Caps and a Blackface AB763 Bandmaster

[Modorange]

I have a question about caps and a Bandmaster amp..

And any other parts that could need replacing. I was considering selling this amp that I found at a pawn shop but my local "tech" (an older guy who has repaired many amps over the years but is retired, and was a TV repairman mostly, but he builds tube radios and such..) replaced many of the components of the rectifier, and I replaced the power tubes which were microphonic. Otherwise the amp was performing generally very well, not too loud, but loud enough. But my son was going to use it for a gigging and practice amp, so I need it to work. THat's why I was considering selling it, because I just bought a "more dependable" Musicman HD130 amp with reverb. But I figured I haven't really given this Bandmaster enough attention before selling it.

Here's what it does. After it's been on for a varying amount of time, sometimes when the guitar plays attacked bass notes, a popping sound happens out of the speaker. Then the volume may drop with another distorted popping tone, and it may kick in a bit later. Also, I plugged some synthesizers into it, and played both channels at the same time, and got this really harsh noise coming back at me after a few minutes with the synth plugged in there. It sounded like feeding back of much of the signal or something. Then the fuse blew. I then decided to get the Musicman that was available. But the tone on this bandmaster is really nice. So I'm not ready to give up. But I took some pictures and found out that the caps that are under the shield plate are Mallory caps dated '66 from the codes. This means this amp probably hasn't had any other work than what my tech already did. We noticed it was very clean inside.

Could the caps which are old and need replacing be part or whole of the above-described problem with this amp?

Any and all comments will be read and I'll try to make sense out of it all and then proceed accordingly. I'm thinking at the very least they should be replaced and the originals put in a bag for collector's sake and then I should see just how good this amp can really sound, to let it help me make an educated decision about its reliability and tone.

Thanks.. Sorry for the long-winded intro.. Here's a few pics including an introductory front of the amp, the wiring inside and the mentioned caps...

[txstrat]

New caps are overdue (don't forget the cathode bypass caps) and the preamp tubes seem to need a replacement too.
This is one of the most sought after amps and I would bet if you get it running properly and once hear what it can sound like, you won't be that eager to sell it.

[Modorange]

OK I called Joe (my tech) and he's a conservative guy and doesn't think I need to "fix" it if it isn't "broken".

So, even though I had intended to take the amp minus the box to his shop, he didn't want to look at it without knowing more about it. He told me that I'd have to see if the problem got worse or better, and tap around on the tubes and components with a non-conductive probe (I used a wooden chopstick) to see if I could get the problem located. He said replacing the caps wasn't necessary and he didn't think they were the problem. So I took the amp back to my son's practice room and plugged the amp into the speaker and the guitar into the amp and played into it, tapping on the various parts, first on the underside.. So far nothing.. Then I turned her over and set her on a gigbag careful not to put too much weight on the tubes, and started tapping on components. I found a resistor that was parallel to the chassis length that had a loose solder joint. In fact it was just touching! Got the soldering iron, called Joe, who said that was probably going to fix it, and I soldered it back in place. It was near the lone white capacitor on the board (possibly non-original?) I soldered it back and left the amp on for several minutes. When disconnected power or volume sagged, and the bass notes oscillated or motorboated some. The resoldering of the connection brightened and tightened the sound of the amp, and the noise on the bass end went away.

I think I'll play her as is for a while, if it sounds good with original 44 year-old components, I think it's a "win-win" to keep it original!

Thanks for the input. I'm actually glad Joe's naturally conservative about tube amps. He knows them really well. They probably make much more sense to him than does much of modern society..

[Chuck H]

Those caps have a life average of about fifteen years. Yours are fourty four years old. If they're not causing an obvious problem now I promise that it won't be long. As for keeping the amp "original" ??? It wasn't shipped with fifty year old electrolytics. You should replace all the electrolytics in the amp. Don't forget the bias supply caps as most cap jobs often do. Unless you do this, your dealing with extra noise and the possibility of failure. A busted amp with all the original parts is worth a lot less than a properly refurbished one. Consider that the electrolytic caps in that amp were never intended to last for fifty years. Would you keep the old oil in a vintage car just to keep it all original??? Those caps are "wear parts" that are suppose to be replaced. Consider what could happen to some less replaceable parts if the bias supply fails.?. A replacement OT would detriment the collectibility of that amp a lot more than replacement electrolytic caps.

JM2C

Chuck

[Modorange]

A busted amp is certainly not wanted! That sounds scary. Exactly how does a faulty cap cause an OT to fail? What are the positive benefits of replacing the caps, if the amp sounds great as it is?

[Albert Kreuzer]

A busted amp is certainly not wanted! That sounds scary. Exactly how does a faulty cap cause an OT to fail?

Shorted bias cap -> tubes redplating -> cooked OT
Shorted filter cap -> cooked PT

What are the positive benefits of replacing the caps, if the amp sounds great as it is?

It will sound even better with new caps (don't forget the 25/25 cathode bypass caps).
I offer my customers to reinstall the old caps for free if they liked them better. Didn't have to by now.

Cheers,
Albert

[Wombaticus]

A busted amp is certainly not wanted! That sounds scary. Exactly how does a faulty cap cause an OT to fail? What are the positive benefits of replacing the caps, if the amp sounds great as it is?

