How do fix a Amp when you Do Not Know Whats Wrong

[Chuck H]

Ok J M, I'll back you up on that. But I'll also endorse my solution above. Because If Bob comes to us with pin voltages, component measurements and follows any test instruction we may be able to tell him "The amp is fine You just don't like it." Then he'd have to get out from under that now very expensive amp to buy something he might like better. And if he hates it after two or twenty hours then he'll have personal issues to face. Maybe he's just too obsessive and can't be satisfied with an amps limitations. As in: "thin sounding and the bright switchs are poping real harsh distortion on the high end and WAY too much bass also". I've known MANY Fender amps that do this... IF YOU SET THEM UP WRONG! Old Fender amps are very non intuitive instruments that require intuition to dial in. Just because a bright switch is there doesn't mean you can flip it anytime with any instrument at any volume and any other control settings with any speakers and expect it to sound good. Same applies to the bass control. Just because it goes to ten doesn't mean that ten is the most bass you can have when the amp is overdriving. Something more like zero to two on the bass control is the most bass you can have with the amp overdriving. Under THESE conditions any higher bass control setting only adds flab to the tone. Change the operating conditions in any way and all other parameters and criteria change too. See, these amps are non intuituve. Modern amps are sort of dumbed down so it's harder to get a bad tone. There are more limitations on the adjustments or other operating conditions allow for the greater variance in operation. That's why they all sound like plastic and people love the old amps. But I've known MANY players that used channel switching master volume type amps for years and then, finally got into a vintage amp. And they usually have complaints like "too much bass" and "scratchy top end" or my favorite, "too loud". These players don't know enough about the amps to operate them in a satisfying way.Or they expect something from them that they can't offer. I think a little of this may be happening here. Too bad really. Since truly great players DO like to use vintage amps often and DO know how to set them up and play to the amps strengths. And then they sound great.

EDIT: It's very tempting, when evaluating an amp, to find some setting or another that sounds bad and then repeat the offense over and over. Blang blang blang. Don't do that. Find settings and play with techniques that your happy with instead. Repeating the offensive and complaining about it is sort of like when your buddy say's "Hey, smell this. It really stinks!" Of course you're going to smell it. But you wouldn't carry "it" around with you sniffing at it and complaining.

[J M Fahey]

Yes, that's the basic idea.
In fact I am saying about the same: gut that amp (if necessary) and rebuild it *exactly* like an original one, and I do not mean with NOS parts, cloth covered wire and such but like **Leo** would have built it if he were alive today, meaning good commercial parts ordered from Mouser and such, if necessary "good, normal" transformers, in the league of, say, Hammonds, etc.
Then *that* is the original sound.
dumbassbob may like it or not, it's his personal decision and I respect him for it, but if not, he should not spend another cent on it, simply forget about this amp .... or ..... learn to use it within its true nature and limitations
I'm not a psychologist but as a layman I can see that there is some insecurity at play here, mainly caused because this amp changes so much every week that it's impossible to have a reference point.
I would also feel quite uncertain about it, that's for sure!!!
Now if the amp goes back to its original, just made state, *that* is a point of reference.

[R.G.]

I actually suspect that DAB knows his amp and is genuinely concerned. It's possible that there is something unexpected happening outside the amp that is causing his problems. It is very human to become extra suspicious of something when multiple instances of odd behavior happen. As the army here says, once is an accident, twice is a coincidence, and three times is enemy action.

[km6xz]

You have been through a lot with the amp and techs, maybe figuring out a way to get the tech to hear what you are hearing and agree that that is what is being complained about. Right now, the amp is generating normal power and most operating parameters are normal so many techs would be predicted to say the amp is OK. Go visit the[or a shop] shop, make an appointment for after hours to you and the tech can hear just what you are talking about and reproduce it at will. A competent tech should be able to fix what he experiences to be wrong, maybe no one has really reproduced what you are talking about. Make sure the amp is the isolated component in the problem. Often we used to get people bringing in mixer or power amp used in studio or pa system the describe symptoms that could be just about any thing with dozens of subsystems in the venue or studio. I was always amazed how sure people where when they brought one unit, had not isolated the problem but instead admitted that they brought the unit that was easiest to disconnect. IS the problem for sure the amp in generating the tone you do not like? How, precisely, do you reproduce the problem in a way that a stranger can reproduce it and recognize it as a problem? This one sounds like a system approach is needed and in person. Don't send the amp anywhere, find someone local who can reproduced the problem at will.

