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SVT PT go boom. How can one protect PT secondaries?

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  • #1

    SVT PT go boom. How can one protect PT secondaries?

    A customer brought in a '76 SVT with a blown PT. This thing was still smoking hot and stinky when it got to me two hours after it's death.
    I tracked down the problem to a KT-88 that had a dead short between it's screen and cathode. I suppose the PT cooked and cooked until
    it shorted internally and took out the fuse? The fuse was a 10A per the schematic.

    I do understand that this fuse is not there to stop what happened, but is there for fire prevention etc, but how would one go about fusing the HT secondaries to protect against this sort of thing? How would you determine the fuse ampacity?

    Another thing I wonder- did the 22 ohm screen resistor and the diode help or exacerbate this failure?


    SVT SCHEMATIC.pdf
    Tags: None
  • #2
    It seems crazy that the 22 ohm screen grid resistor didn't blow!
    Best practice is to fuse every winding.
    To determine capacity, try measuring the currents levels under various conditions and start up modes, make an educated guess, then 'suck it and see'.
    As you've found, it can be a problem when an amp has multiple power tubes, as the current that one shorting tube can draw may be equivalent to the normal full load current; hence sacrificial resistors for each cathode, screen grid might be worth considering.
    Last edited by pdf64; 10-23-2015, 02:39 PM.
    My band:- http://www.youtube.com/user/RedwingBand

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    • #3
      Was that 10A fuse fast or slow blow?
      Are you 100% certain there is no possibility the user tried a bigger fuse (or tinfoil), then put the blown 10A back in to "save face" ?
      Big Ampeg's use fast blow fuses, and also have an internal back up fuse. In this case the back-up is 15A fast. Is it still intact or has it been bypassed?
      Originally posted by Enzo
      I have a sign in my shop that says, "Never think up reasons not to check something."


      ​

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      • #4
        The 15 internal was gone. It had a 10A ceramic in the holder. Not sure if it was fast or slow- I was overwhelmed by the smell of charred insulation....

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        • #5
          Who knows what happened to this amplifier since 1976.

          Hey, when a transformer starts to go, there is really no stopping it if turns are shorted.

          Comment

          • #6
            A replacement is what 10$/lb ($225)?

            Not many things made 39 years ago work at all...
            Last edited by tedmich; 10-24-2015, 04:51 PM.

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            • #7
              Am I correct in thinking that the screen diode just caused this problem to become worse? The screen was a dead short to the cathode. That diode was just conducting current straight through.

              I see that the newer SVTs have no diodes and the screen resistors are 220.

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              • #8
                Yes, there was a service bulletin that suggested removing diodes and replacing 22 Ohms resistors with 220 Ohms.

                Mark

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                • #9
                  But it is unlikely the diodes made the problem worse. They burn up just like a resistor does.
                  Education is what you're left with after you have forgotten what you have learned.

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                  • #10
                    supposedly this was in Kevin O'Connor's TUT-3 section 2-25:

                    "Ampeg also used small screen resistors, or often tied screens to a common resistor as in Traynor amps. Individual 1K screen resistors are an advantage in all of these amps, as Ampeg is another high-voltage afficionado. The SVT and V9 use 22ohm screen resistors to keep the screen supply as constant as possible, in order to get every last watt out of the sextet of 6550's. To protect the printed circuit board, these resistors are shunted by 1A diodes, so the possible fusing potential of the tiny 1/2W resistors is negated and tube failures are inevitable and frequent. Removal of the diodes and replacement of the resistors with 1K 5W units protects the tubes and imparts better sonic aspects to the amp. Most players state that the amp never sounded so good"

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                    • #11
                      Oh no, did he actually admit to "possible fusing potential" of lower wattage screen resistors there?
                      Must have mellowed out a little from earlier days.
                      Originally posted by Enzo
                      I have a sign in my shop that says, "Never think up reasons not to check something."


                      ​

                      Comment

                      • #12
                        Yikes, first time around somehow I didn't notice that the 22 ohm resistors were being shunted by the diodes; I think I just assumed they were going to 0V.
                        Hence the resistor on the bad tube wouldn't blow.
                        Depending on the supply impedance / current sourcing capability / screen diode failure mode, the diodes may make the amp less resilient to such tube failure.
                        Last edited by pdf64; 10-29-2015, 08:27 AM.
                        My band:- http://www.youtube.com/user/RedwingBand

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                        • #13
                          I did find that bulletin and I made those changes.

                          Fresh set of 65s in there and it's kicking again. I just can't believe an old tube shorted screen to cathode and killed the PT. Crazy.

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