Who needs B+ anyway?

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

  1. gadget73

    gadget73 junk junkie Subscriber

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    and it may not happen in something with lower value grid resistors, fixed bias, or a more conventional cathode bias setup either. The way this particular amp is designed, if a tube starts to run away it feeds itself and goes even faster.
     
  2. oldman55

    oldman55 Well-Known Member

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    That's just great.

    Happy New Year to all
     
    Last edited: Jan 1, 2018
  3. oldman55

    oldman55 Well-Known Member

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    I eliminated tubes as an issue as I put the exact set, in the same position, in a working a500 and it ran steady at 28v bias.
    Checked almost every resistor, traced down circuits and cant find an issue.
    Symptoms:
    1) Bias goes past the 30v spec and rises until the resistor smokes at about 80v or so. Cant be good on the heaters of the preamp tubes though voltages drop correctly thru them.
    2) B+ from 117v variac are about 20v low and drop another 20v as bias voltage rises.
    3) I have one set of nos Raytheons that it will run and hold 33v of bias steady, all other weaker tubes cause the run away.

    Xformer? Incorrectly wired (kit version) and worked fine until the tubes got old?
    I now suspect that I didnt cause this issue as it came to me with a blown 400uf/50v cap (that was a replacement from a previous catastrophic can failure) and I blew that same cap initially.

    Anything sound familiar? The bias circuit is way over my head.
     
  4. oldman55

    oldman55 Well-Known Member

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    Finally.:music:
    After tracing every circuit down and finding all wiring and components correct I started checking impedance across all of the circuits. A wire that looked good and soldered, did not have continuity (appeared to have corrosion around the wire when removed).
    The 3.5v coming from the voltage divider on the bias supply went to the first output tube but from there, it did not continue on to the other 3 tubes.
    So only the first tube was properly biased and the remainder were lacking the 3.5v. I have no idea why that causes the bias to run away only that the amp is running with a steady 28.5v bias on weak tubes and playing fine. And whats the deal with my mystery set of tubes that didnt care?
    All HV on the power supply is in line except the last one (feeds the preamp) which is high at 182v vs 160v spec. Guess I should increase the resistor size to get it in line?

    Thanks again to all.
     
    Last edited: Jan 2, 2018
  5. gadget73

    gadget73 junk junkie Subscriber

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    It would make only one tube really try to carry the load. Could make it run away and overheat.

    try swapping the preamp tubes and see if it changes. Could just be the tubes are a little off.
     
  6. oldman55

    oldman55 Well-Known Member

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    Though it is playing fine, I am concerned with plate voltages at V7, first preamp tube. Pin6 (plate 1) should be 25v lower than Pin6 (plate 2) but it is 77v higher instead (177v vs 70v spec).I have tried a half dozen tubes in that position and the tubes down the line and problem remains. Power supply is high (only at this last cap) but when I change the resistor at the cap to get voltage to 160v, Plate1 goes way below the spec of 95v. Next tube in line, V1, is about 20v high on both plates. Resistors are within tolerances.
    V7 voltages.jpg
     
    Last edited: Jan 3, 2018
  7. gadget73

    gadget73 junk junkie Subscriber

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    are you measuring the voltages with the controls in the same positions that the factory did? Looks like the cathode goes somewhere, possible it changes the resistance to ground on the cathode circuit depending where the selector is. If thats the case, the plate voltage would change when the cathode voltage does. High plate voltage means its conducting very little.
     
  8. oldman55

    oldman55 Well-Known Member

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    Plate voltage stays steady at 177v on pin6 regardless of where the controls are.
     
  9. gadget73

    gadget73 junk junkie Subscriber

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    is R46 good ? How about R43 and R44 ?
     
  10. oldman55

    oldman55 Well-Known Member

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    Off a bit but within tolerances. 10k is 9.1k but others are close.
     
  11. gadget73

    gadget73 junk junkie Subscriber

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    shouldn't be enough to bother anything. Dunno, sounds like the tube is biased near to cutoff but unless it has negative voltage at the grid I don't see how that would be. Its not supposed to flow much based on what the original values are anyway, something like 0.2ma, but its flowing enough to develop voltage drop across that resistor. The source voltage being a smidge high shouldn't be enough to bother it, you've got near to no voltage drop across the plate resistor.

    maybe try clipping a wire from ground to the grid on that tube section and see if the voltage changes any.

    of course its also possible that the measurements were taken with a meter that loads down the circuit enough that your readings are never going to match. Does it tell you what they used? a VTVM is something like 10M input impedance, a digital meter should agree reasonably well with that. An analog meter could be anywhere from 1Kohm / volt to 100Kohm / volt, typical decent was about 20k / volt.
     
    Last edited: Jan 4, 2018
  12. oldman55

    oldman55 Well-Known Member

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    Grounding the grid (pin7) does nothing to the voltage reading of the plate, 177v. The grid has no dc voltage but reads 12mvac.
     
  13. gadget73

    gadget73 junk junkie Subscriber

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    does the phono stage sound right? if so I might be inclined to just leave it alone. Seems like it has no obvious component faults, so it may just be a difference in measurement equipment. The amount of current this runs at is so low that it wouldn't take a whole lot of meter loading to skew the results.

    Maybe check if @GordonW can shed some light on it. I know he's got some experience with the A500.
     
  14. oldman55

    oldman55 Well-Known Member

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    I think you may have nailed it. Phono channel B doesnt play and I never tested it on the bench. There are a couple of buried resistors in that corner that I didnt check (resistors/caps are three deep) and I believe that one of the resistors is open. Not sure as it is tough getting a probe down there.
    I will have to pull a few resistors and check for sure.
    Thanks.
     
  15. oldman55

    oldman55 Well-Known Member

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    It was an open cathode resistor, one that you had asked me to check earlier and I thought that I did.
    But, now its playing on weak tubes with no issues. And I spaced out some of the components that the kit builder stacked. I also had another ground wire just pull loose from a cold joint. Guess it is always better to get the factory unit.
    Thanks again for your time and analysis.
    Oldman

    Possible method of testing relative tube strength?
    -Balance a pair of good nos tubes
    -Put in one different tube and see what the voltage differential is
    -Repeat with other tubes and rate based on additional voltage needed to balance with the nos tube
     

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