[dumbassbob]

Perhaps it's cursed, or haunted.

I can see one or two baffled or differently-clued techs, but this...

I have a maxim that I remind myself of every now and then - if what is happening doesn't make any sense, it's because you don't understand all the rules or conditions. It is possible that you have the worst luck with amps AND techs in the known universe. It is also possible that there is a mysterious flaw hidden inside the amp that competent (and less competent) techs cannot find. And it is possible that something about your location/setup is causing problems. I think the comment about your guitars being the cause was uncalled for.

But it occurs to me that the techs were not testing the amp with your speaker cab(s) or your AC power line, both of which can cause issues if they have certain conditions. An intermittent open in the speaker wiring can do ugly things to a tube amp. So can an AC power line with a lot of nasty transients, or one which has a high-impedance neutral.

I have no idea which of these is right - if any - but I do think it's important to keep thinking.

The Bias Cap has popped 2 times, even after they said it was fine

[dumbassbob]

I'm sure everyone here actually believes that most of the problems with repair techs in these threads are due to gross ignorance, that we are all above it and could easily poke a couple of probes into your amp and send you home grinning in an hour. To be sure, there are some problems that get by even the clever among us now and then. Unfortunately no one is as clever as they see themselves in their minds eye. The real pisser is that a bad tech can actually CAUSE problems, not just fail to fix them. Why in the H E double hockey sticks is that amp back with GW?!? Even a self proclaimed dumbass can work with the advice here to fix their own problem. The best advantage to that is we keep each other in check. This is a pretty effective method for getting better than average information. At least here. Where there is a greater than average number of very good techs that seem willing to police errors. This HAS GOT to be better than continuing with the "tech" you've been using. Who's name I won't mention. But it rhymes with Werald Geber.

I am now thinking Power Tranny is Going?

[Tom Phillips]

The Bias Cap has popped 2 times, even after they said it was fine

Replacing failed parts is only one part of the repair process. It is also necessary to find and correct the root cause of the failure.

[dumbassbob]

I actually suspect that DAB knows his amp and is genuinely concerned. It's possible that there is something unexpected happening outside the amp that is causing his problems. It is very human to become extra suspicious of something when multiple instances of odd behavior happen. As the army here says, once is an accident, twice is a coincidence, and three times is enemy action.

Thanks I have owned this amp since 1971 when I bought it from Hollywood Parks and Recreation (Hollywood Bowl)It was a backline amp there from 1965 to 1971 It always had a great sound.I always set it to Vol 5-6..Tone 3 ...Bass 3 I only use a alittle Boss SD1 and some DDL. The Bias cap has popped 2-3 times since I first got the amp back the first time....All my other amps work OK with the 120 AC house votage...Maybe the Power Tranny is going...My girl friend sent it to GW 11/2 yrs ago as a Xmas present to have the filter caps changed ...Now 6 trips back and over $1,000 itis just getting worse..At least before it would work ,and soung great for a week,and then it would start to sound like a blanket was over it, and I would adjust the tone,and Bass way up to 8-9....ThanksRG for your time and Geofx I made a Ice picky noisy,vintage CB wha sound warm because of you..

[dumbassbob]

The day that Gerald Weber failed to fix my silverface Bassman for the second time but fixed the problem of having several hundred dollars in my wallet was the day I vowed to learn how to fix my own amps. For what you've paid on that Bandmaster, a good tech would have been able to replace every part, INCLUDING the transformers!
1. Is the problem tube related? Switch all the tubes with known good backups.
2. If it isn't tubes, start at the output section and work your way back to the preamp.
with a problem like this it is more like checking off what you know is good until all you're left with is the problem.

I should of quit when I was only out $460..

