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#1
Just got a Fender Bassman 100 from 1975 and finally got a cab for it, and took it for a spin. After maybe an hour of playing pretty loud (I had noticed significant volume loss, but just thought it may have been the caps or tubes) the fuse blew, and a puff of smoke came from the chassis. I opened it up and saw burned areas where the big transformer of some kind (sorry I'm not to experienced with this stuff) goes into two strings of diodes in parallel. There was also a burned area on the first tube socket. It looks like the contacts for that tube socket had shifted and may have arced. Anyway if anyone could give me a ballpark idea of what could cause this, that would be great.


Here are some pics, the first has red circles showing the burned areas.


This one is where it burned just before going into the diodes.
Quote by Cathbard
Quote by Raijouta
Unless its electronic drums.

BURN THE WITCH!!!!!
#2
ouch. take that to a pro, dude.
Gear:
Epiphone G400/L6 Spider III 15
Squier CV J-Bass/Acoustic B20
Misc. pedals

Currently saving for:
Acoustic B200

Funds:
$125/$350


Quote by m4l666
I play hard, like death metal hard

#4
Quote by chaazums
ouch. take that to a pro, dude.


+1

Especially if you don't know 100% whats wrong with it...poking around inside an amp could be the last thing you do
#5
Quote by chaazums
ouch. take that to a pro, dude.

It's a definite possibility, but I want to look into it first. My friend is an electrical engineer, and he looked with me. He said there is nothing in the chassis that he couldn't remove and replace no problem. He said it may just need all the contacts cleaned since the ones that burned look shoddy.

I know a tech is the best idea, but if a simple clean up and maybe new tubes/caps would do the trick he can do that...

If nothing else I'd like to know what the problem is when I take it to the tech so I don't get ripped.


Quote by eyebanez333
+1

Especially if you don't know 100% whats wrong with it...poking around inside an amp could be the last thing you do


My friend promises me he knows how to work on it without being killed. He designs and builds stuff like GPS and alarm systems, so he said this is a cake-walk for him. He just doesn't have the specific to amp knowledge, so I'm trying to find some info to help him know what to replace/resolder.

Quote by acoustielectric
I don't have the slightest idea what to do I don't know about amps. Did this happen during a gig?


Then why did you click "Amp Help PLEASE!" J/K

No I was just jamming with some drums.
Quote by Cathbard
Quote by Raijouta
Unless its electronic drums.

BURN THE WITCH!!!!!
Last edited by tubetime86 at Jun 25, 2009,
#6
Hrm...bunch of things that could go wrong. Go back and take in focus pictures of everything that went wrong, and report. HArd to tell what's up. That "loss of volume" is NOT a good thing at all.
Quote by kcdakrt
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#7
Quote by DLrocket89
Hrm...bunch of things that could go wrong. Go back and take in focus pictures of everything that went wrong, and report. HArd to tell what's up. That "loss of volume" is NOT a good thing at all.

Ok I'll get you some good pics tonight. All I had was my cell phone at the time, sorry. Any idea what a FULL restore on this would run?
Quote by Cathbard
Quote by Raijouta
Unless its electronic drums.

BURN THE WITCH!!!!!
#8
I did a full restore about a year ago on my old 66 Super Reverb and i beleive it was about around 350-400. Re-capping, new tube sockets, replaced 2 prong with 3 prong cable, check all connections and pots, new power tubes.
#9
Quote by Chaosinborn
I did a full restore about a year ago on my old 66 Super Reverb and i beleive it was about around 350-400. Re-capping, new tube sockets, replaced 2 prong with 3 prong cable, check all connections and pots, new power tubes.

Thanks, great info. So Tube sockets are commonly replaced? I thought they were one of the 'diehard' parts of an amp, but I definitely blew one.
Quote by Cathbard
Quote by Raijouta
Unless its electronic drums.

BURN THE WITCH!!!!!
#10
Like I posted in the other thread...

Remove the cover on the back of the chassis (on the end with the input jacks). The filter caps are under that. Do they look normal/intact?
#11
Quote by tubetime86
Thanks, great info. So Tube sockets are commonly replaced? I thought they were one of the 'diehard' parts of an amp, but I definitely blew one.


It's one of "they don't make them like they used to" things. I just rebuilt a 1953 Fender Pro, sockets were all fine. Just replaced a socket on a Fender combo made in 1998 because the terminals inside the sockets were aluminum and one of them got crumpled by the pin as the tube went in.

Ug.
Quote by kcdakrt
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#12
Quote by mr_hankey
Like I posted in the other thread...

