Bad Pot and Schematic

[ToddSmee]

Hello everyone, I'm new here. I have a couple questions in regards to my amp. I have a 1965 Fender Bandmaster, the sticker in the head says AB763. Please see attached picture. I included the schematic.

I downloaded the schematic from Hoffman, but for some reason, I don't believe the schematic, and my amp match up. I have no reverb on my amp, yet there is one on the schematic. I do have vibrato though.

Was the AB763 used in multiple amps? The problem I'm having is that I have a pot that appears to be shot. The pot in question is the volume on the normal channel. Are all the pots the same in this amp? Can I replace this one with any off the shelf pot, or do I need a specific Fender volume pot? Also, I'm pretty sure it's not just dirty. I get no sound through the normal channel at all, and turning the knob causes a shockingly loud crack that I was sure destroyed my speakers, but fortunately did not!

In general, I do get a lot of odd sounds from time to time. Sometimes the sound cracks, and is weak, but most of the time it sounds alright. We all know however that most of the time is not good enough.

I don't believe the head has ever been in for service, even just a general cleaning. Aside from a good cleaning, what specific parts should I be looking at as likely culprits for my sound issues?

I'm capable of dismantling the head, and making basic repairs requiring soldering. I'm aware of the caps holding some serious charge, and also how to drain them prior to working.

What I really need is some advice on the schematic matching my amp, likely culprits causing sound issues, and also whether or not I need a special potentiometer to replace the volume.

Thanks in advance for your help.

[92Volts]

Yes, these circuits were used in multiple amps with "minor" changes such as add/remove reverb or even different power tubes. Here is an AB763 bandmaster: http://el34world.com/charts/Schematics/files/fender/Fender_bandmaster_ab763_schem.pdf
And an AB763 Deluxe without reverb:
http://el34world.com/charts/Schematics/files/fender/Fender_Deluxe-AB763-layout.pdf

Maybe one of those more closely matches your amp?

There is no need for a special pot, as long as the overall diameter and style of shaft will fit in the chassis and fit the original knob. I'm not familiar with this amp in particular, but it's likely 24mm diameter and there are only 3 styles of "normal" shafts, with ridges, round, or with one flat side. You can visually check which is used before you buy a replacement. The A stands for "audio" taper, which is important for it to behave like it's supposed to. The other common option (which you don't want) is linear taper.

[ToddSmee]

I appreciate your advice and links. Would "deluxe" be stated somewhere on my amp if it were in fact deluxe?

[tubenit]

Would "deluxe" be stated somewhere on my amp if it were in fact deluxe?

The Deluxe has 6V6 tubes.  The BandMaster schematic that was posted has 6L6 tubes.

IF your amp has 6L6 tubes and actually says BandMaster on it, then I would presume that BandMaster schematic is correct.

with respect, Tubenit

[ToddSmee]

If I understand your direction well enough, then I can be certain mine is the deluxe, correct? See the attached image for clarification please.

[ToddSmee]

I found the below schematic. This one appears to match, from what little I know about schematics anyway. Would you agree?

[ToddSmee]

I found what I believe is the correct potentiometer. Does anyone know how to find the replacement on this site? I don't seem to have a simple search box available, if there is one on this site? Please see attached image.

[66Strat]

I found the below schematic. This one appears to match, from what little I know about schematics anyway. Would you agree?

Yes. That is the correct schematic for your amp.

I found what I believe is the correct potentiometer. Does anyone know how to find the replacement on this site? I don't seem to have a simple search box available, if there is one on this site? Please see attached image.

You may or may not need a new volume pot. If the pot is noisy, you may have a leaky capacitor in the tone stack. Take a DC voltage measurement between the wiper of the pot and ground. It should be 0 or a couple of millivolts at most. Also, if the filter capacitors and bias supply capacitor have not been changed in this amp, they could be causing noise issues.

[ToddSmee]

I appreciate the info. I know this is a tall order question, but is there any chance you could mark up that schematic, identifying the parts in question, or at least try to talk me into their positions on the schematic?

• A leaky capacitor in the tone stack (I know what the cap is, but the tone stack is a new term to me).
• The filter capacitors and bias supply capacitor.

