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    Mk-3 high voltage to pins 3 and 4 on both output tubes both amps...

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    GlacierJohn

    Posts : 38
    Join date : 2014-04-17

    Mk-3 high voltage to pins 3 and 4 on both output tubes both amps...

    Post by GlacierJohn on Mon Apr 21, 2014 10:51 am

    I posted this question on my other bias thread, but since I have fixed the runaway bias problem, I thought it would be simpler to separate this issue from the bias thread. Both amps are sounding great, lots of good clean power! That's good, but in the course of troubleshooting the runaway bias problem I took a lot of measurements and discovered those voltages were quite a bit higher than the manual calls for. The manual calls for 475 volts at these points with the 6550s removed, mine measures 566 volts + or - one volt, both tubes both amps. Having four tube sockets within one volt seems more than a coincidence, almost by design.

    My amp has the SDS cap mod, the old quad cap removed from the circuit, 100uF 160 volt caps in place of the two stock 50 uF caps on the bias circuit. Most of the resistors underneath measure close to original except the two 1,000 ohm resistors between pins 5 and 6 of the output tubes measure 750 ohms. Mine came stock with a small diode where I have seen some MK 3s have a small selenium rectifier. I kept the original Dynaco driver boards and didn't take any measurements there.

    Three questions;
    1) Are these voltages dangerously high for the amps well being?
    2) Will it effect the sound quality?
    3) What could be the cause or causes?

    Sorry, one more question, I'm thinking about replacing my driver boards with the tubes4hifi boards, how do they sound compared to the old Dynaco boards?

    This is a great site, thank you everybody.

    DynakitParts
    Admin

    Posts : 181
    Join date : 2008-11-30

    High Voltage measurements

    Post by DynakitParts on Mon Apr 21, 2014 11:11 am

    Hi John..
    The 475 voltage spec in the MK III manual at pins 3 & 4 are based on the tubes installed. Without the
    output tubes installed these voltages will be much higher...like you measured.

    Kevin @ Dynakit

    GlacierJohn

    Posts : 38
    Join date : 2014-04-17

    Re: Mk-3 high voltage to pins 3 and 4 on both output tubes both amps...

    Post by GlacierJohn on Mon Apr 21, 2014 11:21 am

    DynakitParts wrote:Hi John..
               The 475 voltage spec in the MK III manual at pins 3 & 4 are based on the tubes installed. Without the
    output tubes installed these voltages will be much higher...like you measured.

    Kevin @ Dynakit

    Thanks Kevin, that's good news indeed. Just for fun I'll have to measure them with the tubes installed.

    John

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