Help identify this circuit

[hebaton]

I picked up Yesterday a "1969 Bansmaster Reverb" It does have the "drip edge" look.
I think there have been a least 3 versions AA1069; AA270 and AA768

I don't know which one I have or Which one is the "most desirable"
If you can help my find out, please do. I noticed immediately it is cathode biassed, THat will be changed to adjustable fixed Bias. I also noted it came to me with a 5u4gb, I will be using a GZ34 and modern 6L6s. All electrolitics are to be replaced and I will verify the value of the resistors and replace any that difted to far away...

[sluckey]

You have the AA768 version. It has a fixed bias balance circuit with a bit of cathode bias provided by the 150Ω/7W resistors. It looks clean and unmolested. The only obvious work seems to be that single .022 orange drop cap. It does have the "brown turd" coupling caps that many people consider to be inferior to the more popular blue molded caps used in earlier Fender amps. I don't hold anything against the ugly brown turds.

[hebaton]

THank you !
What would be involved in getting rid of the 1500 OHMS resistors ?
I am also wondering about installing a SR OT ...

[sluckey]

It's a fine vintage amp that has not been molested. Why start now?

[hebaton]

DId you identify the circuit from the layout or is there some writing I am missing somewhere on there ?

Thanks

[sluckey]

I compared schematics to your hi-rez pics until I found the correct schematic. Took less than 5 minutes. Those 150Ω 7 watt resistors were the first obvious clue. Then the bias balance circuit nailed it down to only one model. Could not have done it without your good pics.

[hebaton]

SO this would likely have been the latest itration right ? The other two where adjustable fixed bias ?
I tend to prefer how that behaves over cathode biaseed amps. Probably the only way I would "molest" the thing.
It does make loud noises. intermittently when playing. IF you had to replace coupling caps in a vintage Fender, Which ones would you choose/suggest ?
Orange drop seems to be used a lot... but there are those "boutique" caps pretending to be recreations of Blue molded and such. Care to share your views on that ?

[sluckey]

All three of the Band Master Reverb amps use a bias balance circuit. The AA768 uses a 3-lug pot but the other two models use a 4-lug pot. LOOK AT THE SCHEMATICS.

A cap is a cap is a cap. I would only replace faulty caps and I would use Xicon or Mallory 150, mainly because I have those in bench stock.

[hebaton]

THank you !
I am starting a different thread as I have questions regarding the PI long pair difference between this and AB763 circuits

[TUBEDUDE]

You have the AA768 version. It has a fixed bias balance circuit with a bit of cathode bias provided by the 150Ω/7W resistors. It looks clean and unmolested. The only obvious work seems to be that single .022 orange drop cap. It does have the "brown turd" coupling caps that many people consider to be inferior to the more popular blue molded caps used in earlier Fender amps. I don't hold anything against the ugly brown turds.

I've had several amps with brown turd caps. Replacing them made a remarkable difference. But! I had a virgin '66 Super Reverb across the bench full of turds and it was the best sounding stock SR I ever heard. Maybe the brown turds often degrade before total failure and that's why we disliked them, not because they were poor caps to begin with.

[dragonbat13]

If that amp is working and doesn't hum like crazy (I doubt it, although a filter cap repair would probably be enough) then I sure would hate to see it modified.

If you are looking for something to mess with them I would bet you could trade it for a "more desirable" amp that wasn't working and mess with that.

[Smitty]

I didn't see this earlier. I am assuming you've done what you have done and have moved on. This post is intended for future readers.

The cathode resistors provide about a quarter of the total bias. There is a balancing cap between them. I tend to leave the resistors in place and change the balancing cap between the two resistors to two individual caps to ground. 22/25 works well. 100/25 even better. I then turn my attention to the bias circuit. Increase the value of the bias load resistor on the board (1K?) until you've got around six volts across each of the 150 ohm resistors. I just worked on a Twin from the same era and replaced the 1K resister with a 4K7. This took idle current from 15ma per tube to 40ma. These few things really wake up the amp.

Replacing all the electrolytics will help too. Unsolder them rather than cutting them and save them. I think the white Mallory cathode bypass caps are what people hear when they replace capacitors in the chocolate drop amps. They replace both the white Mallory cathode bypass caps and the chocolate drops and hear a difference. Those white Mallory caps are low quality. I use vintage Sprague TE series for the cathodes in old Fenders. The chocolate drops seem to measure the same as Sprague mylar orange drops for signal capacitor characteristics. I'd leave those in. Hope this helps.

For chocolate drop nay-sayers, I would ask that you replace all electrolytics with good quality caps first. Then give the amp a listen before replacing the chocolate drops. Might need to break in those caps for a few hours first. That could be what you're hearing too. Then, if you are still compelled to replace the chocolate drops, use good quality mylar film foil (Sprague 6PS) and then listen again.

If you replace any mylar with a polypropylene cap you'll hear a difference. Mylars sound nice with your nose stuffed in the speaker. Polypropylene sound better from the third row at the show (clearer). I don't know many players who make the effort to listen to their setup from the perspective of the audience, though. Finding someone who plays just like you do is hard. If find a lot of players pick much harder than me and get a lot of 'splat'.