[dumbassbob]

You have been through a lot with the amp and techs, maybe figuring out a way to get the tech to hear what you are hearing and agree that that is what is being complained about. Right now, the amp is generating normal power and most operating parameters are normal so many techs would be predicted to say the amp is OK. Go visit the[or a shop] shop, make an appointment for after hours to you and the tech can hear just what you are talking about and reproduce it at will. A competent tech should be able to fix what he experiences to be wrong, maybe no one has really reproduced what you are talking about. Make sure the amp is the isolated component in the problem. Often we used to get people bringing in mixer or power amp used in studio or pa system the describe symptoms that could be just about any thing with dozens of subsystems in the venue or studio. I was always amazed how sure people where when they brought one unit, had not isolated the problem but instead admitted that they brought the unit that was easiest to disconnect. IS the problem for sure the amp in generating the tone you do not like? How, precisely, do you reproduce the problem in a way that a stranger can reproduce it and recognize it as a problem? This one sounds like a system approach is needed and in person. Don't send the amp anywhere, find someone local who can reproduced the problem at will.

He is in Texas I live in California..He is supose to be a Fender Amp Guru...I have owned this amp since 1971 I know how it sounds,and when it is sick Maybe the Power Tranny?

[JoeM]

.....He is supose to be a Fender Amp Guru....

In his mind??

The suggestion to take the amp locally and have the tech reproduce the (bad) sound you're hearing is right on the money.

[R.G.]

The Bias cap has popped 2-3 times since I first got the amp back the first time....All my other amps work OK with the 120 AC house votage...Maybe the Power Tranny is going...

My normal advice is that PTs are hardly ever the issue in an amp. But losing a bias filter cap more than once in an amp's lifetime is beginning to sound like one of those "hardly evers".

One confounding issue with bias caps is that sometimes they have a voltage rating that's too low. I don't suppose you remember the voltage rating on the popped ones, do you? There's no reason you should, but I have to ask just in case.

The way diagnosis works in many cases is that the human brain is a pattern-matching machine with no known equal. I can hear the first few notes, and sometimes the first two notes of a song I'm familiar with an I know the song that's coming. Good techs, doctors, and mechanics can see one or two symptoms and know pretty well what is wrong. They do this by imagining all the things in their experience that could cause that symptom, and eliminating the things it should not be. Doctors have a saying that when you hear hoofbeats in the hallway, don't go looking for zebras - it's almost certain to be a horse. That's the practical - and cheapest in terms of pain and money - for almost all patients.

But sometimes you get a zebra.

It's really hard to give good advice on this one because it's been worked on so much; in fact, that's one of the problems. You don't know exactly what's in there now, and there could well be more than one problem. Worse yet, I think you're in the unfortunate position of not being able to do any of your own work inside the amp, so you have to buy someone else's expertise.

What the amp needs is to be traced out and compared to the original schematics so you know what is stock and what is not. Then each part needs at least partially tested or looked at to see if it's good, bad, or questionable. Then the thing should be fired up, the tube pin voltages and power supply voltages measured, as well as the DC voltages on each pin of each control and switch (other than power and standby). Then, the tubes replaced with known good ones and voltages measured again. Once all that pick-and-shovel work is done, one could sit down with the info and make some informed guesses about possible issues and things to set right.

For many of the people on this forum, that would be an enjoyable evening or Saturday afternoon diversion. For a tech you have to pay, it's several hours of PITA, and you'll get charged for it.

Popping the bias cap is an odd one. Very strange. That almost has to be either (a) bias caps replaced with new caps at too-low a voltage rating or (b) some problem in the circuit or PT that is putting too much voltage on the bias cap sometimes. I can speculate that some internal quirk in the PT might also fry coupling caps or something that would change the tone, but that's a bit far fetched; however, sometimes when you grasp at straws, you catch one.

In short, like what I do with Thomas Vox Super Beatles, what it needs is a labor of love, not a commercial transaction. I think the best thing for you to do is find some nut... er, I mean "amp repair enthusiast" who lives near enough to you and who can be um, encouraged with enough flattery and foaming mental lubricants to go through this mess with you thoroughly. There's no guarantee of success, but, just like George Washington's axe, it can be made fully functional again.

I have George Washington's own axe, the one he chopped the cherry tree down with. 'Course, it's been repaired over the last couple of centuries; it's had three new handles and two new heads.

