1980 Fender Deluxe Reverb issue

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Laurent Brondel, Jan 18, 2015.

  1. Laurent Brondel

    Laurent Brondel Supporting Member

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    It's the second time I have this problem with my SF Deluxe Reverb, as soon as I turn on in play mode I hear this 60 Hz hum in the speaker, and no sound. There is also a tiny bit of smoke and a hint of that lovely burnt smell.
    The last time it happened I noticed the two 100ohm resistors going to ground from the pilot light 6.3v heater wires looked burnt (I had already changed them before). I saw nothing wrong inside the amp, tried another 5U4GB rectifier and everything seemed back in order for a few days. Then today the same thing happened again at practice.
    I noticed that those 2 100ohm resistors are absent on the AB763 schematic and appear on the AB868 and later schematics & layouts. My other Deluxe Reverb (from 1979) seems totally fine, the original 100ohm resistors look and test good. I also have a 77 Princeton Reverb on which I had to replace those same two 100ohm resistors, they were so burnt, they were severed in half.
    Maybe I'm chasing the wrong thing here, any idea?
    Are those resistors necessary for the SF amps, and not for the BF ones? Slightly different PT? Nothing got damaged yet, but I would like to know what the recuring issue is, and if anybody experienced the same problem.

    [​IMG]
     
  2. smolder

    smolder Gold Supporting Member

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    Those two resistors to ground are used on transformers without a center tap built in for the heater winding. My guess is that those two schematics will reflect different numbers for TR1. You might check to see if your power transformer is stock.
     
  3. Laurent Brondel

    Laurent Brondel Supporting Member

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    Ok, I see on the layouts. Yes, the PT is original to the amp.
     
  4. pdf64

    pdf64 Member

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    Use of the standby switch puts a big stress on the rectifier tube.
    It's bad design practice to 'hot switch' a capacitive load on to a tube rectifier.
    It was probably done for reasons of economy, so that 450V B+ caps could be used.
    Try to avoid using standby.
    When next re-capping, get 500V B+ caps fitted in there, though good 450V caps should cope with a few seconds of over-voltage at switch on, between the 5U4 ramping the VB+ up and the 6V6 warming up enough to conduct.

    About the only thing that can cause that is a power tube short, in which B+ current is sent to ground via those resistors.
    This is one of the few examples where it's a good idea to uprate resistors (I use >3watt wirewound) because there's the potential for high collateral damage in this situation.
    If the heaters lose their ground reference and get pulled up towards VB+, the heater-cathode insulation of all the tubes in the amp may get damaged.
     
  5. mark norwine

    mark norwine Member

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    might have some carbon-tracked sockets, too....
     
  6. Laurent Brondel

    Laurent Brondel Supporting Member

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    I replaced the 1/2w 100ohm resistors with 1w ones, is that OK or should I go higher? It looks like the 1/2w ones caught fire and burnt the heater wires insulation.
    I re-capped this amp in 2013 with 20uf/600v Sprague Atoms if I recall.
    The tube sockets look clean, should I try to clean them anyway?
    So, you think it is better practice to leave the standby switch in play mode at all time, and just use the on/off switch?

    [​IMG]

    [​IMG]
     
  7. mark norwine

    mark norwine Member

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    those resistors are not the problem....they're the result of the problem.

    Your problem is a.) at the power tubes and/or sockets or b.) your PT is breaking down internally.
     
  8. pdf64

    pdf64 Member

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    I wouldn't open the amp up especially to do it, but for the next time you're in there, buy >3W wirewound resistors with a high surge voltage rating (eg 500V, or at least 350V).

    I would look to clean up that carbonised heater wire insulation too. It may be conductive and bits may break off and cause trouble.

    Yes, for any amp, especially one with a tube rectifier, and especially squared when it's arranged like this.
    I've had my 60s JMI built Vox AC30 for over 30 years; the rectifier is fine and could well be the original. No standby on a real AC30.
    It's enlightening to monitor the B+ current (between the winding CT and ground) and the VB+ across the reservoir cap, at power up.
    After about 20 seconds of warming up, over the next 30 seconds both begin to ramp up smoothly to the working level.
    A 5U4 will ramp up more steeply, but the point still stands.
    Consider muting the amp by pulling the input jack plug out halfway.
     
  9. Laurent Brondel

    Laurent Brondel Supporting Member

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    So, the amp worked and sounded fine out of the cab after I replaced those resistors. The first time I turned it on after putting it back in the cab it did the same thing again. I turned it off, and on again, it worked for a minute or so, but sounded a bit fuzzy, then stopped working with that 60hz hum.
    I will carefully inspect the power and rectifier tube sockets tomorrow, I do not remember any tube arcing on that amp and it's been working fine for the past year and a half or so.
    Power transformer breaking down internally? What does that mean and is this common?
     
  10. Laurent Brondel

    Laurent Brondel Supporting Member

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    I found the issue this morning: cold solder joint on pin 2 of the 2nd 6V6 socket, the heater wire / solder was intermittently touching pin 3, about 420v DC… Hence the burnt resistors at the pilot light, they must have acted as fuses.
    I feel completely stupid for this, I should have looked more closely… Thanks Mark for pointing out the obvious, and all for suggestions. I am glad nothing got damaged in the process, as far as I can see or hear.
     
  11. TimmyP

    TimmyP Member

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    As to whether those resistors are needed: Maybe. They act as a faux center tap on the heater winding. Some amps are fine without them, some amps have a bothersome amount of hum without them (i.e. a SF Deluxe of ours). Instead of the resistors, Some amps have a pot to adjust the hum out, as some tubes will be fine at 50/50, and some won't, depending on the amp (i.e. JCM900-4100).
     
  12. pdf64

    pdf64 Member

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    I can't think of any amp design that allows its heater circuit to freely float?
     
  13. TimmyP

    TimmyP Member

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    The above-mentioned SF Deluxe Reverb was floating (or it had a bad CT - I forget now).
     

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