conditions of V2.
Pete

Cool, I'll look for that. Thanks for the input everyone.

[nateflanigan]

Alrighty, so as it turns out it's an AB165 circuit board despite the AA165 label. That explains a lot.

I've worked through the various mods outlined on these sites...

http://www.tone-lizard.com/Bassman_Modifications.htm
http://fenderguru.com/amps/bassman

... FWIW, I prefer the oddball NFB, implementing the standard 820r with the other mods just got too gainy for me. However the other mods were big improvements to my ears, especially the local feedback on the driver tube and futzing with the coupling cap before the PI. For that I hooked up a decade box, my friend settled on .003, I like .01

So, I just gotta clean up some wiring and replace the 12' orange extension cord some one hooked up to it.

Thanks for the help guys!