Why would a tube socket arc?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by pfrischmann, Aug 26, 2006.

  1. pfrischmann

    pfrischmann Member

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    I've been working on my homebrew '68 era non master Marshall for a while. I've had two instances where two different tube sockets have arced between pins 2 and 3 on the el-34's. The first time I noticed a little scoring on the v5 socket when I got the amp but didn't figure out what it was until it was too late (an expensive lesson as it took out the OT)

    The second time it was (v4) a brand new NOS amphenol socket. It arced so badly, the socket actually felt lighter than the others from all the carbon displacement....Luckily the ot seems fine.

    I've just replaced the socket and have talked to a few people but I'm looking for some thoughts.

    About the amp....

    This was a mkII suberbass. It now has a ptp metro board, new power tube sockets. New caps a MM OT and a MM 10H choke.

    ***The original PT is in there. It has some outside rust and is pretty covered in varnish on the outside of the laminates ****

    The last time this happened.
    The amp was biased at 35ma (490 volts on the plates)
    I had my OCD pushing the front end of the amp. the amp was dimed and going into a Hotplate....then fzzzzzzt! both times this has happened the amp has been on for a while. I can play regularly for a while, 2+ hours with no problem.

    I've talked to a few techs...here's what I've heard

    I bet it's parascitic oscillation
    I'd bet you have too much current going through the heaters
    shrink wrap the heaters
    replace the PT, that will solve it
    It's more likely an impedance issue. If the tubes are arching it may be a mis match when the amp is under an extreme load.......check to make sure your impedance selector is wired correctly. (it's very easy to do this in correctly)

    any thoughts???
     
  2. jbltwin1

    jbltwin1 Member

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    You don't specify the one thing that I feel is critical-what kind of tubes are you using. Almost 500v is a lot to suppress with 35ma of idle current. I usderstand that they "supposedly" will take it but dimed into a hotplate is pushing it, I'd try cooling the bias down to around 30mA and installing ceramic tube sockets and try it.
     
  3. jbltwin1

    jbltwin1 Member

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    Also, you need to make sure that the HOTTEST tube is only pulling 30ma-not just an average for the two.
     
  4. pfrischmann

    pfrischmann Member

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    These were JJ's....You make a good point. I biased them a while ago. I'm not sure if I counted for the top and bottom range in the bias.

    I talked to ED at THD. He mentioned cooling the bias down as well.
    More like 25ma. All things being equal (I know there not). That would put the tubes at about 12 watts stattic dissipation.

    Seems cold to me....but what do I know
     
  5. VaughnC

    VaughnC Supporting Member

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    It almost sounds like to me that some time in its life the amp was played without or too high of a speaker load causing flyback. For starters, I'd replace the tube sockets with high quality ceramic one's.
     
  6. hasserl

    hasserl Member

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    The most common reason for arcing at the tube socket is from high flyback voltage due to incorrect impedance loading.

    The tech who told you that is correct, the others are full of crap. With all due respect to Ed, he is not a tech, a great guitar player, but not a tech.

    The OT failure you mentioned was not caused by the arcing tube socket, it was killed by the same high flyback voltage. YOu've got a serious impedance mismatch there and until you correct it you are going to have these problems.
     
  7. hasserl

    hasserl Member

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    One other thing, once the arcing happens at the socket a trace of carbon is deposited, which is conductive and will continue to give you problems even after you get the impedace mismatch corrected. Ceramic sockets may be better for this, but can still give you problems, leading to another socket replacement as the final cure. But you've got to get that load correct first.
     
  8. VacuumVoodoo

    VacuumVoodoo Member

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    One more thing: recommendation to bias the tubes colder in this case is wrong. With colder bias the tubes draw less current and present a lighter load to the power supply so the B+ voltage will go up. With higher B+ the flyback voltage will also be higher and therefore more prone to arcing. Arcing threshold is a function of voltage between the points and physical distance between them. As said by others, get your load impedances in order, change sockets and bias to normal 70% plate dissipation or a bit higher.
     
