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My First Tube Amp Repair


unclej
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well i took in my first tube amp for repair and could use some direction. my customer was playing with a processor plugged in to the #1 channel. he said he wasn't playing overly loud and after 5 minutes or so he smelled and saw smoke comming from the back. he shut it down and brought it to me.

when i powered it up at the shop i didn't get any smoke but when i struck a chord on the guitar i got a distorted sound that faded away to nothing quite quickly.

i removed the chasis and managed to bleed the filter caps without hurting myself. upon visual inspection i found a burnt resistor (first and second pic). by process of elimination i believe it to be a 15kohm/10%/1watt resistor.

the resistor is connected on one end to an element that i can't identify. it's the silver tube in the third pic with mallory type fp written on it. you can see the top of this element in pic two. it's rust collored and you can see where the wire leading to the resistor has been over heated.

this all leads to several questions.

1) is this resistor most likely carbon film or metal film?

2) what sources do you use for resistors, caps, etc?

3) what is the long silver component and how do i test it to see if it's failure

caused the resistor to burn?

4) are there any other probable causes that i'm overlooking?

and one last thing that i just discovered. there's a potentiometer mounted to the back of the chasis. it has no knob but a screwdriver slot on the shaft. the outside lugs each have a wire that go to one of the pins on two different 7591 tubes. the center appears to go to ground. the pot smells burnt and there's evidence that the wires have overheated. could it have anything to do with the burnt resistor and how would i test it? it doesnt' have any writing on it at all to tell me what it's rating is.

here's a link to the pics and by the way, if my questions during my learning curve become irritating please let me know. i won't quit asking them but i will grovel and cowtow more. :D

http://www.villagephotos.com/pubbrowse.asp?folder_id=1119580

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First off, what is the make and model?

It does look like you've got a fried resistor alright. The silver thing that you are referring to is a metal can capacitor. It looks like something that could be associated with a tube rectifier. Does this amp have a tube rectifier?

That knobless pot that is adjustable with a screwdriver sounds like the bias adjustment for the power tubes, the 7591s.

It's looking to me like something has went awry in the power tube bias circuit. Possibly a bad rectifier tube if applicable or a bad power tube, internally shorted or something.

EDIT: whether or not that resistor is carbon comp is a moot point. It's not in the signal path, so there's no mojo value involved there. Resistors are identified by color codes. In this case, it's going to be hard to tell what it is since it has been fried. Measuring it with a DMM will probably give you a false reading because it has been damaged. You really need to round up a schematic for that amp, if possible.

Edited by Paul Marossy
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I concur with my colleague - you have a burnt resistor there! :D

All joking aside, it's hard to tell anything without knowing what we're looking at. I'm assuming that since you mentoned 7591s, it's an Ampeg of some color (or not - they were used by a few other manufacturers), and my first guess would be that you've got a bad cap or rectifier somewhere, but that's just a wild guess. As Paul noted, without a schematic to reference, you're pretty much sailing into the wind. FWIW, I think that the pot you mentioned is probably a hum balance control, from the days before matched output tubes - lots of old Fender amps had them, theoretically so you could adjust the relative bias between power tubes to eliminate the hum from unmatched tubes. I would tend to think that the burnt resistor is a symptom of a larger problem, and would try to identify and repair it before worrying about the resistor - like Paul said, I'd be very suspicious of the bias supply. Get an accurate schematic, figure out where the power to that resistor comes from, and the culprit may become obvious. Hopefully, it's not anything too serious (or expensive). Good luck, and be careful!

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please note that i have no practical experience with building/trouble shooting tube amps (yet :D) and the following knowledge is gleaned from what ive read and my own knowledge of electronics.

from the way that i see it, if the resistor is hooked up to the smoothing capacitor and the pot that could be responsible for biasing is fried as well then it may have been a power tube go imho

tube goes -> short circuit -> draws way too much current through the resistor in the B+ line and also through the pot -> fried resistor and pot

this could have damaged the rectifier posablly the power tranny and if you're unlucky the output tranny but im not totally sure. however since you say it turns on and you get sound for a bit i dont think any of these are. id still check tho.

if the amp has an output before the power amp section you could always take the power tubes out (to eliminate any posability of damage to output tranny) and then run a signal from the pre-amp to another power amp. that way you can tell whether the rectifier/PT is ok

with regard to the resistor, like paul said its notin thesignal chain so it wont matter what type it is. from what it seems its in the B+ supply to hekp with smoothing so just pick a resistor that can handle the power.

i hope that at least some of thats helpful. like i said, my practical knowledge doesnt extend to tubes yet. its a case of reading alot of books/websites but not having got my hands dirty lol

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I think that the pot you mentioned is probably a hum balance control, from the days before matched output tubes - lots of old Fender amps had them, theoretically so you could adjust the relative bias between power tubes to eliminate the hum from unmatched tubes.

Yeah, that could be a hum balance pot as well.

I would tend to think that the burnt resistor is a symptom of a larger problem, and would try to identify and repair it before worrying about the resistor - like Paul said, I'd be very suspicious of the bias supply.

