Modding an AB165 Bassman. Question about the 470 ohm resistor??

[cluster]

Hello All, I've been working on black facing an AB165 Bassman and have a question about the 470 ohm sister that follow the 220k mixing resistors. Can anyone tell me it's purpose? I notice if I remove it I get a serious volume drop. Here is a layout with the resistor outlined in red. Thanks again.

[Justin Thomas]

That's the negative feedback resistor for that local stage. See how it goes from the plate back to the grid? Just like the 220k that go from the Power tube plates to the grids. In theory, anyway. No idea why the volume drop; I can try it on mine when I get back to my amp. I never tried it. But that little loop is a defining factor in every 50W Bassman head that followed, it never left.

Just out of curiosity, check the wiring on all your feedback loops/OT wires. I don't know if you noticed, but the OT wires on an AB165 are wired backwards form usual. When you mod the PI, you may need to swap OT leads. Main point: there are a lot of tiny changes in the AB165 that add up, but you don't normally think about when Blackfacing. Maybe as it is right now your distortion is from some kind of positive feedback/oscillation caused by feedback phase problems? Positive feedback doesn't ALWAYS manifest as uncontrollable screeching and howling... I learned these while trying to return a effed-up Bassman 50 back to stock- every wire location counts!

Just some thoughts.

Justin

Edit: is that a 470R in your amp? Supposed to be a 470K...

[pdf64]

There should be a massive gain / volume boost when the 470k feedback resistor is lifted.
As there's a volume drop, I guess it may have gone into ultrasonic oscillation.
Maybe add a 220pF or 470pF plate to chassis 0V cap on that stage, to see if that stops the oscillation and gets the volume back.

[cluster]

That's the negative feedback resistor for that local stage. See how it goes from the plate back to the grid? Just like the 220k that go from the Power tube plates to the grids. In theory, anyway. No idea why the volume drop; I can try it on mine when I get back to my amp. I never tried it. But that little loop is a defining factor in every 50W Bassman head that followed, it never left.

Just out of curiosity, check the wiring on all your feedback loops/OT wires. I don't know if you noticed, but the OT wires on an AB165 are wired backwards form usual. When you mod the PI, you may need to swap OT leads. Main point: there are a lot of tiny changes in the AB165 that add up, but you don't normally think about when Blackfacing. Maybe as it is right now your distortion is from some kind of positive feedback/oscillation caused by feedback phase problems? Positive feedback doesn't ALWAYS manifest as uncontrollable screeching and howling... I learned these while trying to return a effed-up Bassman 50 back to stock- every wire location counts!

Just some thoughts.

Justin

Edit: is that a 470R in your amp? Supposed to be a 470K...

Yes, my mistake 470K. Also, I did reverse the plate wires on the board. Is that the same as reversing the speaker wires? Anyways, it worked at getting rid of the oscillation. Curious if yours suffers the same volume drop when you remove that 470k?

[Justin Thomas]

Like I said, I never tried it, cuz I loved the amp as is, and I don't mess with a good thing. There are two things about mine not stock - it has a 100W Twin Reverb OR so needs an 8R load,and the 220K feedback resistors on the 6L6s are removed. I never tried the amp with them installed. I have changed a lot of resistors out, but with same values. One thing - if I muck about changing bad parts, if the wiring isn't JUST SO because I bumped it, it does break into instability. And while working on the B-man 50, I learned that the physical locations of your ground wires matters. A LOT.

Look for Machine Gun Amps, he does a great writeup on keeping the best of the AB165 and integrating the best if the AA864. Basically, you keep the AB165 preamp and use the AA864 bias circuit & power amp.

[cluster]

So far I've done:

1) Modded the Bass Channel Tone Stack to Marshall specs.
2) Put shielded wire to all the preamp tube grids.
3) Converted the PI to AA864 specs. Kept the 100k bias feed resistors instead of using 220k. Reversed the plate wires at the board.
4) Replaced all electrolytic including all the crappy by-pass caps.
5) Converted the bias balance to bias adjustment.
6) Removed the cathode bias on the output tubes. Don't know who did that but I rather a fixed bias for this amp.
7) Cleaned the tube sockets as well as the pots.
8) Removed the 220k feedback resistors off the output tube plates as well as the 2 disk caps coming off the plates of the preamp tubes.

Things I might do:

1) Replace the carbon comp plate resistors to reduce crackle.
2) Move the tone stack in the bass channel, make it a cathode follower and add another gain stage. In other words, make the bass channel more like a Marshall.
3) Play with the tone stack of the Normal channel.
4) Put in a Mid pot where the ground switch is now.

Any thoughts or ideas? I got this amp as a platform to learn all the the little mods one can do, so any hair brain ideas will be welcome.

thx,

joseph

[cluster]

Like I said, I never tried it, cuz I loved the amp as is, and I don't mess with a good thing. There are two things about mine not stock - it has a 100W Twin Reverb OR so needs an 8R load,and the 220K feedback resistors on the 6L6s are removed. I never tried the amp with them installed. I have changed a lot of resistors out, but with same values. One thing - if I muck about changing bad parts, if the wiring isn't JUST SO because I bumped it, it does break into instability. And while working on the B-man 50, I learned that the physical locations of your ground wires matters. A LOT.

