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

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

  1. nj pheonix

    nj pheonix AK Subscriber Subscriber

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    If you're restoring one from scratch, you can get the sds boards from triode.
    It takes care of high voltage on the caps.
    Also it gives you the option of going SS on the board for B+ (which i did but if I had to do it again I think I'd use plug in rectifier).
    This frees the rectifier filament winding to be used as a bucking winding. Kills 2 birds with one stone pretty much for free. The PS boards cost little more than new can cap and resistors.
    In the vein of the last comment they sell a pretty slick bias/balance control.
    I thought maybe it was a bit over the top (especially if you went through the trouble of buying matched outputs) but I like many others got carried away during the build.
     
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  2. crispycircuit

    crispycircuit AK Subscriber Subscriber

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    Tubes don't stay matched no matter how much you pay. So to get long life from tubes, you need Bias/balance controls. If you run your amps for long periods this is a must....
     
  3. peterh

    peterh AK Subscriber Subscriber

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    Ok ...
    I have 4 JJ 6550 that has 3560h on them. Currently playing. And still holds bias.

    They were installed new and matched. Still matched.
     
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  4. petercapo

    petercapo AK Subscriber Subscriber

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    A failure is a failure. But, tubes that are properly burned in and matched when new should remain reasonably well matched over their usable life, or so that has been my impression. The OP has a very old pair of amplifiers that could have any number of issues due to age and much use. Plus we have no idea of how his tubes were originally screened before being put into use.

    To avoid a lot of guesswork, the best way I can think of to try to ascertain the basic health of a Mark III is to use the chart of reference voltages in the original manual and take some readings to compare.
     
  5. peterh

    peterh AK Subscriber Subscriber

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    All about is True.
    I would like to add that new tubes ( at least kt88/6550) is needed to take out any more guessing.
    Tubes are consumables, and if they redplate or loose control of current they are subject to trash.
     
  6. Lucky Pierre

    Lucky Pierre AK Subscriber Subscriber

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    This is all true. Old amps, different tubes in each. Each amp was gone through by a reputable tech before they were put into use, though.

    I am planning on buying a matched quads. I think I will stick with KT88s--if for no other reason than that is what was originally spec'd.

    What is the deal with burning in tubes? I have read a lot of different opinions regarding this. If I buy from a reputable vendor will they have been burned in before shipping? One thing I will not do is leave the amps on unattended.
     
  7. petercapo

    petercapo AK Subscriber Subscriber

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    I would still want to take readings and compare to the reference values in the chart. And, hopefully this event did not cause any damage to the amp(s) that could cause further failure even with a good, new set of tubes. Tube amps, especially fifty year old ones, need hand-holding. For instance, swapping tubes around can loosen the socket contacts, which can cause problems. The socket contacts need to be clean and fit very snugly onto the tubes' pins.

    I'd suggest contacting Jim McShane for a burned-in and matched quad of either 6550 or KT88: http://www.mcshanedesign.net/tubes.htm
     
    Last edited: Jan 2, 2018
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  8. nj pheonix

    nj pheonix AK Subscriber Subscriber

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    That was my understanding:dunno:
     
  9. petercapo

    petercapo AK Subscriber Subscriber

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    My concern for the OP is that there might possibly be some other cause for the red-plating, and he could end up ruining a new set of tubes. Maybe try to get the cheapest (though NOT Chinese-made) matched quad of 6550 or KT88 he can find to see if the amp is working okay. If it can be reliably determined that the amps are working right, then he can get a better set. Just one approach.
     
    Last edited: Jan 2, 2018
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  10. crispycircuit

    crispycircuit AK Subscriber Subscriber

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    Part of the problem is testing & grading isn't done at 500vdc. MK3s run close or at 500vdc on tube plates. I don't know of anyone who tests & grades at that voltage. High B+ is stressful on tubes. Ideal set up would test /grade tubes at your B+.
     
  11. sKiZo

    sKiZo Hates received: 8641 Subscriber

    ?

    The Mark III uses two tubes per amp - just switch the sets and see if the problem follows?

    Just keep a close eye on things to make sure you don't screw the pooch on both amps ... <G>

    (And yes - DO buy a cheap quad or pair as well - Spares are good.)

    ** I'd still check the operating spec voltages prior to getting too crazy there. Fire up the amp with just the rectifier, then add the drivers, and finally, the power tubes per the recommended "new build" start up sequence. I'd also check wall current to make sure the rectifier isn't getting pushed too hard in the first place.
     
  12. petercapo

    petercapo AK Subscriber Subscriber

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    I believe the OP did that and the problem did not recur. However, red-plate problems can be phantoms - now you see it, now you don't. Not necessarily readily repeatable, even with the same tube.

    The octal socket contacts must be cleaned and retensioned, with extra attention to contacts #5 for the 6550/KT88.
     
  13. whoaru99

    whoaru99 Epic Member

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    FWIW, I experienced a bad tube causing runaway bias. It had been working at lower voltage ~335V net plate, but then I juiced up the amp to ~530V B+. Everything seemed fine but then after several minutes of operation the bias on the tube would begin to creep up (become less negative). Slowly at first then faster and faster as the minutes went by, ultimately resulting in red plate. The symptom followed the tube, replaced it, no further problem.
     
  14. Lucky Pierre

    Lucky Pierre AK Subscriber Subscriber

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    Thanks everyone for the advice and great information.

    I'll put the amps on the bench this weekend (or Friday if we get walloped with snow) and check to make sure everything is in spec. Then, new tubes.

    FWIW, I ran the suspect amp for 4-5 hours yesterday with a pair of 6550s. Bias held at 1.56 v and it sounded great.
     
  15. Chris McColm

    Chris McColm New Member

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    Silicon Diodes will have less voltage drop and can provede more voltage . Some of ther newer manufactuer tubes have a higher rating .. Ck with the manufacturer .. Your high quality tubes are much more tolerant especially at high levels . I have a Dynaco st-70 , With Russian outputs , been working well for 15 years , no burnouts . I have destroyed poorly made tubes in hours .... I have cked NEW tubes that are defective ... grid emission , shorts , . A tube tester is essential if you are listening to tubes . Todays quality control is not ike it was 60-70 years ago . I always ck new tubes before installation . I ordered one time 10 12 AX7s for a tape recorder . 6 out of 10 were bad .And...bad coupling caps will destroy tubes real fast with that DC coming though . Hope this helps .
     
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  16. nj pheonix

    nj pheonix AK Subscriber Subscriber

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    I'm sure some sellers vet their tubes better than others.
    I recently bought new tubes and was surprised how good they were.
     
  17. whoaru99

    whoaru99 Epic Member

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    Pesonally, I think I would go with matched pairs of the EH KT88s. I suppose one could also do a matched quad but not sure why since it's two tubes per amp (unless lower cost than two matched pair).

    McShane describes them sonically as 90% of the best for 1/2 the price, tough and reliable too. $135/matched quad. These are what I have for my Kegger KT88 PSE amps (if I ever get them finished).

    Considering the situation it seems prudent to me to consider these first. Run the legs off them, then if everything seems kosher spend more later if you wish too.

    As far as caps, some mods required but Nichicon has 600V snap-ins. Unless the voltage is way up there they can do away with needing series caps and balance resistors in many cases.
     
    Last edited: Jan 4, 2018
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  18. peterh

    peterh AK Subscriber Subscriber

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    A matched quad has the benefit that up to 2 tubes may go west one still has a matched pair.
    JJ, EH or whatever. KT88 is somewhat more resilient, but 6550 works well too.
     

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