Could be the end result of a failing power and/or rectifier tube. Maybe that resistor just couldn't take it anymore and croaked. If you're lucky, it could be a simple as changing out that resistor. But, it's more likely that something caused it to fail.

Get an accurate schematic, figure out where the power to that resistor comes from, and the culprit may become obvious.

Having a schematic is like turning the light on in a dark room!

tube goes -> short circuit -> draws way too much current through the resistor in the B+ line and also through the pot -> fried resistor and pot

More or less my line of thinking... :D

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the amp is an ampeg gemini II and i'll fess up..i have the schematic. that's why i said that by process of elimination i believe it to be a 15kohm/10%/1watt resistor. there's only one other resistor the same physical size in the amp and it's a 15kohm/etc. and the schematic shows two.

i'm just learning to read the schematics so it'll take me a while to figure it all out but i did track down the pot on the back and it's labled "balance" and "100 ohm"

i'm assuming that if i disconnect the wires from the pot i should be able to test it like any other but my concern is that if it's fried what do i replace it with? it has no markings on it at all and i'm not 100% sure that the 100 ohm label is for the pot.

i've found a local tv repair shop that will test the two 7591's for me since my tester doesn't list them so that's taken care of.

i guess i'll plod on today and see what i can figure out and as always, thanks for the invaluable help.

edited to say: woohoo..that was fun. i confirmed the that burnt resistor was the other 15k. who'd a thunk it...once you positively identify one or two components on the schematic you can just follow those little lines and they're just like the wires in the amp and you can find all the stuff. :D

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who'd a thunk it...once you positively identify one or two components on the schematic you can just follow those little lines and they're just like the wires in the amp and you can find all the stuff. :D

Ain't it a good feelin'? :D You're starting to think like a techie already! B)

You shoudn't have any problem getting replacement parts, but if you can't find something in particular, I'll bet somebody around here can locate it!

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I believe that http://www.hoffmanamps.com sells those hum balance pots. Also, http://www.ampwares.com but I have heard mixed reports about their performance with filling orders.

EDIT: Actually, neither of those sites show one of those. I know I have seen them before on the net somewhere... :D

Edited by Paul Marossy
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having not seen a pic of the pot i dunno if im totally right but is it one of those cermet multi turn pots that looks kinda like this

68016001.jpg

if not i know that aikenamps uses special screw pots for the biasing in his amps so in a pinch you could email him for info

Aiken Amps

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No, that's not the kind of pot in question here...

You can get rid of that pot altogether as Doug Hoffman suggests

http://www.hoffmanamps.com/fenderservice4.htm

OR

You can get a 5W 100 ohm replacement pot from here:

http://store.yahoo.com/triodeel/controls.html

Of course, that's only applicable if you actually need to replace it. Might not be a bad idea to replace it according to what Doug Hoffman says about those hum balance pots...

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well, i just hate it when business gets in the way of my learning and having fun. i took in four repair jobs today plus what i was finishing up from yesterday plus all those dadgum customers commin' in and buyin' stuff and i didn't get a chance to do anything with the amp at all. but i'll come in early tomorrow and check out that pot. the way it smells it almost has to be bad so i appreciate the links paul. i also scored some attic stuff that i'm going to start a separate thread about.

one of the really cool things that happened today though was that a long time customer brought in a repair job and he saw the amp in question and asked me about it. found out that he's an old time designer of tube amps and is supposed to be an expert. he's retired, likes me and has offered to come in and help me if i need him...cool, huh?

later

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That is cool. I used to work a few doors down from a guy that owned the coffee shop on the corner, Fred Gotz was his name. He used to maintain tube radio equipment when he was in the navy in the 60s, and then he was an instructor at DeAnza College when they had an electronics program and he also worked for lots of other companies doing electronics design and stuff. Anyhow, he was a great resource in helping me with amp problems and designing stuff. My latte breaks used to be 20 minutes long! Just a little question Fred...

Too bad they moved away and I got a job at a different engineering firm. Life is always changing, darn it... :D

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id really love to find a tube guru around me but unfortunatelly i doubt id find any without taking the train to london. theres always my uncle who's pretty nifty with electronics but he's definatelly not within walking distance

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well the "silver thing" is indeed a can capacitor and i'm suspicious of it because the burnt resistor hooks directly to it. i've tried all morning to find a source for a replacement if it turns out bad but can't find the exact one. this one is a mallory type fp, 40,40,40 500v. so first off, how do you test one of these rascals and if it's bad any ideas where i might find one?

by the way, the hum balance pot is totally fried. it smelled so burnt i went ahead and disassembled it and it was garbage inside. so that will be on order.

so now i'm curious about cause and effect. i'm guessing that the can capacitor is part of the power supply circuit as are the 7591 tubes. the burnt resistor is connected to the can and the burnt balance pot is tied to the two 7591's. if this was all a domino effect sort of thing which one is most likely to have started it?

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well the "silver thing" is indeed a can capacitor and i'm suspicious of it because the burnt resistor hooks directly to it. i've tried all morning to find a source for a replacement if it turns out bad but can't find the exact one. this one is a mallory type fp, 40,40,40 500v. so first off, how do you test one of these rascals and if it's bad any ideas where i might find one?