Remove the cover on the back of the chassis (on the end with the input jacks). The filter caps are under that. Do they look normal/intact?

I'm at work now, but I will look and take pics later. There was a capacitor on the master volume that was only connected on one side. There are a lot of shoddy looking connections in there. My electrically knowledgeable friend said a few spots in there just look awful. Namely the connections on the tube sockets, and some other randoms.

DL: The contacts on the tube socket have a lot of side to side play, and appear to have possibly arced. Would that have fried the spot going into the diodes? I'm thinking a tech is gonna be my best option, but I'd like to rule out the possibility of my friend cleaning it up first. As I said, he told me he could remove everything and put it back no problem, if necessary. He said that won't be necessary, I'm just saying he is confident working on it.
Quote by Cathbard
Quote by Raijouta
Unless its electronic drums.

BURN THE WITCH!!!!!
Last edited by tubetime86 at Jun 25, 2009,
#13
Quote by DLrocket89
It's one of "they don't make them like they used to" things. I just rebuilt a 1953 Fender Pro, sockets were all fine. Just replaced a socket on a Fender combo made in 1998 because the terminals inside the sockets were aluminum and one of them got crumpled by the pin as the tube went in.

Ug.


meh.

I think the new ceramic sockets are a lot better than the old brown bakelite ones.

To TS:
The big white cap, at the top in the middle of your pictures: what's that silvery stuff next to it?
Last edited by mr_hankey at Jun 25, 2009,
#14
One of the tube sockets had to be replaced because something happened to the tube and it arc'd and melted part of the tube socket. Or maybe the tube socket melted and that caused the arcing i dont know.
#15
Quote by mr_hankey
meh.

I think the new ceramic sockets are a lot better than the old brown bakelite ones.


They've made ceramic sockets since the beginning of tubes.

The black plastic ones SUCK (which is what that newer Fender had BTW), ceramic ones are OK. Their propensity for chipping sucks though. My favorite ones are the brown bakelite-like ones that Belton makes. Great sockets.
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#16
Quote by DLrocket89
They've made ceramic sockets since the beginning of tubes.

The black plastic ones SUCK (which is what that newer Fender had BTW), ceramic ones are OK. Their propensity for chipping sucks though. My favorite ones are the brown bakelite-like ones that Belton makes. Great sockets.


I don't know which those Belton ones are (new? old?), but old Fender bakelites arc much too easily. I'm not familiar with what Fender uses now.


So what's your take on this? I'm mostly worried about the filter caps, but it also sounds like a tube arc.
Last edited by mr_hankey at Jun 25, 2009,
#17
Quote by mr_hankey
meh.

I think the new ceramic sockets are a lot better than the old brown bakelite ones.

To TS:
The big white cap, at the top in the middle of your pictures: what's that silvery stuff next to it?

I can't remember what he called it, but my friend said it's just like hot glue, it holds stuff in place.
Quote by Cathbard
Quote by Raijouta
Unless its electronic drums.

BURN THE WITCH!!!!!
#18
Quote by mr_hankey



So what's your take on this? I'm mostly worried about the filter caps, but it also sounds like a tube arc.

Would a tube arc cascade the problem back to the transformer? Because if it were just the one burn spot, I'd say no doubt about it that's it. But it also burned just after the transformer. I don't know which transformer is, I'm kind of new to this, but it's the 2nd biggest, and its output goes through some diodes then to the tubes.
Quote by Cathbard
Quote by Raijouta
Unless its electronic drums.

BURN THE WITCH!!!!!
#19
Quote by mr_hankey
I don't know which those Belton ones are (new? old?), but old Fender bakelites arc much too easily. I'm not familiar with what Fender uses now.


So what's your take on this? I'm mostly worried about the filter caps, but it sounds like it could be a tube arc as well.


Belton: http://www.tubedepot.com/sk-b-vt8-st.html

Um...I'm not favoring arcing as the explanation right now, as the TS heard that declining volume thing. Possibly a filter cap, maybe also a the OT. That burnt stuff by the rectifier resistors would indicate WAAY too much current being pulled, which is consistent with a bad OT (that was starting to die). Maybe a bad output tube.

There was mention of arcing around a socket. I'd like to get a good look at that before going too much further into it.
Quote by kcdakrt
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#20
Quote by DLrocket89
Belton: http://www.tubedepot.com/sk-b-vt8-st.html

Um...I'm not favoring arcing as the explanation right now, as the TS heard that declining volume thing. Possibly a filter cap, maybe also a the OT. That burnt stuff by the rectifier resistors would indicate WAAY too much current being pulled, which is consistent with a bad OT (that was starting to die). Maybe a bad output tube.

