Fender showman power tube overheating

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Mike7, Sep 18, 2006.

  1. Mike7

    Mike7 Member

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    New to this board kinda and due to my own circumstances need to try to fix my amp by myself, hopefully with some help,advice and knowledge from the posters here. So if any of you people can offer some assistance and instruction it would be greatly appreciated. My problem as I wrote in the title is one of my power tubes is now over heating, I just got this amp, the seller said it was just gone through and when I first pluged it in and turned it on the power light lit but the tubes did not power up at all. That was a loose wire from the power light to the first power tube socket. I reconnected that and all seemed fine for about two hours. Brought the amp home went to practice and got a loud noise like static happening when I turned it on. Shut it down turned it back on worked ok for a little while, started with the noise again. I can hear what sounds like a faint tremelo regeneration in the background when no signal is being sent. The tube isn't the problem, I tried two others and get the same over heating, all glow a bright orange while the other three tubes are fine. Anybody seen this before? And is this something that a new to amp repair person could/should attemp to fix?
     
  2. doctorx

    doctorx Member

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    No, it isn't something a newbie can fix. It sounds like your bias gone bad, or you have a bad tube, or a bad tube socket, or your heater supply is awry. That wire from "the power light" is your heater supply and the tube will not work without it. You may have connected it wrong.

    I would stop trying to do repairs and take it to an experieanced tech to have the amp fixed.
     
  3. TheAmpNerd

    TheAmpNerd Member

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    +1

    A guy got in touch with me, kind of in the same but different boat.
    He has a 1994 Matchless HC-30 head that is making noise, hums,
    staticy etc... He sent me a few pics and it has been hacked
    inside crummy job that someone did with the amp.

    SO

    He doesn't know squat about electronics, has no experience and has set off on his own to build a bench, aquire soldering gear and tools, then he is off to repair the amp himself!

    He should start with a Champ or something simpler that isn't
    real point-to-point, as opposed to Fender amps with eyelets
    or Ampeg, Marshall, etc.

    He volenteered me to help him out and coach him in this process too.

    Thinking...

    ...go ahead, make my day
    /month/year/decade

    Check out the new/old clips on Mypage.
    Some of the local cats doing in the jam thing.
    All guitar and bass amps I worked on too. : )

    NOTE: These are video clips that I used to get some sound
    from my Sony Cyber-Shot P7 Pocket camera. It is nothing fancy
    but really isn't bad for a little pocket camera that has sound on it.


     
  4. Mike7

    Mike7 Member

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    Blue Strat I see what you mean in looking at the layout drawing, I only put it back to where it most definately was already attached, the tube socket that I attached the wire to, that tube is fine, the socket where I am experencing the problem is the third from the left, I also neglected to mention and I don't know if this makes a difference but mine is a SF showman from 1969.
    Unfortunately for me Doctor X taking the amp to a tech at this moment is not an option as I spent all available funds on the purchase. I was told by the seller "The amp has been gone through, it has new grid resistors, the original iron, & the caps are good. Notice it has the coveted blue Mallorys too." So this is actually somewhat dissappointing to me but I realize the seller probably took the word of his tech and has no idea of the problem. Repairs in my area are extremely expensive with minimum bench charge starting at $65, before anything is even fixed.
    Can anybody recommend a book or books where i could gain some knowledge in the field of tube amp repair and at least attempt to educate myself in the field.
     
  5. doctorx

    doctorx Member

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    If you have a digital camera, take some good clear closeups of the insides of the amp, maybe we can help you that way.
    For the moment, I still think you should refrain from doing anything with regards to repairs. If you keep swapping tubes you are liable to fry one if they are redplating.

    You could go over to the new Ampage and ask for help, there are a lot of smart guys over there.
     
  6. Mike7

    Mike7 Member

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    Thanks for the Ampage tip Doc, I will certainly give it a shot over there, and I will take some pics and post them tonight.
     