The cap might be fine believe it or not, but then again, it may have been the cause of the mess in the first place. I believe that the 40, 40, 40 refers to three seperate sections within the cap - three at 40uF each. A lot of those metal can caps were designed to be multi-purpose in terms of capacitance. Depending on how you connect it, you could have a 40uF, 80uF or 120uF cap. I would be willing to bet that it is connected as a 120uF cap, though.

by the way, the hum balance pot is totally fried. it smelled so burnt i went ahead and disassembled it and it was garbage inside. so that will be on order.

According to Doug Hoffman, those pots fail all the time. :D

so now i'm curious about cause and effect. i'm guessing that the can capacitor is part of the power supply circuit as are the 7591 tubes. the burnt resistor is connected to the can and the burnt balance pot is tied to the two 7591's. if this was all a domino effect sort of thing which one is most likely to have started it?

Definitely sounds like a domino effect. It's hard to say just what started it all, though. It could have been a power tube, the hum balance pot or the cap failed. If it's a bad power tube, then this could all happen again. You really need to determine if those power tubes are good, with no internal shorts or anything... B)

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i'll find out about the tubes tomorrow.

i had already read about the can cap having three sections but i can't find one that starts off with the 40,40,40. it has seven tabs on top with one going to ground so i'm guessing that i should be able to get readings by measuring across two at a time but how would you know which two go to which section?

the good news is being able to narrow down the problems to a couple of possible sources and actually beginning to understand what they are. :D

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7 tabs plus one ground, huh? That's not at all what I expected. OK, I would try placing a post at the Ampage Guitar Amp Forum at http://www.firebottle.com

I'm sure that someone there has some experience working on those old Ampegs and can hopefully help you get a replacement if needed, or at least help you on how to test that strange thing. :D

EDIT: OK, I just thought of one thing. The schematic. It should indicate how many caps there are and their values. That might help you to figure it out. Another thing is that those types of caps may not be available anymore, and if so, you will have to improvise by replacing that with modern axial/radial caps. But don't take my word for it, do some more research. I just know those can caps are really hard to find these days...

Edited by Paul Marossy
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by the sound of it the metal can actually has 3 seperate caps in it of 40uF each. 6 connectors for the caps and the 7th will be a ground for the case. it is still possable to get cans which contain multiple caps but like paul said it would probablly be alot easier to replace it with new caps.

this is assuming of course that thats whats blown. personally i think it would be ok since i dont think an excessive current draw through the resistor would have fried the cap since i dont think it would have gone above the rated voltage and i cant think of how else it would have damaged it. id still check it tho lol

if you have a good multimeter then that might be ableto measure it tho looking at mine it only goes up to 20uF

im sure if i thought about it i could think of a way to do measure it with a pulse generator, a smallish resistor and an oscilloscope. but that would probablly be a bit of adance to sort out and you'd be much better off asking the TV repair guys.

in general im still of the reckoning that its power tubes that went first and started everything off but then theres that "opinion backed up by little experience" thing ive got going so dont take my word for it.

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well now i believe that's what i just said pilgrim. :D

http://www.villagephotos.com/pubbrowse.asp?folder_id=1119580

take a look at the first picture. that's the top of the can. the twisted tabs evidently hold it in place but are also the tabs for the caps. a couple of them are wired together with a jumper and the others have either one or two wires connected to them so it's hard for me to imagine how the three caps are all connected.

anyway, my new guru will be picking up his guitar sat and i'll pick his brain about it.

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Well, gee, I didn't know there was a picture of the top of that thing. :D

It looks to me like two sections are wired together for ~80uF and the other one is a standalone at ~20uF. The other four lugs appear to be connected to the case, which is probably connected to ground. You could confirm that easily with a continuity checker. Those types of caps usually had labels on them that would show you how to connect them up various ways - that's what the circle, square and triangle indicate.

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wow a picture lol. i think Paul is right on this one. seems that its gotta be a common ground with the four outer lugs, otherwise they would all have to be connected somewhere.

no all you gotta do is check the sucka

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That cap is bad as well as at least one output tube. If you want that exact cap as a replacement, go to the oldest, smallest TV repair shop you can find and they will have a dusty cardboard box full of misc. Sprague canned caps like this in variuos voltages, values, etc. Get one with the same or higher(not more than 50%) mfd rating and the same or higher voltage rating. This replacement will also be bad. It wil need "reforming" which is done by applying the operating voltage/current slowly while it reforms the oxide layer that lets it capacitate. New word, capacitate. Cool. Put a lightbulb socket in series with one wire of the AC line cord. Screw in a 15 watt bulb and turn it on for a day. Then screw in a 50 watt bulb and run it for a day. Don't do this if it has a rectifier tube instead of silicon diodes as it could crunch the rectifier tube.

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thanks dr. i like the word capacitate. i guess when they go bad they're un-capacitated, eh? one of the power tubes did test bad and my tube expert's coming in tomorrow and i'll find out about the can. and thanks for the tip on powering up.

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