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Who needs B+ anyway?

Discussion in 'Tube Audio' started by oldman55, Dec 19, 2017.

  1. oldman55

    oldman55 Well-Known Member

    Messages:
    782
    Finished recapping a HK a500. Left the B+ wire off of the new diodes (trans still connected). Fired it up, power tubes arced and red plated and bias cap blew.
    Found the loose wire and replaced cap but I must have damaged something else as B+ voltages are all 10-20% down. Bias supply is high at 75v (vs 30v spec). The 330 ohm resistor at the bias balance pot started smoking (6.5 watt resistor replacement for a 3 watt) and scratching my head. I didnt have B+ so I dont know how I could have damaged anything on the power supply line. Since this is kind of cathode bias, that should isolate the problem I would guess but have no clue where else to look. Tubes were all replaced with a working set after the red plate. Any suggestions?
    Thanks
    oldman who blew it again
     
    Last edited: Dec 19, 2017

     

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  2. knockbill

    knockbill Addicted Member

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    Which wire did you leave off? Got a schematic?
     
  3. oldman55

    oldman55 Well-Known Member

    Messages:
    782
  4. knockbill

    knockbill Addicted Member

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    Seems you wouldn't have B+ at all with that wire disconnected... are the diodes still good?
     
  5. oldman55

    oldman55 Well-Known Member

    Messages:
    782
    Exactly. No B+. Didnt worry about diodes as they are before the problem.
     
  6. gadget73

    gadget73 junk junkie Subscriber

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    Southern NJ
    with no B+, there should have been no voltage at that 330 ohm resistor so there should not be any reason for it to smoke. No way for anything to arc, or red plate either. You must have had B+ connected to something, if the connection was off where your schematic shows, there wouldn't have been any power to anywhere to arc with.
     

     

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  7. Tubeglowpio

    Tubeglowpio Active Member

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    Did the wire touch the chassis? Is the chassis earth grounded?
     
  8. Tubeglowpio

    Tubeglowpio Active Member

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    336
    Location:
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    You said new diode, was it put in the wrong way?
     
  9. oldman55

    oldman55 Well-Known Member

    Messages:
    782
    It was correct. Was only connected to trans.
    The smoke was after I corrected the wiring and replaced the 400/50v bias cap.

    I think the wire was just flopping in the breeze but sure was hidden to me.
     
  10. oldman55

    oldman55 Well-Known Member

    Messages:
    782
    Only the 7355s were lit, the preamp was dead. I guess it was fed thru transformer wire?
     
  11. gadget73

    gadget73 junk junkie Subscriber

    Messages:
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    just doesn't make sense. If there was no connection from the output of the voltage doubler to the first filter cap, there isn't any plate voltage. No plate voltage at the 7355 stage means no current through those tubes, so the preamp tubes won't light.

    Only reason that 330 ohm resistor would smoke is extreme current flow. If V1, 2, or 3 are bad you'd have the output tube using that as the cathode resistor but with the value as high as it is, it shouldn't flow enough current to bother anything.
     

     

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  12. oldman55

    oldman55 Well-Known Member

    Messages:
    782
    This happened with the tubes that were bought. From what a sencor Mighty Mite 4 can test, they were good.

    But yes, they were lit up and red plating without the B+. And I could hear arcing through the speakers. Christmas miracle or wiring issue?

    old and glad to make another Christmas.
     
    Last edited: Dec 19, 2017
  13. kve777

    kve777 AK Subscriber Subscriber

    Messages:
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    I think your chassis fed B+ to everything through grounds by way of the Red/White wire, this kind of feedback happen in cars I work on several times a year when a power or ground lifts open, current 'finds' a way out.
     
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  14. knockbill

    knockbill Addicted Member

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    If that disconnected wire was touching the chassis, then B+ would back flow through the chassis(ground system), completing the ckt, in reverse...
     
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  15. oldman55

    oldman55 Well-Known Member

    Messages:
    782
    The trans wire connected to the diode is the same color as the wire leaving the trans and going to pin 3 of the output tubes. Fed through there?

    But if it was back fed as such, it blew the bias cap and must have done some other damage.

    It is now put back together correctly and I am currently having the problem now of too high voltage at bias supply and something is causing the circuit to fry. Since I replaced all of the tubes and the cap since the "problem", what could be causing this issue? It seems to be dragging down the B+ on the whole power rail. That particular circuit just feeds the heaters on some preamp tubes and the bias. Dont know where it is getting its power (if the B+ was gone) or what could possibly cause this problem.

    hot circuit.jpg

    Still scratching my head.
     
  16. gadget73

    gadget73 junk junkie Subscriber

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    make sure the 47 ohm resistor is in spec, same with the 330 ohm. The bias on this is set by feeding some positive voltage back to the grids of the output tubes. If those resistors are off, it can feed too much voltage in, which makes the current higher, which feeds more in, etc. Would also be worth confirming that the tubes in the cathode circuit are good with no heater-cathode shorts especially. The tubes can have weak emission as long as they aren't shorted or just open. I had that problem on my Fisher 600, which uses a basically similar bias arrangement. One of the tubes shorted to the grounded cathode, basically half the heater went away and it caused high current through the output tubes.

    Also verify the heater wiring to those tubes, if something is shorted or connected wrong it will mess it up. I'd probably go for old production tubes in those spots just in case modern tube heaters act a little different than originals.
     
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  17. oldman55

    oldman55 Well-Known Member

    Messages:
    782
    The resistors are both still on the money. Tubes are rechecked and were pulled from a working unit, nos Teles. The power tubes are nos Sylvania and test strong also. Heater wires are all good on all tubes.
    I have a short window of time to test before overheating if there is any data that would help identify the problem.
     
  18. gadget73

    gadget73 junk junkie Subscriber

    Messages:
    36,556
    Location:
    Southern NJ
    unhook either end of the 330 ohm resistor and see if it stops. That should cause very low current flow through the output tubes since they will not have the positive voltage fed back to the grid.
     
  19. oldman55

    oldman55 Well-Known Member

    Messages:
    782
    Wow. Disconnected the 330 ohm going to ground thru the 46 ohm. Played clear music but bias voltage kept climbing without distortion. Shut it off after it went over 70v (think spec is 44v).
    Didnt think that circuit would perform without that ground reference.
    What should that be telling me?
     
  20. kward

    kward AK Subscriber Subscriber

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    With that 330 ohm resistor disconnected from one end, try removing the output tubes and powering up the unit. It won't play music of course, but you're looking to see what that bias voltage is doing. Put the positive end of the meter probe on the side of the 47 ohm resistor that connects to the 330 ohm resistor, put the negative probe to chassis. Look to see what that voltage reading is with the output tubes removed. It "should" be zero volts DC. If it's higher than that, that points to another problem somewhere. (leaky coupling caps perhaps).
     

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