Power Transformer gets HOT

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by matt1969, Apr 16, 2015.

  1. matt1969

    matt1969 Member

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    I have a 1974 marshall superlead 100 watt. I noticed that the power transformer gets really hot but not always. doesn't matter the location happens at gigs, practice.....

    last night it got really hot, too hot to touch. two nights ago it didn't get so hot - same location & set up. nothing different.

    I realize that us humans are whimpy when it comes to temperature. Usually we can stand up to about 130 140 Deg f without having to let go.

    I know there are a lot of variables....

    Does anyone have or have had any experience with this issue & can lead me in the right direction?:confused:
     
  2. VacuumVoodoo

    VacuumVoodoo Member

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    Standard norm for a transformer built in box is 65-70C (ca 150-160F) maximum on the surface of core.
    That's really hot to the touch, if you touch it lightly you'll pull back your hand within 3 seconds. Personally, I don't like running transformers at their max allowed temperature.
    The warning that something might be wrong is in the different behaviour at different times.
     
  3. Blue Strat

    Blue Strat Member

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    What's the bias setting on your power tubes?
     
  4. pdf64

    pdf64 Member

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    Have you got the correct HT fuse in place?
    Can you correlate any change in sound / tone to the PT heat level?
     
  5. matt1969

    matt1969 Member

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    EL34s are biased at 35 mA

    I think it's the right fuse. lol
    I can not tell any sound difference.

    thanks for replies! So any ideas besides checking the fuse?
     
  6. pdf64

    pdf64 Member

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    The HT fuse should be 1 amp. On the early 70s models it is after the reservoir cap, so a F / quick blow type might be tolerated.
    Is it always used with the same speaker cab and cable?
     
  7. schmidlin

    schmidlin Member

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    I've posted this before, but certain Marshall models are prone to thermal runaway, depending on the conditions. You may have it set at 35ma, but that could double after 30 minutes of playing, again depending on conditions. Install a fan. At least go to Big Lots and get a chintzy clip-on fan. Could save you a lot of heartache and $s.
     
  8. EFK

    EFK Supporting Member

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    You may have an intermittent oscillation going on that is above the range of your hearing but is forcing the amp to pull a lot more current, so even under the same playing conditions, when it's not oscillating the PT may run room temp or just warm, and when oscillating it may really heat up. All you may audibly hear is a slight increase in 'hiss' or white noise type of sound… something else to look into.
     
  9. matt1969

    matt1969 Member

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    Interesting.
    What would oscillate? I'm new to this kind of an issue. I wouldn't know where to start troubleshooting. would the tranny itself be oscillating? I been maintaining, modding & repairing this amp for about 15 years now so know my way around this particular amp pretty well. Obviously not well enough.

    thanks for all of the replies!
     
  10. Diablo1

    Diablo1 Member

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    The transformer won't induce the oscillation, but other parts of the amp circuit can. Possible causes for oscillation are wire lead dress or a bad grid stopper resistor or bad filter capacitors between the stages. The only way to tell if it's oscillating is to put a scope on it.
     
  11. Blue Strat

    Blue Strat Member

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    ^^^ Having Diablo weigh in on a "heat thread" is priceless! :D
     
  12. matt1969

    matt1969 Member

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    Thanks! The problem with testing is that it is an intermittent problem. I suppose that while it's hooked up to a scope, I should "chop stick test" wires and components to see if it goes into oscillation, right?
     
  13. zenas

    zenas Member

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  14. donnyjaguar

    donnyjaguar Member

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    I like the oscillation theory but if you think about it, any ultrasonics are going to meet with a rather high impedance on the output of your amplifier (think speaker coil inductance) and really not draw a lot of current. That said, most every oscillation I've had in a guitar amp has been in the audible range and had an adverse affect on tone. In other words, you probably would've noticed something not quite right. Thermal runaway on the output tubes is probably more likely. Marshall amplifiers generally have a bias circuit that drops the bias voltage to maximum if something goes awry though, but I suppose the issue could be more upstream or a defective output tube.
     
  15. EFK

    EFK Supporting Member

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    Exactly this, and also¬Ö

    "I suppose that while it's hooked up to a scope, I should "chop stick test" wires and components to see if it goes into oscillation, right?"

    Yes. However if you don't have a scope, you can do some preliminary testing by monitoring the bias current and/or plate voltages while you chopstick and see if you can get either to jump around. Best to inject a signal though while doing this. Kind of rudimentary but fairly often you can spot a problem in this manner.
     
  16. matt1969

    matt1969 Member

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    Thanks. I'll have to bring it in to work when things cool down, we're really busy. I can do all of this in the electronics lab where I work.
     
  17. matt1969

    matt1969 Member

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    So I played two gigs this weekend. Friday night my Power tranny got so hot, I could have fried an egg on it.

    Sat. night It started to get hot so I put a fan on it and then everything was fine. Amp sounded awesome both nights.
     
  18. matt1969

    matt1969 Member

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    I just checked. I have a 4 amp slow blow in there instead. Is that a problem or can it be one?
     
  19. zenas

    zenas Member

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    An updated fuse means it's been blowing the original rated fuse right? Amps blow fuses for a reason.
    This amp still running original electrolytics by any chance?
     
  20. matt1969

    matt1969 Member

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    I did not blow any fuses. I had the 4 amp slow blow in there forever.
    I replaced the filter caps about ten years ago.
     

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