[mresistor]

It would be best to take the amp chassis out of the head cabinet and inspect and take some pictures of the insides and under the cover called the dog-house that houses the main filter capacitors. The fog-house cover is on the outside of the chassis where the tubes and transformers are mounted. it has 4 screws securing it to the chassis.

If the main filter electrolytic capacitors are all the old paper mallory caps (orange) then you need to replace all of them Asap if you plan on playing this amplifier. There are two 70uf and 3 20uf caps under the cover.
There are also 5 25uf 25v electrolytic capacitors on the main board and a 25uf 50v on the bias supply near the on lamp right behind the front panel and near the power transformer on the inside of the chassis.  Those need to also be replaced. Replace the 25uf 25v caps with 25uf 50v replacements. Replace the bias cap with either a 47uf 100v or a 100uf 100v that latter is what I use, Extra filtering in the bias circuit doesn't hurt a thing.  You can replace the main filter caps with F&T 20 uf 450v and 80uf 450v caps.

[mresistor]

The tone stack is the resistors capacitors and potentiometers that filter the bass mid and treble frequencies right after the first input 12AX7 node. (the first half of the 12AX7).  There you find the treble and bass potentiometers.  It's pretty easy to recognize a tone stack.

[ToddSmee]

I really appreciate your knowledge on this. I play this amp everyday, so it's going to be tough to have it down, but it's something I've been wanting to do for a while now. I was going to bring it in, and have it done, but I really want to understand a tube amp a little more, so I figured I'd take on the challenge myself. My grandfather left me this amp, along with his 1951 Gibson. I have a sentimental reason for wanting to really understand what makes it tick. I figure the worst that can happen is that I have to bring it in and have it done, and it might cost me more to repair any damage I might do in the process, but it's how you learn, is it not? I have a goal to build my own in the near future, with all external tubes. I have a stack of books on the subject, and nothing but time on my hands, so why not?

I hope you don't mind if I check back in with you time and again for further questions I might have while going through this.

Thanks again for your help.

[ToddSmee]

The tone stack is the resistors capacitors and potentiometers that filter the bass mid and treble frequencies right after the first input 12AX7 node. (the first half of the 12AX7).  There you find the treble and bass potentiometers.  It's pretty easy to recognize a tone stack.

It probably wouldn't hurt to just replace all the components within that stack then, would it?

[mresistor]

I would make sure you don't have a bad pot first. Get some "deoxit"  and clean the inside by squirting some into the opening by the terminals on the pot and then turn the pot shaft back and forth to work the cleaner into the pot inner surfaces. I have never had to replace any component in any original Fender tone stack yet other then the potentiometers. And I have had quite a few of them. The volume pot gets a lot of usage and can wear out over time. Especially if it has not been cleaned before. If cleaning it doesn't fix the problem them consider replacing it.


BTW I sent you a PM .... 


The main thing to be concerned about is the electrolytic capacitors which are like 52 years old if they are original. They can make alot of noises in the amp. And they can destroy your tubes and power transformer if you leave them in there and play the amp and one of them shorts out. The amp will sound dead with those caps in there and lack the great Fender tone.

[ToddSmee]

I'll pick some up and give it a shot. I'm about 99% sure it's shot though.

[mresistor]

The chassis is going to have to come out to do any of this work. So....can you take it out and take some pics for us?

[ToddSmee]

Yes. I'll document the entire thing for you guys. It'll be a couple days before I can begin this project though. I will definitely take plenty of picture though, and post them back here when I begin.

[mresistor]

When you get to replacing the cathode bypass capacitors inside (the little 25uf 25v electrolytic caps) you're going to be surprised to find that Leo used caps with very long leads, long enough for one negative lead from one of the caps to go through the eyelet on the board and all the way to the brass grounding plate. Also, those caps could be double caps, two caps in one case. I'm not sure when exactly Fender quit using those. It is possible that someone has already replaced the electrolytics but based on what you said the amp could very well be untouched.

[ToddSmee]

Haha, I'll be surprised at everything I see in there, not just that.

[mresistor]

Todd  - here is a great resource for identifying and learning about Fender guitar amplifiers   


http://www.thevintagesound.com/ffg/