  9. pfrischmann

    pfrischmann Member

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    Fly Back voltage makes sense. Is it possible it could be something other than the OT/impedance selector?

    Could the Hot plate affect this at extreme volumes?
     
  10. hasserl

    hasserl Member

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    Absolutely, that is exactly where the problem lies. There is a mismatch between the tubes/OT/attenuator/speakers. When you mentioned Ed at THD I suspected you were running a Hot Plate. Is the Hot Plate impedance matched to the amp/speakers? i.e. a 4 ohm amp into a 4 ohm Hot Plate into a 4 ohm speaker cab? Or is one of the items not matched?
     
  11. pfrischmann

    pfrischmann Member

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    Yup,
    16 ohms on the amp, 16 ohm HP into a 16 ohm cab...assuming I wired the impedance selector correctly..

    It is possible I didn't bias to the hottest tube so one of the tubes could have been 35+ up to 38 MA...I just don't remember
     
  12. pfrischmann

    pfrischmann Member

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    BTW it's a 100 watt amp
     
  13. John Phillips

    John Phillips Member

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    I would definitely NOT do that with this amp. 70% is way too hot, and although it might not be the cause of the arcing in this case it's still bad for the tubes. These amps are extremely hard on the tubes (especially the screens) anyway, and biasing them as hot as that just makes it worse, especially into an attenuator - have a look at what happens to them under load when they're biased like that. Biasing cooler does make the plate voltage go up, but so slightly it isn't important - a few volts at most, and it's not the static voltage which is the problem for arcing, it's the induced dynamic voltage... which might actually be higher if the tubes are biased hot since the overall current draw through the OT is slightly more.

    I agree with hasserl, but I would also suspect the impedance selector if its the original. Check your speaker cable (the one from the amp to the attenuator) too.

    Replacing the sockets with ceramics might stop the sockets arcing, but unless you find the cause of the voltage spikes, it may just do it somewhere else instead... like in the OT.
     
  14. hunter

    hunter Supporting Member

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    Certain 8 pin JJs have been manufactured with undersized pins. I don't recall specifics but Jim McShane over on the Weber tube page can provide some insight. May even still be a thread on the subject. Sounded like a bunch were - you might have to measure yours to know for sure. If so, then even a new socket is subject to arcing. If the pins on your tubes are undersized, the solution might be to either not use the JJs or tension the new sockets to grip the smaller tube pins (which could lead to problems later on when you use a standard spec'd pin diameter tube).

    hunter
     
  15. pfrischmann

    pfrischmann Member

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    :messedup
    What????

    I'm not saying this is the problem but this seems very irresponsible...

    Thanks for the insight.
     
  16. pfrischmann

    pfrischmann Member

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    O.K.....I think I found my problem, if so. I feel really stupid.

    I switched out the impedance selector for one of the new ones. I may have wired it wrong.

    Does anyone know if the Mercury Magnetics color codes follow the same color scheme as Marshall. If so, I goofed.

    I believe it should be
    Green 16
    yellow 8
    Black 4

    If this is the case, I switched the black and green wires. I did a continuity check on the selector..I had the green wire going to the 4 ohm and black to the 16....

    Can anyone confirm the MM secondary color codes?
     
  17. John Phillips

    John Phillips Member

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    Marshall color codes vary a bit, the one I was working on yesterday is yellow 4, green 8, grey 16, ground black.

    It's fairly easy to check it - just use a multimeter on the output taps (to the ground connection). The DC resistance of the windings is very small, probably less than the zero error of the meter, but you should get three distinct readings (eg .5 ohm, .6 ohm, .8 ohm). The lowest is the 4-ohm tap and the highest is the 16.
     
  18. pfrischmann

    pfrischmann Member

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    Cool stuff,
    thanks Jason. I believe I typically overlook stuff like this.
     
  19. hunter

    hunter Supporting Member

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  20. pfrischmann

    pfrischmann Member

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    I'm afraid my DMM isn't good enough to be sure of the difference. Fluke for my birthday I guess.
     

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