  7. Blue Strat

    Blue Strat Member

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    Actually, the layout drawing doesn't show the heater (green) wires. You are correct that there should be a connection to the power tube sockets.

    You've probably got a loose pin "grabber" in that socket which is not providing a good connection to pin 5 of that tube. This would result in major overheating of any tube you put in that socket.

    Another possibility is an open "grid stopper" resistor (1.5K) which is the smaller resistor mounted between pins 1 and 5 on the socket.
     
  8. donnyjaguar

    donnyjaguar Member

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    Bias resistor?
    Coupling capacitor?
    Tube socket?
    Grid stop resistor?

    Only four things I can think of. If you're feeling ambitious, and your life insurance is up-to-date (kidding), I can tell you how to test for each of these. All of which can be done with the amplifier unplugged and a basic multimeter.

    DJ
     
  9. Blue Strat

    Blue Strat Member

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    I'm glad Donny has stepped up. My liability insurance might not cover the lawsuit;)
     
  10. Mike7

    Mike7 Member

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    Thank you guys for the replies and yes DJ I'll take you up on testing those things. I'm buying a few things at Radio shack tonight, so I'll be ready to proceed. Also I was thinking about replacing all my tube sockets, it doesn't look to difficult and the pin "grabbers" if I'm using the right term are getting loose in the ones that are in the amp. Am I correct in assuming that ceramic tube sockets are not actually not much different from the plastic ones? Except perhaps they are more expensive?
     
  11. Blue Strat

    Blue Strat Member

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    Installing new sockets isn't a bad idea but BE SURE you get ones that fit the chassis. There are probably 8 types of octal sockets and only one will fit Fender chassis.

    Probably best to order them from Hoffman amps as he sells stuff exclusively for guitar amps.
     
  12. donnyjaguar

    donnyjaguar Member

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    If cash it tight, check out Walmart's automotive section for el cheapo multimeters. I bought one for 10$ (actually, no I bought it at LIDLs in the UK but its the same one as Walmart) and it works fine for this kind of thing. Just get one that will test DC Voltage, AC voltage, Resitance and DC current. Mine also has a transistor checker (which actually works) and a diode check function.

    The receiving pins, or whatever they're called, on the octal tube sockets feel "loose" when the tube is not inserted. Once the tube is in they tighten up. This can be checked with a pair of needlenose pliers. If one feels loose, or is visibly sitting higher than the rest, that's the culprit. If you're crafty like me, you can remove one of the unused pins from another octal socket and replace it. The rectifier is often a good donor here. :) That'll give you something to check first. Standard cautions about working on any high-voltage circuit apply.

    I used the ceramic ones one time but they were crap. They made it very difficult to insert the tube and the key hole was too small and broke off one of the keys on the tube I used. :( YMMV.
     
  13. Mike7

    Mike7 Member

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    Thanks for all your help guys,but I'm going to have to wait to get started on this repair. I also neglected to mention it does not always happen. Sometimes the amp works fine as long as I don't move it around alot, once it works it seems to be ok. I was gonna take some pictures tonight but my kid took the camera back to college so that will have to wait. I'm moving next week so I think I'll wait till I'm in the new space to start on this project. Thanks again
     
  14. tom grossheider

    tom grossheider Member

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    Mike, something got banged loose in the shipping most likely. Where are you located? PM me if you want.
     
  15. Blue Strat

    Blue Strat Member

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    This is a pretty good indication that the problem is a loose socket "grabber" for pin 5. Seen this lots of times.

    When you're confident that the caps are discharged (use a meter!) you can retension the socket in question. Use something like a dental pick and pry the contacts closer together....not too close, though, or you won't be able to insert the tube. Also use some contact cleaner.
     
  16. tom grossheider

    tom grossheider Member

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    Could be a bad tube also, swap it to the other socket and see if the problem follows the tube or stays with the socket.
     
  17. Mike7

    Mike7 Member

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    Definately the socket, all the other sockets are fine and any tube in that one overheats when it does happen.
     

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