[Chuck H]

Ok... Since you've owned the amp a long time and were always happy with it and you still like your other amps I can scratch "Bob is in crazy town" off the list. Rethinking the symptoms now... The amps is OK for awhile, then sounds weak and/or thin and the bias cap keeps blowing up. This is fairly indicative of an intermittent bias problem!?! Unfortunately, with so many techs in there that have reversed mods, altered lead dress, changed resistors and caps or otherwise drifted from the original circuit, you may never find your way back to the exact, original sound of the amp. That doesn't mean it can't be repaired to work correctly. If you get my meaning.

That bias cap is likely blowing because of excessive voltage. If the issue were excessive current you would probably have resistors blowing also. It's probably not a tube. But it may be a tube socket. Like a carbonized track on the socket or an unsoldered wire strand touching something else. It might also be a carbonized track on the board. It might be a bad bias circuit diode. Less probable but possible is a failing resistor in the bias circuit. If it's an adjustable bias (or has been made adjustable) it could be an intermittent pot, depending on how it's wired.

EDIT: I just looked at the schem and I see that the trem circuit is also using the -V supply right from the top of the cap. I never noticed this hinky system with the AB736's before now! So, to the list above I would add the possibility of a bad oscillator tube a bad cap in the oscillator circuit, or some other problem that might allow the oscillator to inject +HV onto the bias cap.

As per the other responses, it could simply be an under rated cap has been installed. Or also possible, an over rated cap has been installed backwards!!! I can also see by the schem that the amp should have a power tube grid voltage of -48. And the cap, placed BEFORE the divider in the bias circuit, is rated at 50V. So this cap undoubtedly has over voltage on it. Perhaps the replacement caps are simply not up to it like the old ones given the extra voltage from modern wall mains.

Still thinking.

Bob, you do some soldering, right? Own a meter?

[dumbassbob]

Ok... Since you've owned the amp a long time and were always happy with it and you still like your other amps I can scratch "Bob is in crazy town" off the list. Rethinking the symptoms now... The amps is OK for awhile, then sounds weak and/or thin and the bias cap keeps blowing up. This is fairly indicative of an intermittent bias problem!?! Unfortunately, with so many techs in there that have reversed mods, altered lead dress, changed resistors and caps or otherwise drifted from the original circuit, you may never find your way back to the exact, original sound of the amp. That doesn't mean it can't be repaired to work correctly. If you get my meaning.

That bias cap is likely blowing because of excessive voltage. If the issue were excessive current you would probably have resistors blowing also. It's probably not a tube. But it may be a tube socket. Like a carbonized track on the socket or an unsoldered wire strand touching something else. It might also be a carbonized track on the board. It might be a bad bias circuit diode. Less probable but possible is a failing resistor in the bias circuit. If it's an adjustable bias (or has been made adjustable) it could be an intermittent pot, depending on how it's wired.

EDIT: I just looked at the schem and I see that the trem circuit is also using the -V supply right from the top of the cap. I never noticed this hinky system with the AB736's before now! So, to the list above I would add the possibility of a bad oscillator tube a bad cap in the oscillator circuit, or some other problem that might allow the oscillator to inject +HV onto the bias cap.

As per the other responses, it could simply be an under rated cap has been installed. Or also possible, an over rated cap has been installed backwards!!! I can also see by the schem that the amp should have a power tube grid voltage of -48. And the cap, placed BEFORE the divider in the bias circuit, is rated at 50V. So this cap undoubtedly has over voltage on it. Perhaps the replacement caps are simply not up to it like the old ones given the extra voltage from modern wall mains.

Still thinking.

Bob, you do some soldering, right? Own a meter?

Yes,I have a cheap DVM but I don't know really where to start and allI know is how to drain a amp and solder a few caps in a boss pedal and a wha..Guitars, I am pretty good at setups grind and polish frets... I do take good care of my gear,Keeping it clean and not abusing it

[Chuck H]

I have George Washington's own axe, the one he chopped the cherry tree down with. 'Course, it's been repaired over the last couple of centuries; it's had three new handles and two new heads.

Sigged