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Will a bad output tube cause runaway bias? Dynaco Mk III

Discussion in 'Tube Audio' started by Lucky Pierre, Jan 1, 2018.

  1. Lucky Pierre

    Lucky Pierre AK Subscriber Subscriber

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    I was listening to my Mk IIIs the other day when I heard a "crunching" noise. I immediately turned off the amps. Yesterday, I powered them up without the cages and watched. After a minute or two, I heard the same noise and saw flashes in one of the output tubes (Electro Harmonix KT88).

    I swapped the tubes to the other sockets, powered up the amp, and watched while measuring the bias. There was no crunch or flashes, but once the bias hit 1.56 v, it hovered there than promptly soared over 2 v, at which point I powered down the amps.

    I checked the bias with another recitfier. It still rose, but not as fast. I swapped in a pair of 6550s, and now it is holding 1.56 v.

    Empirically it seems I had a KT88 go bad. What else should I check?

    Thanks!
     

     

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  2. Lucky Pierre

    Lucky Pierre AK Subscriber Subscriber

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    BTW, the amp was check by a tech and recapped last year.
     
  3. jaymanaa

    jaymanaa RIP 1961-2018

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    Yep, that would be my guess.
     
  4. primosounds

    primosounds Powered with pure tube sounds. Subscriber

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    Yep, as the tube gets weaker or less output, it starts to draw more current to try and keep up. Good thing you were paying attention. Time for some new tubes. You might consider kt90 or kt120 which are higher output tubes and may be better in a MK3. Also if all your tubes were put in service at the same time it would be best to get a new quad. Anyway it would be less stressful than waiting for the old pair to go nuclear on you.
     
  5. jaymanaa

    jaymanaa RIP 1961-2018

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    Yeh, especially if he could find some of those old EI KT90s, but two matched pairs of those would be tough to find probably. I haven't heard anything bad about the new 90s and 120s though. I wonder if they are a "kinkless" as the original KTs.
     
  6. primosounds

    primosounds Powered with pure tube sounds. Subscriber

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    Right, people say the "new" kt90s are not the equal of the old ones. I don't have any first hand experience with those, so i won't comment on that but it seems that their price is stable which indicates that they are not being dumped on the market. The ones in my amps are mostly used and marked "VTL" , and are still working strong.
    KT120 are pretty good sounding. I have a pair in a SEP amp that i use all the time. In comparison vintage KT66 sounds a little more precise and polite, but the power behind the 120 yields an overall more satisfying sound.
     

     

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  7. Lucky Pierre

    Lucky Pierre AK Subscriber Subscriber

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    Thanks everyone. It looks like I am buying a new quad. Right now I am running KT88 in one amp, 6550 in the other.

    I don't think I will go with the KT120. So, it will either be a new set of 88s or 90s. OR I could go 6550 on both amps. I understand the 6550s will knock down the power some.

    If I am going to stay in the $50-75/tube range, will I notice any real difference between the 88s or 90s? Gold Lions or Mullard reissues? Or do I try to find NOS 6550s? There is a lot of info and opinions out there.

    Please bear in mind these amps are unmodified. My preamp is a factory wired PAT-4; speakers are Dynaco A-25. If anyone else is in the house, I can't turn up volume past the 9:00 position (7:00 = no volume). If I am alone, I can go to ~10:00 before it is too loud. I am certainly not lacking for power now. I should also state that my living room is small, roughly 11 x 12'.
     
  8. sKiZo

    sKiZo Hates received: 8642 Subscriber

    Any reason to avoid the KT120? Excellent sound AND value. Been going strong for several years now in my Custom ST120 ... great price at less than $50 a pop. Your iron should be more than strong enough to drive them.

    [​IMG]
     
  9. Lucky Pierre

    Lucky Pierre AK Subscriber Subscriber

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    This is my concern.
     
  10. peterh

    peterh AK Subscriber Subscriber

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    Grid current avalanche happens. It's the result of several things, could be debris that have accumulated
    on the g1, resulting in emitting electrons and raising the grid potential, which will accelerate the avalanche.
    Tubes are dead, replace with new ones.

    What may have caused it ? Could have been overheating, could be gas inside that will turn to ions
    that crash into the cathode and trow cathode material onto the surroundings.

    MkIII have 100k grid resistors, this is a bit to much for 6550 tubes, but ok with kt88.
    A more conservative 50k grid resistors _might_ have helped here, or at least prolonged tube life somewhat.

    Note that tubes are consumables, don't expect them to run forever. But treat them well and they
    will run long enoght ( 3-5000h for ordinary powertubes), treat them bad and they will die sooner.
     
    Pio1980 likes this.
  11. gadget73

    gadget73 junk junkie Subscriber

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    I believe the KT120 wants more heater current, on an original Mark III it may not have the extra capacity in the transformer to feed them. If these are a modern re-issue Mark III there should be no issue. Its not going to gain you any power, but they are a good sounding tube. I have a set of EH KT88's and Tung-Sol KT-120's for my modified Bogen amps. I prefer the KT120 sound. The circuit used on those Bogens is similar to a stock Mark III circuit with the 6AN8 tube. I've got loads of extra heater capacity on those amps though.