Look for Machine Gun Amps, he does a great writeup on keeping the best of the AB165 and integrating the best if the AA864. Basically, you keep the AB165 preamp and use the AA864 bias circuit & power amp.

Thanks, I did remove those 220k resistors off the the plates of the output tubes as well. Did you mean a 100W Twin Reverb OT or OR?

[Justin Thomas]

If that amp came with the cathode/fixed/mixed bias on the 6L6s, you have an AC568, not an AB165... Nobody did that - it came from the factory that way. Part of me wants to apologize if I failed to catch that earlier, that you should have Haas an AD568 schem to work with... Don't ave the difs memorized, but the only ones of any Fender that came with that bias arrangement are all ended in ---68 or ---69. Might make a difference!

Justin

PS: OT. $&@#ing Autocorrect...

[cluster]

If that amp came with the cathode/fixed/mixed bias on the 6L6s, you have an AC568, not an AB165... Nobody did that - it came from the factory that way. Part of me wants to apologize if I failed to catch that earlier, that you should have Haas an AD568 schem to work with... Don't ave the difs memorized, but the only ones of any Fender that came with that bias arrangement are all ended in ---68 or ---69. Might make a difference!

Justin

PS: OT. $&@#ing Autocorrect...

That's interesting you say that since that cathode arrangement did kind of look like a factory job. Anyways, I pulled it out and turned it into a fix bias. I also made it an AA864 PI. BTW, how is the AC165 different?

I just looked at the AC568 schematic and there is no 470k feedback resistor in there. Also the PI in the AC568 is not what came in this amp. The PI in the AC568 looks more like an AA864. Maybe my amp was a transitional model. Dates to 1967 on the transformers (drip edge), so who knows?? I'll see what it says on the paper sheet inside the head cab.

Thx

[Justin Thomas]

Go by the circuit, not the tube chart. Fender never wasted anything, even if it did give bad information. If it came with a mixed bias setup, it's a 68/early 69. Yes, you probably do have a transitional model. And are you sure the amp was not halfway missed when you got it?

The schem for the AC568 I found has the 470k on that third "gain stage," after the channels are summed... right ahead of the input to the 12AT7.

Okay, there are 3 BF Bassman circuits, and they all use the same PI resistance values. The difference is in where the negative feedback gets reinjected. Here, I mean the SPEAKER secondary negative feedback. Not any of the local networks. On the AA864 the -ve FB 8is inserted into the cathode of the PI, kind of in the middle. In the AA165, it's into the middle again. Starting with the AB165, it's inserted into the GRID of the PI input. And it stays that way through all the rest of the SF Bassmans. The AC568 also has the same PI values, except the plate loads for the 12AT7 are now 47k. Then there's the AA270, same PI. The real SF PI shows up in the AA371, and it stays that way til 1980.

The PI should have been the same as the AA864 all the way to 1971. Another note- the AB165 came in both BF AND SF cosmetics. Mine IS a blackface, cosmetically.

Justin

Justin.

[cluster]

Thanks for that. Yeah, mine must of been a transitional model. Even the PI had an extra bypass cap in the feedback.

Here is a picture of where it is now after I cleaned it up:



Standard AA864 PI, Bias Adjust, No 470k feedback, replaced caps.

Here r some shots of the original layout. You can see the cathode bias resistors and bypass cap. Also, that little ceramic disk in the feedback at the bottom of the PI. As well as the 470k at the 3rd gain stage.











The amp is very loud now and the hiss is more apparent. Swapped out the tubes with no improvement which tells be that I need to replace the CC plate resistors as well as the decoupling CC resistors in the power section. Probably use metal film. Not sure I believe in the CC mojo.

[pdf64]

Lifting the 470k feedback resistor gives more gain now?
With gain comes hiss.
Yes a bad CC eg on a plate load will hiss and sizzle, but in an amp, there's little to distinguish the noise level of a good one from any other type, it's mainly determined by the circuit.
Any hiss from CC in the power supply will be bypassed to ground by the power supply caps.
But it's a good idea to have flame retardant / proof resistor types in the power supply.
If replacing them, the new type needs to have high voltage and current surge rating, as at power up these resistors are stressed, eg 500V rated MO types.
CC are good with surges, not so good for flameproof.

[cluster]

Lifting the 470k feedback resistor gives more gain now?
With gain comes hiss.
Yes a bad CC eg on a plate load will hiss and sizzle, but in an amp, there's little to distinguish the noise level of a good one from any other type, it's mainly determined by the circuit.
Any hiss from CC in the power supply will be bypassed to ground by the power supply caps.
But it's a good idea to have flame retardant / proof resistor types in the power supply.
If replacing them, the new type needs to have high voltage and current surge rating, as at power up these resistors are stressed, eg 500V rated MO types.
CC are good with surges, not so good for flameproof.

Just replaced all the preamp plate resistors, power section resistors as well as the screen resistors and the noise floor is definitely lower now :-) Purrs like a kitten and is ready to rock. Not much i can do now unless I want to used that spare gain stage and convert that bass channel to a cathode follower. any ideas?? maybe jus leave it alone?