1967 AB763 Bandmaster cap mods

[Randall]

What a trip this amp is! Customer drives to buy a brownface Showman 212 combo for $500. But when he brings it to me, it is actually a Bandmaster with a Showman face plate, in a custom 212 dirty blonde cabinet with a blonde style Fender logo on it. WTF? The filter caps were changed to IC caps 25 years ago, the trem roach was missing, the bias cap was changed, but everything else is stock except for two orange drops. In the Vibrato channel the .1 cap in the tone stack is changed to a .047, and in the PI the 500pF on the top grid is .02.

The Vibrato channel sounds more bassier than the other, so I assume this bigger cap is why. Is the same thing going on with the PI cap? I'm torn between leaving it since it sounds good, or putting it back to stock. I don't want to put a lot of time into having the customer do listening test while it's here. He actually described it as sounding like a million bucks, so there's that.

What say you?

https://www.thetubestore.com/lib/the...-Schematic.pdf

[Chuck H]

First, where is there a .01 cap in the tonestack of an AB763? Did you mean the .1 cap? If you did, and the bass cap was changed from .1 to .047 I wouldn't even sweat it. I'd be surprised if you hear a difference between them.

2) The .02 cap feeding the PI always seems like a good idea to modders. It's usually not. AB763 amps have AMPLE LF in the preamp without letting it all get to the PI. SF amps used a .01 here, but with other changes to the PI. I usually opt for something like .0022 in my own starter designs and I found a .001 sounded best (to me) in an AB763 build I did a few years ago. YMMV, but I'd change it a .001 and call it done.

And C) Those IC caps are probably bad or going to be soon enough that a cap job should be a consideration.

JM2C

[Leo_Gnardo]

He actually described it as sounding like a million bucks, so there's that. What say you?

With a review like that I'd leave the tone stacks & cap to the PI grid right where they are. Maybe freshen up hi voltage & bias filters, any other regular maintenance like clean pots & brite switches, clean pre tube pins & output tube socket electrodes. Check bias & power too, just for an observation. Unless the bias is way way off I'd leave it where it is too. "Sounds like a million bucks" maybe you can make 100 more or less.

Decades in the past, when crustomers wanted more bass I'd swap out that 500 pF for a much larger cap. I'm guessing that's what happened to this one. But then the amps sound swampy. These days I'd max that at 0.0022 uF or 2.2 nF or 2200 pF any way you wish to write it. Since crustomer likes the tone, leave that alone.

[Randall]

I meant the .1 tone cap, not .01, I fixed that.

"sounds like a million bucks" was while he was checking out the amp before he bought it. It had a ticking noise, no trem, an intermittant oscillation, etc, so I'm not putting too much weight on that. After changing the electrolytics on the board (the originals were WAY off), a few out of spec cathode resistors, the drifted grid and screen resistors, cleaning, and tightening up a few dodgy solder connections (one in the HV section), the amp sounds quite a bit better than it did.

I think I am going to go back towards the direction of the original PI cap value.

edit: I put in a 0.0022, four times larger than the stock 500pF one, but way,way less than the 44 times larger 0.02 that I took out. I don't really think I hear a difference.

[Chuck H]

edit: I put in a 0.0022, four times larger than the stock 500pF one, but way,way less than the 44 times larger 0.02 that I took out. I don't really think I hear a difference.

There's a difference. You might only notice it when the amp is pushed close to and beyond it's max current spec when conducting signal though. The 2.2n/.0022uf/2200pf cap rolls off at an appropriate rate below typical guitar frequencies where a 22n/.022uf cap remains essentially flat down to DC (even as the system approaches clipping and the input impedance falls). So the 2.2n cap is everything you need, and nothing you don't. Modern 7/8/+ string guitars not withstanding The consideration here is that overtones and "beating" in the lower frequencies can detriment performance since the speakers can't reproduce those frequencies anyway and they require A LOT of the amps potential to reproduce. Potential that could be preserved for frequencies the speakers CAN reproduce.

[Randall]

Thanks for explaining that Chuck. I opted to ditch the original modder's choice of such an extreme choice, but still give a nod to what he may have been going for, albiet at a much less and more reasonable value of 0.002. This amp has reconed speakers that look like low power CTS or maybe Oxford, so they are breaking up anyway, any making it difficult to hear the difference. I think this case is closed for now, at least.

[J M Fahey]

Don't overthink it.
Donīt turn a cash cow into a bottomless time, money and effort pit; "sounds like a Million bucks?" ... ok, **just** solve the annoying stuff such as tickin tremolo, buzz, etc. and donīt touch any f*ckng thing which "might" change tone.

Just remember dumbassbob and avoid getting there.

[Randall]

Sorry Juan, I did change some things that might change tone. That enormous PI cap, the way off carbon grid and screen resistors, any cathode resistors that were more than 10% over, and a double cathode bypass cap that measured 5 and 8 nanoF (I can't live with that). I also fixed the tremolo, repaired a buzzy speaker, and got rid of the odd noises, scratchy pots and jacks, and slowed down the tremolo. It actually does sound very good now. I have never had anyone complain about putting a blackface back to stock, and I am confident with this one as well. I did leave in the 0.047 bass cap.

But, thanks for playing!

[Justin Thomas]

Just remember dumbassbob and avoid getting there.

To be fair, DAB did okay once he got his amp to a technician instead of a marketing manager (to put it politely)...

I have actually run into DAB on Farcebook, telling the sordid tale of the Bung-Fued Band Master & who NOT to send your vintage amps to. I answered his comment to check on his amp but didn't hear back.

I remain confident that Randall will not let this become GW's BM, Round 2.

Justin & Jusrin

[J M Fahey]

I only fear Customer saying: "It does not sound like before, put it back as it was" and then you wasting the next 15 years endlessly modding it, finally saying: "look, I swear itīs exactly as before, I even put the shorted capacitor back" and him saying (for the 1000th time):"no, it does not sound as before".

[Justin Thomas]

"Send it to This Texas Guru Guy; he'll get it fixed up for you!"

Jusrin

[Randall]

"I only fear Customer saying: "It does not sound like before, put it back as it was" and then you wasting the next 15 years endlessly modding it, finally saying: "look, I swear itīs exactly as before, I even put the shorted capacitor back" and him saying (for the 1000th time):"no, it does not sound as before"."

That's possible I suppose, but I'm going to stick with my belief that if he thought it sounded good when it was compromised with bad parts, then he will like it even better after it was squared up. I like my odds here. This guy is knowledgeable, reasonable and experienced, and he only played the amp for a couple of minutes before I got it. I'm not worried about it.

[Leo_Gnardo]

he only played the amp for a couple of minutes before I got it. I'm not worried about it.

Ah, we didn't know that part. I thought he'd played thru it for quite a while maybe years.

A million bucks doesn't go as far as it used to. Let's hope his next review is "sounds like twenty million bucks!"

[eschertron]

Just out of curiosity, you did write down the drifted values of those

.. way off carbon grid and screen resistors, any cathode resistors that were more than 10% over..

just for scholarly research purposes, of course. Could come in handy if the unthinkable happens and the customer balks

[Leo_Gnardo]

Just out of curiosity, you did write down the drifted values of those

just for scholarly research purposes, of course. Could come in handy if the unthinkable happens and the customer balks

Or if you want to make the Randall custom "million dollar amp." Heck why not, Dumble's only 1/10 of the way there.