    Power output from KT88 vs 6550 should not change by any real amount.

    but yeah, peterh has it nailed. Grid leakage / grid emission which allows it to run away with itself. Possible those tubes might work in an amp that runs lower voltages or lower grid resistors, or not. I wouldn't trust it though.
     

     

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  12. sKiZo

    sKiZo Hates received: 8642 Subscriber

    From Bob and Co ...

    This tube should work fine in any Dynaco Mark III or VTA ST-120. It could also be used with any Dynaco ST-70 or Mark IV as long as you have an upgraded power transformer.

    Tungsol KT120 output tube for Dynaco amps

    Don't quote me, but I've heard you should see around a 10% gain - in the VTA ST120 anyway. Not sure if this holds up across the line. More efficient design? Don't know, but I do know they bias up at lower pot settings than any KT88 I've tried.

    Each KT120 will draw 1.7 - 1.95 amps per tube heater current. A KT88 draws about 1.5 or 1.6 amps per tube.


     
    Last edited: Jan 1, 2018
  13. Lucky Pierre

    Lucky Pierre AK Subscriber Subscriber

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    Perhaps I wasn't clear. These are original, unmodified Mk IIIs, recapped with new RCA jacks. Other than that, they are as they were put together 50 or so years ago, sans the selenium rectifiers.

    I am not planning on doing it, but I am curious about what is required to upgrade the power transformers.
     
  14. sKiZo

    sKiZo Hates received: 8642 Subscriber

    Not sure it'd be worth the effort. As long as you haven't cooked the original iron, that was plenty strong to take most anything you can throw at it.

    * Only real complaint I've heard about the Mk III is that the capacitors in the power supply are not rated at a high enough voltage for the power transformer. This results in the quad cap being run over it's rated voltage every time the amp is turned on. If anything, I'd look into either a new capacitor bank (new ones are smaller and you should be able to fit more capacity in the same space, and old caps need replacing anyway as a rule) and/or a new quad cap. Maybe the gurus can chime in here.
     
    Last edited: Jan 1, 2018
  15. nj pheonix

    nj pheonix AK Subscriber Subscriber

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    The question is what do you gain?
    Updated PTs is a straight swap.
    You're not going to see an increase in power. I have no beef with the KT 120s I used them for a VTA ST 120 for years.
    I prefer the gold lion reissue KT88s ( I'm not saying they're better, just personal preference).
    Seems like a lot of hassle just to run a different output tube.
    Based on what was said before and the fact that the MKIII was designed to run a preamp you more than likely have the current to run the KT 120s with the stock PTs
     
  16. gadget73

    gadget73 junk junkie Subscriber

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    yea I wouldn't bother with changing the transformers either. There just isn't any real gain to be had there.
     

     

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  17. crispycircuit

    crispycircuit AK Subscriber Subscriber

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    Just make sure your running the correct size fuses.... A bias/balance circuit would be good mod for these amps.
     
  18. fredtroy

    fredtroy Member

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    "Only real complaint I've heard about the Mk III is that the capacitors in the power supply are not rated at a high enough voltage..."

    Rectifier tube supplies voltage before outputs conduct. There is a simple thermistor fix for this.

    These are good amps, recommend the Poseidon driver board update.
     
  19. petercapo

    petercapo AK Subscriber Subscriber

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    Available power output from 6550 should be same as KT88. For channel-to-channel sonic consistency, I suggest running the same type and brand of tube in both amps.

    Changing to a new power transformer probably won't make a difference, except that the new power transformer's primary might be wound for either 120 VAC or 122 VAC, which would drop the voltages inside the Mark III compared to a 117 VAC PT.

    KT120 may work okay, but consider space requirements if the KT120 is wider than KT88/6550. They should probably not be too close to each other. I think some tube manufacturers of yesteryear even specified minimum distance between power tubes.

    The capacitors in the Mark III are fine for the power transformer, except that maybe over a long period of time turn-on surges might take a toll, but this should not happen often. The more significant problem is the level of the AC mains. It needs to be within the range originally specified by Dynaco. With the original 117 VAC PT, the Mark III was good up to an AC mains of 124 VAC, per the Dynaco manual. Later Mark III had a 120 VAC PT, which, extrapolating, should mean it would be good to 127 VAC mains.
     
    Last edited: Jan 1, 2018
  20. LexDM3

    LexDM3 AK Subscriber Subscriber

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    I have fixed a few nicely restored Fisher amps for other people recently that had output stage problems due to boutique coupling caps shorting or at least conducting a lot of high voltage DC. Three occurrences due to failed caps that were not more than 5 years old, one Mundorf and two Jensens. You might want to check the coupling caps before going too much further. PIOs are my friends now...

    Your issue may very well be a tube issue or something else but checking all of the the coupling caps is now part of my standard diagnosis.
     

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