There was mention of arcing around a socket. I'd like to get a good look at that before going too much further into it.

If it could be helpful, the guy who sold it to me had it on with no speaker for a minute or two before I bought it...
Quote by Cathbard
Quote by Raijouta
Unless its electronic drums.

BURN THE WITCH!!!!!
#21
Quote by tubetime86
If it could be helpful, the guy who sold it to me had it on with no speaker for a minute or two before I bought it...


****.

Theory: That weakens the transformer (say it arcs between two coils). Generates more heat as you crank it. Heat = less efficient magnetics, so you start losing volume. Finally, OT gives out, scorching the recto diodes along with causing that arc.

*shrugs*

hrm...now, in reverse:

Tube socket gets hot, arcs, burns recto diodes.

Hrm. Either one is possible.

How noticable was the volume loss before it died? If you were cranking it, it might've been your ears getting fried instead (if you listen to loud music for awhile, it seems quieter). There might not have been actual volume loss at all, at which point the arcing first theory is most likely correct.
Quote by kcdakrt
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#22
Old filter caps will cause volume loss.

If there was a short somewhere along the B+ (so tube arc or filter cap failure), that could cause the rectifier to toast itself.
Last edited by mr_hankey at Jun 25, 2009,
#23
Quote by mr_hankey
Old filter caps will cause volume loss.

If there was a short somewhere along the B+ (so tube arc or filter cap failure), that could cause the rectifier to toast itself.


Indeed, very true.
Quote by kcdakrt
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#24
By volume loss, I meant consistent volume loss. As in, it was much quieter than a 100 watt should be. It didn't lose volume all of the sudden or anything. Actually it did the opposite, it took probably 5 minutes to fully heat up, and I only had it on standby for maybe 3-4. So when I turned it on, it got gradually louder until it was finally fully warmed up. But even then I had the MV and the volume at 8 and my guitar volume at 8... and I could stand in front of it comfortably. That's not normal for 100 watt tubes in my experience.
Quote by Cathbard
Quote by Raijouta
Unless its electronic drums.

BURN THE WITCH!!!!!
#25
Quote by tubetime86
By volume loss, I meant consistent volume loss. As in, it was much quieter than a 100 watt should be. It didn't lose volume all of the sudden or anything. Actually it did the opposite, it took probably 5 minutes to fully heat up, and I only had it on standby for maybe 3-4. So when I turned it on, it got gradually louder until it was finally fully warmed up. But even then I had the MV and the volume at 8 and my guitar volume at 8... and I could stand in front of it comfortably. That's not normal for 100 watt tubes in my experience.


Yep, that's what I mean by volume loss.
#26
Just got this email from the electronics guy:

"The filter cap at the Main Volume knob was not attached which is a bad connection. The other cap connections looked good but until we clean it and reheat the solder connections we cant be sure.

The tube socket did looked arched. I would replace that and then see if we would need to get a new tube. It would probably be smart to get four new sockets if there cheap. The explanation of the transformer being weak makes sense especially since the output wires from the transformer was looking really bad (beginning of the diode section).

Id get the transformer, new sockets at least two, the caps we can get in town."
Quote by Cathbard
Quote by Raijouta
Unless its electronic drums.

BURN THE WITCH!!!!!
Last edited by tubetime86 at Jun 25, 2009,
#27
No offense to your friend, but you should really get some who knows about guitar amps to look at it.
#28
Quote by mr_hankey
No offense to your friend, but you should really get some who knows about guitar amps to look at it.

None taken, he admits he knows about 99% of the components in there, but not much about tubes, OTs and not much about the application of them in amps. So I should just bite the bullet and take it to a tech?
Quote by Cathbard
Quote by Raijouta
Unless its electronic drums.

BURN THE WITCH!!!!!
#29
That whole gradual loss thing is not good. Filter cap if you're lucky, OT if you're not. And those diodes? Those are on the power transformer, NOT the OT.

EDIT: replace the socket, those older sockets can become conductive when they char (talk about the definition of STUPID), so yeah....not good.
Quote by kcdakrt
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#30
Quote by tubetime86
None taken, he admits he knows about 99% of the components in there, but not much about tubes, OTs and not much about the application of them in amps. So I should just bite the bullet and take it to a tech?


Yeah. Modern EE degree means exactly jack when talking about tubes. They're just in a different world than transistors.

Where are you located roughly?
Quote by kcdakrt
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#31
Quote by DLrocket89
Yeah. Modern EE degree means exactly jack when talking about tubes. They're just in a different world than transistors.

