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The Dynaco Tube Audio Forum

Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Tubes4hifi VTA tube amp and preamp kits and all Dynakitparts.com products


2 posters

    unstable bias M-125 monoblock

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    Gerry Borghesi


    Posts : 1
    Join date : 2024-08-07
    Location : Melbourne Australia

    unstable bias M-125 monoblock Empty unstable bias M-125 monoblock

    Post by Gerry Borghesi Sun Aug 25, 2024 6:52 am

    One of my M-125s (output tubes KT-120) has developed an unstable bias, with 4 tubes in place, I can increase bias to about 0.4v and then the reading becomes unstable. Tubes all tested (e-tracer) and good. With speakers connected I get a 4 beats a second sound. Quad cap is good an no other obvious faults - thoughts welcome!
    Seamus
    Seamus


    Posts : 92
    Join date : 2020-03-17

    unstable bias M-125 monoblock Empty Re: unstable bias M-125 monoblock

    Post by Seamus Sun Sep 08, 2024 5:13 pm

    What happens with one tube per side?

    Did you build the amp?

    Have you checked for a poor solder connection on the driver board?

    If you fiddle with bias a lot, the pot could be shot.
    Cermet pots have a rated life of about 200 cycles.

    If you replace the pots, use multi-turn as they are easier to adjust
    Bourns 3339P-1-503LF


    Last edited by Seamus on Sun Sep 08, 2024 5:14 pm; edited 1 time in total (Reason for editing : add line)

      Current date/time is Thu Nov 21, 2024 1:21 pm