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    ST-70 with glowing plates

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    j beede

    Posts : 328
    Join date : 2011-02-07
    Location : California

    ST-70 with glowing plates

    Post by j beede on Wed Jun 01, 2011 12:19 am

    I finished debugging and restoring a pair of Mk IIIs and am quite happy with them. Now I am rebuilding a ST-70 for the first time. Two of the four EL 34 outputs (one from of each pair) show dull red glowing plates. This is barely visible with normal lighting, but quite clearly visible in the dark. I lowered the bias to 1.4V and see (maybe) a small change but 2 of the four still show some red glow. The amp is stock and has the stock bias circuit in place. I am assuming that any plate glow indicates a problem. The tubes are marked Dynaco and are presumably the original matched pairs. I don't have a curve tracer to use. Any ideas?

    I also have unacceptable hum from one channel that seems to follow one of the 7199s. Is there a source of (quiet) 7199 anywhere these days? I am not considering a board upgrade for this amp at this time--even though that usually allows separate biasing of all four outputs. I plan to keep this particular amp very stock.
    ...j

    j4570

    Posts : 141
    Join date : 2010-08-30

    Re: ST-70 with glowing plates

    Post by j4570 on Wed Jun 01, 2011 10:36 am

    I sent you a PM about a 7199. I recently upgraded an ST70 and few original Dynaco parts in the parts box.

    Bob Latino
    Admin

    Posts : 2413
    Join date : 2008-11-26
    Location : Massachusetts

    Re: ST-70 with glowing plates

    Post by Bob Latino on Wed Jun 01, 2011 11:13 am

    Hi J,

    When a tube "red plates" it means that the tube is drawing too much current. Since you mention that it is one tube on each channel, I would say that either the coupling cap for that tube socket is bad OR the tube itself is bad. The tube itself is usually the issue so try replacing the two tubes that red plate with two new tubes. You could also move (one at a time) each red plating tube to a new tube socket. If it red plates there - it is the tube itself that is bad. If it doesn't red plate then there is a problem on that coupling cap or on that particular tube socket.

    Bob

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