Where are you located roughly?

Eastern Shore of MD, pretty slim pickings here for amp techs. The local shop does it, but they have a 6-8 week turnaround, and they are way overpriced on everything so this will be no exception, I'm sure.

How about the filter cap on the master volume, he noticed last night it is not connect on both sides... Could that be it?
Quote by Cathbard
Quote by Raijouta
Unless its electronic drums.

BURN THE WITCH!!!!!
#32
Quote by tubetime86
Eastern Shore of MD, pretty slim pickings here for amp techs. The local shop does it, but they have a 6-8 week turnaround, and they are way overpriced on everything so this will be no exception, I'm sure.

How about the filter cap on the master volume, he noticed last night it is not connect on both sides... Could that be it?


That's probably a bright cap, not at all related to this. Someone probably removed it to make it less bright. Get a pic of it.
Quote by kcdakrt
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#33
Quote by DLrocket89
That's probably a bright cap, not at all related to this. Someone probably removed it to make it less bright. Get a pic of it.

I'll get you tons of pics when I get home, but for now; it was a nickel sized flat capacitor actually attached to the master volume pot itself. One side was soldered on, the other had a bead of solder from where it was soldered on, but was pulled back a little.


I think I'm just gonna take it to a tech...

Quote by Cathbard
Quote by Raijouta
Unless its electronic drums.

BURN THE WITCH!!!!!
#34
Quote by DLrocket89
Yeah. Modern EE degree means exactly jack when talking about tubes. They're just in a different world than transistors.

Where are you located roughly?


Yep. I'm doing EE (just finishing the first year), and nothing about tubes has come up. Hardly anything about high voltage, either.
#35
Well I saw an ad on CL for a company that does sound and communications systems installs in the area. Apparently the owner is hanging his shingle out for tube amp repair. This should be a better, and cheaper option than the local shop (I have had so many bad experiences with them it is ridiculous.) How do I verify a tech's qualifications?
Quote by Cathbard
Quote by Raijouta
Unless its electronic drums.

BURN THE WITCH!!!!!
#36
well if your a tech and you bring it to a super tech you can tell after talking to him for 5 minutes if he knows how to fix an amp. good amp techs have a list of things that they do. its easy for them to get right to the problem. most of them try to sell you on refurbishing parts that are not broken but highly recomended for repair/new parts. this is a good idea and if you trust the tech, I would listen to everything he tells you and do it.

I think its very likely that you have dry solder in a few places. although it really could be so many things that I cant say unless I'm working on it. you wont kill your self working on your amp if you bridge the big caps with a screwdriver (rubber handle is better). some amps dont hold anything in the caps after a few minutes of off time. like the mesa tech was telling me about my mesa.
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#37
Quote by tubetime86
Just got a Fender Bassman 100 from 1975 and finally got a cab for it, and took it for a spin. After maybe an hour of playing pretty loud (I had noticed significant volume loss, but just thought it may have been the caps or tubes) the fuse blew, and a puff of smoke came from the chassis. I opened it up and saw burned areas where the big transformer of some kind (sorry I'm not to experienced with this stuff) goes into two strings of diodes in parallel. There was also a burned area on the first tube socket. It looks like the contacts for that tube socket had shifted and may have arced. Anyway if anyone could give me a ballpark idea of what could cause this, that would be great. This one is where it burned just before going into the diodes.
Old amps have old caps. And old amp without knowing the history should be treated very cautiously. I would have removed all the tubes, added a resistor in series with the output of the SS rectifiers (to monitor the current) and connected the amp to a variac.

Ancient caps need to be "re-formed" before applying full voltage to them. The electrolyte inside is conductive, but the metal elements develop an oxide coating on them. This must be done slowly, allowing little current to flow during the process, lest the electrolyte boil from localized heat from excessive current.

Most cap jobs can be avoided if the caps are brought back slowly. But once a cap has been "whacked" by excessive internal current, it's usually toast.

Likewise some tube sockets are porous. In a humid environment they can absorb moisture and be conductive. Suddenly applying high voltages to them can cause them to arc. Once arcing occurs, it generally gets worse. The carbon tracks left behind by the arc are even more conductive.

Quote by tubetime86
If it could be helpful, the guy who sold it to me had it on with no speaker for a minute or two before I bought it...
There likely wasn't a problem caused by this. Fender uses shorting jacks on the speaker connections. This prevents excessive voltage swings in the primary of the transformer and on the plates of the output tubes if a signal is applied with no speaker connected.

