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    Mk III zero bias voltage on one tube

    solderblob
    solderblob

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    Post by solderblob Wed Jun 16, 2021 12:48 pm

    Before I tear into it, I thought I'd see if anybody knows an easy way to troubleshoot my problem.  Amp is a Mk III with VTA octal board, otherwise stock, no autobias.

    The bias voltage on the right KT88 reads zero while the reading on the other KT88 is normal ~0.5 v.  The reading is zero, not just a low value.  

    Other that that, the amp sounds fine although I have my preamp's balance control at about the 12:30 position in favor of the subject amp.  No hums or buzzes.

    I haven't tried swapping tubes left for right -- I'll try that and report back later.

    Any hints or tips sincerely appreciated.

    Dave
    peterh
    peterh

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    Post by peterh Wed Jun 16, 2021 3:29 pm

    solderblob wrote:Before I tear into it, I thought I'd see if anybody knows an easy way to troubleshoot my problem.  Amp is a Mk III with VTA octal board, otherwise stock, no autobias.

    The bias voltage on the right KT88 reads zero while the reading on the other KT88 is normal ~0.5 v.  The reading is zero, not just a low value.  

    Other that that, the amp sounds fine although I have my preamp's balance control at about the 12:30 position in favor of the subject amp.  No hums or buzzes.

    I haven't tried swapping tubes left for right -- I'll try that and report back later.

    Any hints or tips sincerely appreciated.

    Dave
    If you have separate bias measuring points for each tube it's not a dynaco MkIII.
    Please supply schematics or describe the amp.

    Swapping tubes is a good suggestion to reduce the possible faults.
    solderblob
    solderblob

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    Post by solderblob Wed Jun 16, 2021 4:25 pm

    peterh wrote:If you have separate bias measuring points for each tube it's not a dynaco MkIII.
    Please supply schematics or describe the amp.

    Swapping tubes is a good suggestion to reduce the possible faults.



    Well, it's a Mk III with Roy/VTA's octal/6SN7 driver board.

    I swapped the tubes left for right and the problem went away.  I think I cleaned and retensioned the sockets a while back but not sure...Will be keeping a closer eye on the bias from now on.

    Dave
    solderblob
    solderblob

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    Post by solderblob Sat Jun 19, 2021 11:23 am

    OK, the problem recurred last night.  And now I recall hearing a clicking or popping sound before and again last night -- not very loud and I had thought it was something outside my room...

    I swapped out both tubes with a spare pair and all seems fine.

    Time for a new set of tubes.  The one that failed was an old Sovtek which I had for a long time but hadn't been operated that much except for the last 6 months.

    Roy has some Electro Harmonics KT88s.  Maybe he'll sell some to me...

    Dave
    solderblob
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    Post by solderblob Wed Jun 30, 2021 1:52 pm

    I got 4 KT88s from Roy yesterday.  Before I installed them I cleaned the sockets with some Deoxit then installed the tubes.

    When I powered them up both amps show the same problem -- no bias voltage on any KT88, one KT88 gets hot, the other in the pair is cold, each rectifier tube is cold.  The 6SN7 drivers appear to be operating normally.  

    I haven't opened up the amps yet -- I'm suspecting tube sockets pushed out of place?  It's an odd combination of things and odd that both amps are acting the same.  Any ideas?  As background, I recently (9 months ago) rebuilt the amps with Roy's VTA octal board and new quad caps.

    Dave
    peterh
    peterh

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    Post by peterh Wed Jun 30, 2021 3:25 pm

    solderblob wrote:I got 4 KT88s from Roy yesterday.  Before I installed them I cleaned the sockets with some Deoxit then installed the tubes.

    When I powered them up both amps show the same problem -- no bias voltage on any KT88, one KT88 gets hot, the other in the pair is cold, each rectifier tube is cold.  The 6SN7 drivers appear to be operating normally.  

    I haven't opened up the amps yet -- I'm suspecting tube sockets pushed out of place?  It's an odd combination of things and odd that both amps are acting the same.  Any ideas?  As background, I recently (9 months ago) rebuilt the amps with Roy's VTA octal board and new quad caps.

    Dave
    A Real MkIII has one resistor for both power tubes, and also ONE pickup point to measure the combined bias ( from both cathodes) there is no external socket to measure individual cathode voltage.

    Your amp is no MkIII, thus more information is needed to diagnose the problem. It seems to have separate sending resistors for each tube. A blown sensing resistor normally shows the bias to be several volts. A zero volt at a cathode idicates that no current flows.

    Schematics is needed, else you are on your own.
    solderblob
    solderblob

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    Post by solderblob Thu Jul 01, 2021 10:54 pm

    OK, I'll turn this into a quiz.

    One KT88 lights up, second KT88 does not light up, 5AR4 does not light up.

    Before that, I removed all above tubes and cleaned the sockets.

    I then inserted tubes.

    Clue: no wiring problems or failed components.

    2nd clue: I am a dumbass.

    dave
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    quad44

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    Post by quad44 Fri Jul 02, 2021 12:31 am

    Did you install the 5AR4 and the one KT88 in the correct positions?
    If the 5AR4 and one KT88 does not light up and the other KT88 does, my first thought would be to look there. You say you have not opened it up so I am assuming that nothing inside has been touched.
    You don't mention in your last post about the 6SN7's so I am assuming they are lighting up. Do not mean to impugn your efforts. Good luck.
    solderblob
    solderblob

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    Post by solderblob Fri Jul 02, 2021 9:15 am

    quad44 wrote:Did you install the 5AR4 and the one KT88 in the correct positions?
    If the 5AR4 and one KT88 does not light up and the other KT88 does, my first thought would be to look there. You say you have not opened it up so I am assuming that nothing inside has been touched.
    You don't mention in your last post about the 6SN7's so I am assuming they are lighting up. Do not mean to impugn your efforts. Good luck.

    You got it!  I put them in the wrong positions.  With the MkIIIs, it's easy to do if you're not paying attention...

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