65 deluxe power tranny troubles

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by frank52, Dec 11, 2004.


  1. frank52

    frank52 Guest

    just got this amp back from the shop for a tune up and 5 minutes after i fired it up i smelled electric fire. i shut it down and pulled the chassis. the primary leads on my output tranny are melted. is it shot and what may have caused this? it still works but is way too hot and smells. everything else is in top working order. do these transformers just simply fail sometimes or is their an explaination for this? thanks.
     
  2. John Phillips

    John Phillips Member

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    Yes, it's shot.

    They do fail by themselves sometimes, especially on older amps. Or, it could have been caused by a fault either on the output tube side, or speaker side causing arcing in the primary side.

    Are you running the correct tubes? Correct fuse? Has it had a cap job done (including the bias cap)? Have you had the bias checked? Have you checked the speaker wiring and speaker for intermittent faults?

    What was done to it at the shop?
     
  3. frank52

    frank52 Guest

    it has the correct tubes, all nos and tested on a hickock. it has new caps where needed and the correct fuse. i had it in the shop for a check up and all that was done was cleaning and tightening of the socket clips. the shop gave the amp a clean bill of healh and five minuites after i got to the studio it started smelling and got very hot. the speaker lead is fine. what bothers me is it has worked fine for months and now this. it does still work but gets very hot and smells. so i am not using it until i find the problem. i am sick about this as the amp was in almost perfect condition and all original.
     
  4. John Phillips

    John Phillips Member

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    Does sound suspicious...

    Are both sides of the primary fried?

    What do the tubes look like when it's running?

    Have you called the shop?


    Edit: in the title it says power tranny, in the post it says output - which is it?
     
  5. frank52

    frank52 Guest

    the tubes are good. tested after the fact. they look fine. sorry for the mixup. it is the power transformer. and it is the center taps (red and green} that are partialy melted. the ones that are grounded to the chassis. i had the tech look at it yesterday and he is uncertain about what caused this to happen. he is very qualified and has built many amps form scratch. can these be rebuilt? if not is the weber a good replacement? thanks
     
  6. John Phillips

    John Phillips Member

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    Both center taps melted... look for a plate-to-filament short in a power tube or more likely its socket - pin 2 to pin 3, or pin 6 to pin 7, screen-grid-resistor supply to filament - these are the most common shorts in a power tube socket.

    In fact, if someone has been poking about in the sockets and used contact cleaner or any other form of oily substance in there, I'd say the chances of this happening are significantly increased.

    I would check the power tube sockets immediately and replace both if you are at all suspicious. (I'd use ceramic sockets, and replace the 1K5 grid stopper and 470-ohm screen-grid resistors with metal-film and wirewound equivalents, respectively - these don't alter the tone, but they do much improve reliability... and shouldn't be considered a major loss of originality either.)

    The good news is that the tranny may not be fried after all (I thought we were talking about the OT). If the amp is still working, neither winding has failed, and probably the reason it's still overheating is because current is still being drawn via this 'wrong' circuit.

    I hope so, anyway... although you're not out of the woods yet.
     
  7. aeolian

    aeolian Member

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    Contact cleaner is insidious stuff. Spraying cleaner and scrubbing with a brush do not clean things. If there's junk in there, you need to remove it. Not put it into a solution, move it around and leave it there when the cleaner evaporates. It's real easy to create a conductive path where there wasn't one before.

    Once you have the gunk in solution in the cleaner, wick it out of there with something absorbant like a KimWipe or paper towel. The easiest way is to wet the area, scrub it a bit with a brush to losen stuff up and then put the absorbant wipe over the area and blot it with a clean wet brush. This will wick the solvent (with the gunk in solution) into the wipe. The idea is to blot, not scrub. You are trying to get the gunk to transfer into the wipe. Keep moving a clean area of wipe over the dirty area and blotting again and again. It may take 5 or 6 times but you can get all the junk out of there. I've cleaned up some pretty bad flux residue in my years as an electronics manufacturing engineer. I've also seen folks that I showed how to do this, scrubbing though the wipe thinking they were making it better. All that does is leave fibrous residue from the wipe. Soak and wick away. That's the idea.
     
  8. frank52

    frank52 Guest

    thanks guys. the tech has assured me that he used deoxit and did it right. i have my doubts. this amp worked perfect for months and now i have big trouble. i am real friggin pissed of and i am demanding my money back for his so called tune up. no sense in asking him to get it done right because he claims he already has. what he has done is @#$% up what once was a perfect example of a 65 deluxe. i put the entire blame for this on him. he says he can "fix" it for more money. obviously if the tranny is shot the amp would not work at all.
     
  9. John Phillips

    John Phillips Member

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    On a bakelite/resin-type tube socket (which the ones in old Fenders are), once a short has been established by dirt/damp/cleaner/whatever, it tends to carbonise the socket material itself, and no amount of cleaning will ever fix it. You have to replace the socket.