Chuck and the others are right. Replacing the electrolytic caps on this is a no-brainer. There is no way that the orginals aren't shot by now. Keep the old ones if you want after the repair for the sake of collectabilty, but you will almost certainly experience a dramatic increase in bass response and tightness, accompanied by very noticeble decrease in hum - perhaps some of the other weirdness you are experiencing too, which could very well be from the bias caps.

You should also have the tech check the carbon composition caps for any badly drifted values. These also are subject to very marked changes from spec after many years of being subjected to high voltages and extreme heat.

By the way, I would kill for this amp, the particular circuit in it is a real classic. Don't even think of selling it until you have had it competently refurbished.

[Modorange]

How many cooked output transformers have you experienced because of a shorted original bias cap? How many cook PT's as a result of a shorted filter cap? Just askin.. Wouldn't the fuse blow before this happens? Isn't the transformers in fender amps a fairly solid part? I certainly want to learn more and do what is necessary. I asked my tech he told me if the amp sounds good as it is, the capacitors are still working after 44 years. He did say some of them could use replacing but he said not replacing them, when they're working and not blowing the fuse or showing any visual signs of failure, would not cause the transformers to blow.

I powered it up and left it on for an hour and it still sounds good, great, really. Bass response is good for the wattage of the amp, compared to the other head I've got that I was using while this one had the loose solder joint: a Musicman HD130r head.

I think in general my tech agrees it would be helpful to replace them, and the new capacitors are probably better than the old ones, but he said it's more likely that an idiot wrapping a fuse in foil or putting a higher value fuse in an amp that is blowing the fuse would cause a failed transformer.

[JoeM]

Frankly, I'm surprised any tech wouldn't recommend a full cap job on this amp.

[soundmasterg]

Three of those caps in your picture have an obvious bulge on the positive end, indicating that they are on their way to failure. A friend of mine has a vintage Sunn amp that just blew it's can cap. I had recommended that he replace it but he decided not to. The cap leaked corrosive electrolyte all over the inside of the amp and got on other parts, and the tubes cooked, and possibly the power transformer might have a problem now too. Another friend had me look at his Magnatone because it was making a bad noise, and of course the first thing I checked was the original 50+ year old can cap by bypassing it with a new cap. Noise went away. Electrolytic caps are a wear item as the others have mentioned. It is easy enough to change them and if you don't like the sound, change them back. Your tech is good to be conservative, but not when it concerns electrolytic caps that are so old and visually already on their way to failure. I think you will like the sound of the amp a lot better if you replace the electrolytics, and you can be sure that the amp should perform well for years to come if you replace them. I would not replace the signal caps however unless they are in fact leaking DC.

Incidentally, each of those electrolytics can be tested that are in your amp if someone has the rigth equipment. I have a Sprague Tel-Ohmike cap checker that will measure the value of the caps at rated voltage, measure leakage, measure ESR, etc. I can guarantee you that the new caps will measure much closer to what their value is supposed to be than those vintage ones in your amp, and they will also have better ratings otherwise. Cap producers have learned a LOT since those caps were made and the new ones in almost all cases are superior to old electrolytic caps.

Greg

[Modorange]

Hi Greg, my tech said the same thing about the new ones being better than the old ones. He also said that replacing them would improve the amp, but he didn't make it sound as urgent as all the responders here have. He does have the necessary equipment to measure the caps.. So I think the next step for me will be to take the amp in sometime and have him measure the existing caps to see how close to spec they currently are. Thanks for all the input.

[Chuck H]

I don't intend to be an a$$ho!e but you need to take that amp to a different tech.

There's a guy in my area (which is an island, so he may be the only game in town) that local players call something like 'smokin' Pete' who is really conservative when it comes to changing any parts (including electrolytic caps) on vintage amps. I've approached some of these players with advice like "You know, a new set of filters will kill that hum." And I get the response "Well. smokin' Pete said..." I just shake my head and walk away. The guy is a hack. I've heard many amps he's serviced and they sound,... Well, anyway, this guy has people believing in him. I think his jive must be better than his amp sense personally.

Any good tech would change those caps (and the bias caps and the preamp bypass caps) and know to check the old CC resistors for drift without having it brought to their attention. When you consider the hard work that guitar amps do (what with being run loud, often, and sometimes cranked) preventative measures are much better than his 'fix it when it breaks' philosophy. You might be surprised that your amp does sound and perform better and quieter after being properly serviced, Especially since you like it and hear no problems now. As many of smokin' Pete's customers feel too.

Find a better tech. If this guy is a friend then just don't tell him.

My $.02

Chuck

[Modorange]

Thanks Chuck, that's an interesting post. "If he's a friend, just don't tell him." HA!

[JoeM]

I saw those bumps.. Are those signs of wear? I will have Joe test them. He does indeed have all these nifty test equipments.. Not sure what brand it is. He builds tube radios for fun. And he said the same thing about new caps. He said new ones are better and will probably never need to be replaced.

Ever seen the inside of a Fender cap can after one (or more) of the e-caps have blown out their guts? Oh, and even new e-caps will eventually need replacing.

Do you wait until your tires are bald and blow out before you change them?

[Modorange]

No. I try to change the tires when they get worn to the point of it being a safety issue.