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    high voltage on speaker terminals of Mk4?...

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    Pooch

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

    high voltage on speaker terminals of Mk4?...

    Post by Pooch on Wed Dec 02, 2009 1:37 am

    Hi i'm a newcomer to this forum and already i have a problem:) Below is a copy of the email I sent to Roy yesterday. After a very quick reply, I tried what he suggested, but no cigar. Anybody got any ideas?
    I noticed from reading a few posts that the quad cap on a dynaco 70 is wired slightly different (to different values,symbols) than a Mk4 and I'm wondering if that might make a difference. Thanks, Gord

    Hi Roy,

    Last spring I bought a pair of Mk4 boards and parts plus a pair of can caps, the 80-40-30-20 from ce Manufacturing. Well I've finally got around to building them and here's my problem. I get a loud and I mean LOUD squawk out of my test speaker and an audible vibration in the output trans. I measure voltage on the speaker outputs of up to 50 VAC on the 16 ohm. Progressively less VAC on the 8 and 4 ohm. When I lift the feedback wire the squawk and the volts go away and the amp won't work when I put it in my system. I've checked voltages on the board and at the tubes, although not perfect, pretty close. I've also checked all resistors, wiring and soldering ( I rewired with silver plated multistrand 22 gauge). Also checked board caps and diode plus the multi can cap. Just now I swapped the output transformer for the other one. Same results, audible vibration in output transformer, squawk from speaker, high speaker terminal voltage. Do you have any suggestions or have you heard of this problem before? I'm a relative newcomer at this electronics thing, but i can follow directions.

    Thanks,

    Gord Wilson

    Bob Latino
    Admin

    Posts : 2378
    Join date : 2008-11-26
    Location : Massachusetts

    Re: high voltage on speaker terminals of Mk4?...

    Post by Bob Latino on Wed Dec 02, 2009 7:15 am

    Pooch wrote:Hi i'm a newcomer to this forum and already i have a problem:) Below is a copy of the email I sent to Roy yesterday. After a very quick reply, I tried what he suggested, but no cigar. Anybody got any ideas?
    I noticed from reading a few posts that the quad cap on a dynaco 70 is wired slightly different (to different values,symbols) than a Mk4 and I'm wondering if that might make a difference. Thanks, Gord

    Hi Roy,

    Last spring I bought a pair of Mk4 boards and parts plus a pair of can caps, the 80-40-30-20 from ce Manufacturing. Well I've finally got around to building them and here's my problem. I get a loud and I mean LOUD squawk out of my test speaker and an audible vibration in the output trans. I measure voltage on the speaker outputs of up to 50 VAC on the 16 ohm. Progressively less VAC on the 8 and 4 ohm. When I lift the feedback wire the squawk and the volts go away and the amp won't work when I put it in my system. I've checked voltages on the board and at the tubes, although not perfect, pretty close. I've also checked all resistors, wiring and soldering ( I rewired with silver plated multistrand 22 gauge). Also checked board caps and diode plus the multi can cap. Just now I swapped the output transformer for the other one. Same results, audible vibration in output transformer, squawk from speaker, high speaker terminal voltage. Do you have any suggestions or have you heard of this problem before? I'm a relative newcomer at this electronics thing, but i can follow directions.

    Thanks,

    Gord Wilson

    Hi Gord,

    I am assuming since you Emailed Roy that you are using his VTA driver boards on your two amps. When you use the VTA boards on a Mark IV you have to reverse the two leads on the two coupling capacitors OR reverse the leads that go from the coupling caps to each output tube. You can do either but not BOTH. If you do both then the phase of the output transformer will be wrong. I don't know if this is your problem but just check that this is not your problem.

    One thing you could do is put up a photo of the inside wiring of one of the amps here and have some of us look at the wiring. Maybe we can pick up on some wiring issue that may not be immediately apparent to you. The forum is supposed to be able to host a photo but I can't seem to make it work right. You can use Photobucket or some other photo hosting site to host the photo of your wiring and then just put the link to the photo in your next posting.

    Bob

    Pooch

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

    Re: high voltage on speaker terminals of Mk4?...

    Post by Pooch on Thu Dec 03, 2009 5:28 pm

    Thanks, Bob and Roy.
    I guess I was wrong in saying I can follow directions Rolling Eyes I reversed the wires and all is well. Actually better than all well, fantastic! A VERY nice sounding amp. You guys have a winner with this mod.
    I have a couple of other questions, but I'll repost with the question in the title.
    Thanks,
    Gord

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