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    Blown 10 ohm to grnd resistor on El34

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    Pooch

    Posts : 53
    Join date : 2009-12-01
    Age : 66
    Location : Wa

    Blown 10 ohm to grnd resistor on El34

    Post by Pooch on Thu Apr 21, 2011 2:58 pm

    I have VTA boarded Mk4s and for some reason one of the 10 ohm 1/2 watt resistors is blown (the one you measure the bias across). Should I check some other areas as to why this is blown? Or just replace? I'm thinking of uping the value to 1 watt 10 ohm, good or bad idea? I've already measured all the rsistors on the board and they seem good, at least in circuit.
    Gord

    GP49

    Posts : 735
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    Location : East of the sun and west of the moon

    Re: Blown 10 ohm to grnd resistor on El34

    Post by GP49 on Thu Apr 21, 2011 3:07 pm

    If you had a runaway tube in that position, for whatever reason, it could have taken the resistor out.

    Pooch

    Posts : 53
    Join date : 2009-12-01
    Age : 66
    Location : Wa

    Re: Blown 10 ohm to grnd resistor on El34

    Post by Pooch on Thu Apr 21, 2011 3:43 pm

    Thanks for the quick reply.Well one of the tubes was a little red on the plate, so that must have done it. I set the bias only about a week ago. Any reason for it to go into a runaway?

    tubes4hifi
    Admin

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    Join date : 2008-11-30

    Re: Blown 10 ohm to grnd resistor on El34

    Post by tubes4hifi on Thu Apr 21, 2011 9:36 pm

    new tubes aren't made as good as the ones 30-50 years ago
    SOME EL34s I've had, especially the Mullard branded repros, do go into runaway after 5 to 20 minutes.
    The cure is to change the grid pulldown (R29-32) resistor from 270K to 220K and then reset the bias.
    The 10 ohm cathode resistor should be 1 watt

    Pooch

    Posts : 53
    Join date : 2009-12-01
    Age : 66
    Location : Wa

    Re: Blown 10 ohm to grnd resistor on El34

    Post by Pooch on Thu Apr 21, 2011 10:35 pm

    Thanks, Roy. I'm just about to pull those out to check. Mine are 220k from the original build, but I'll check them anyways, out of circuit. I replaced the blown 10 ohm resistor and it too blew after about 30 sec. Something else going on.
    I bought some 10 ohm 1/2 watt and 1 watt today. When I get it figured, I'll replace all 4 with 1 watt
    Gord

    Pooch

    Posts : 53
    Join date : 2009-12-01
    Age : 66
    Location : Wa

    bad tube

    Post by Pooch on Sat Apr 23, 2011 3:31 pm

    After checking everything, and still blowing the resistor, I changed the tube. Problem solved. Kinda strange because this bad tube checked OK on a Precision 10-12 tester.

    Luddite

    Posts : 235
    Join date : 2009-02-04
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    Re: Blown 10 ohm to grnd resistor on El34

    Post by Luddite on Sat Apr 23, 2011 4:52 pm

    Pooch wrote:After checking everything, and still blowing the resistor, I changed the tube. Problem solved. Kinda strange because this bad tube checked OK on a Precision 10-12 tester.

    Pooch,

    You did check for "shorts" between all of the elements, right?

    Best Regards,
    Charlie

    Pooch

    Posts : 53
    Join date : 2009-12-01
    Age : 66
    Location : Wa

    Re: Blown 10 ohm to grnd resistor on El34

    Post by Pooch on Sun Apr 24, 2011 12:23 am

    Yep, I had, but your question got me to thinking. So I tried it again and pin 5 and 8 have a very faint glow, indicating a short. I could barely make it out, so I tried testing with another tester and this time it showed clearly. Old ears, old eyes Suspect
    Regards,
    Gord

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