Quote by tubetime86
My friend is an electrical engineer, and he looked with me. He said there is nothing in the chassis that he couldn't remove and replace no problem. He said it may just need all the contacts cleaned since the ones that burned look shoddy.
The areas that look burned are more likely a symptom than the original problem. Excessive current causes heat and discoloration. An arcing tube socket or a filter cap that went conductive can destroy your rectifiers, shorting them out. This is the beginning of a cascade failure. Re-establishing good contact between the secondary of the power transformer and the input to the rectifier section will make it possible for shorted rectifiers to allow your PT to supply enough current to overheat and short internally.

Find and fix the root problem, don't just focus on the symptoms.

Quote by tubetime86
My friend promises me he knows how to work on it without being killed. He designs and builds stuff like GPS and alarm systems, so he said this is a cake-walk for him. He just doesn't have the specific to amp knowledge, so I'm trying to find some info to help him know what to replace/resolder.
Your friend sounds overconfident. While the circuits a valve amp are based on are quite simple, troubleshooting them without causing even more damage is not. A little knowledge is sometimes more dangerous than none at all.

Make the wrong moves at this point, and you could be causing some very expensive damage. Tube amp iron is not cheap. If one or both of your transformers are not already damaged at this point, it would be sad to make mistakes and ruin them.

Quote by fullsailstudent
you wont kill your self working on your amp if you bridge the big caps with a screwdriver (rubber handle is better).
You can kill an electrolytic cap this way. The rapid discharge causes ridiculously high internal currents.
Meadows
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Last edited by SomeoneYouKnew at Jun 25, 2009,
#38
Quote by SomeoneYouKnew
stuff

Sorry, not going to quote all that, but I did read it... Several times in some instances.

Anyway, I have been talked out of letting him work on it, I'm taking it to a tech. What are the best-case, worst-case scenarios here, and ballpark costs of each?

I am talking to the guy who sold it to me about some sort of compensation. Is it possible this amp is totaled, as in more money to fix than it is worth? I paid $200 for it, and I think I can get $100 back since it blew. I would be willing to spend up to $450-500 on it including what I pay him, if I knew it were reliable after putting that into it. But if there is a chance it is going to be damn near unrepairable within reasonable cost then I will just give it back and get my money back...

Also how picky should I be about techs?
Quote by Cathbard
Quote by Raijouta
Unless its electronic drums.

BURN THE WITCH!!!!!
Last edited by tubetime86 at Jun 25, 2009,
#39
Quote by tubetime86
Also how picky should I be about techs?
About as picky as you are with the mechanic who services the brakes of your car?

I dunno. Tough question, that.
Some kids have skills and are willing to work for very reasonable rates.
If you're lucky, your amp might be worked on by the next Ken Fischer.

Otoh, most techs who work on valve amps have specialized skills and they command high prices. But price alone doesn't guarantee good work.


Full cap job, new SS rectifiers and maybe 1 tube socket replaced, figure $100 in parts and another $100 for labor.

Transformers for a 100 watt amp, maybe $150-ish each. Add about $50 for labor if one or both of your transformers need to be replaced.

Problem is, you won't know exactly what's needed until a decent tech evaluates the amp as it stands right now. I doubt anyone would bother to estimate without $50 or more for their time in doing that. That $50 would go toward the labor costs if you have it repaired. But if you choose to not repair it, they get some compensation for their time.
Meadows
Quote by Jackal58
I release my inner liberal every morning when I take a shit.
Quote by SK8RDUDE411
I wont be like those jerks who dedicate their beliefs to logic and reaosn.
#40
Quote by SomeoneYouKnew
Excellent insight.

The local shop here charges $40 for diagnostics, then calls you when they have a real cost, and lets you elect to or not to. So, from what you said I guess that is reasonable. They are a Fender dealer, and a pretty big chain (3+ locations I think) so they are probably OK, but I have not been very impressed with their gear or prices, so I'm a little unsure.

On the other hand there is a company that does pas and telecommunications and stuff that put a CL listing up for tube amp repair. I guess one of their employees has experience so he's trying to get started using their name.

Both seem like a little bit of a risk. I don't want this amp turned into a 2009 Bassman. I want either the original 1975 Bassman circuit as intended, or I want it blackfaced, depending on cost. I've heard some of these places would rather replace damn near everything with cheap parts than actually find the problem... that is my fear.


Thanks for the help already SYK, DL, and Hankey. People like you make UG worthwhile.
Quote by Cathbard
Quote by Raijouta
Unless its electronic drums.

BURN THE WITCH!!!!!
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