    Luckily, that's only about as serious from an originality point if view as a cap job, or any other minor essential repair. I'd consider it an upgrade if it's done properly...


    BTW - the reason I think it must be this: normally, no current flows in the heater center tap. It's just to 'reference' the filament windings and eliminate hum. It's not part of any circuit, so for current to flow in it (which is the only thing that could have melted it), it must be coming from somewhere else. A filament-to-cathode (ie ground) short in a tube could make current flow in the center tap too, but that wouldn't explain the overheated HT center tap.

    But if one of the high-voltage pins in a tube socket shorts to one of the filament pins, you now have a circuit where there shouldn't be one: ground > HT center tap > HT winding > rectifier > (OT > plate connection) or (choke > screen-grid supply) > SHORT > filament wiring > filament winding > filament center tap > ground.

    It's surprising that it hasn't blown the rectifier (which must be taking a very heavy current, if it's enough to overheat the center-tap wires), and if it's going via the OT that's under a lot of stress too.


    Hope that helps! I can't think of any other cause of this one, and it would fit with both the symptoms and why it should do it immediately after being serviced.

    I don't know if I'd suggest this to your tech or not! He should at least refund you for the 'tune-up', I think. Whether he should charge for any other work depends on whether it was definitely his fault, or just something that was waiting to happen on an old amp... sometimes unlucky coincidences do happen like this. Changing the tube sockets is not a big job though, so it wouldn't kill him to do it at cost.

    Have a look at the sockets (inside the chassis is more normally where shorts happen) under a very strong light and see if you can see any blackening between the pins. It might be hard since they're dark colored, but you may be able to see something.
     
  10. frank52

    frank52 Guest

    thank you guys for the info and your time. i will have the sockets replaced. should i go with ceramic? also the center tap wires have the coating burned off but are still intact. can i use shrink wrap or some other material to cover the bare wire?
     
  11. John Phillips

    John Phillips Member

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    I would definitely go with ceramic - they're just better, and won't let this sort of thing (assuming it is the cause!) happen again since they're immune from carbonising.

    It's almost certainly better to replace the grid-stopper resistors while you're at it, and very definitely better to do the screen-grid ones. Both are carbon-comp as stock, and being mounted right over the tube socket, suffer from heat cracking. Use metal-film for the 1K5 stoppers, and 3W or 5W 470-ohm wirewounds for the screen-grids - they're much more tolerant of short-term overloads (like when a tube blows).

    You can just sleeve the burned wires, they'll be OK. To replace them you'd have to get the transformer apart, which you don't want to do... still hoping it's undamaged.


    Actually, before you do anything, try a simple test: pull the rectifier tube and power the amp up (including the standby to on), preferably while looking inside to see if the wires smoke and the PT overheats. If I'm right, nothing much will happen - the false circuit will be broken by doing this.
     
  12. TheAmpNerd

    TheAmpNerd Member

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    Hey John,

    That is some good stuff you posted here.

    Those OLD carbon comps can really be a problem.
    I've had more than I can count, disintagrate on me
    when testing, desoldering, or just checking their
    integrity.

    AND

    Not just on the output tube sockets, also on the
    imput jacks too, which really surprised the hell
    outta me, cause there isn't a lot of heat there.

    While ceramic sockets are very nice, I've been
    starting to wonder about them, especially when
    using them in MI (music instrument) amps.

    I'm beginning to form the opinion they (ceramic)
    transfer too much mechanical energy to the tubes when
    an amp is used or transported when compaired
    to Micanol, Phenolic, Micalex, Celanex, or other types.

    The other types of sockets are less rigid and probably
    provide a softer interface to the chassis then ceramics
    provide. After playing through amps where I changed
    the sockets, I noticed changes in harmonics and note
    decay, not all of them plesant at least to me.

    It appears to be different then just a resistor change,
    which I do a hell-of-a-lot more frequently than changing tube sockets. I can say that I have not noticed the same
    changes in harmonics and note decay with the less
    rigid sockets.

    When you change from the CC resistors to Wirewound,
    MF or CF on the output tube sockets, you will be surprised
    at how this can clean up the tone on amps. You are literally removing a layer of **** from your tone.
     
  13. frank52

    frank52 Guest

    thanks to john and amp nerd. after quite some time i have got the amp running right again. the power transformer was gone. i put in a new mojo and a different set of power tubes. left everything else alone. so far it is working fine. i am still not sure what took out the original transformer.
     
  14. John Phillips

    John Phillips Member

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    Hmmm... internally shorted then.

    :(

    It might just have happened with age, but I'm still suspicious of something that happened during the 'clean up'. Make sure you have the right fuse in the amp and watch out for signs of trouble.
     
  15. frank52

    frank52 Guest

    yes this whole ordeal took the fun out of having an original 65 deluxe. it does and always has had the correct fuse. i can't help but wonder what kind of person whould do work that might destory a vintage transformer. i conceed it is possible that it just "wore out" but not likely.
     

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