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    Filament voltage mismatch

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    j beede

    Posts : 316
    Join date : 2011-02-07
    Location : California

    Filament voltage mismatch

    Post by j beede on Fri Feb 18, 2011 1:48 pm

    I am slowly bringing my Mark IIIs into a "real" audio system as I work through their issues and updates. I see a visible difference in filament color across all the tubes in the left versus right amplifier. I will get them on the bench later to confirm voltages. Assuming there is a difference... Are there options to consider? The filament connections don't show up in the schematic... I assume that all but the 5AR4 are getting 6.3V ac directly from the PT?
    ...j

    Bob Latino
    Admin

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

    Re: Filament voltage mismatch

    Post by Bob Latino on Fri Feb 18, 2011 4:50 pm

    Hi J,

    The filament voltage on a Dynaco Mark III is 6.3 volts AC. There should be two GREEN wires coming from the power transformer that go to pins 2 and 7 on V2 which provide the 6.3 VAC.

    Below is a link to our online (large!) pictorial of the Mark III's pictorial which does show the connections from the power transformer to V2. If you have a difference in colors on the filament wires between your two amps, it is possible that under the bells of the power transformer the wires may be spliced to a different color. This could have been done because the original wires were frayed.

    Mark III pictorial

    Bob

    j beede

    Posts : 316
    Join date : 2011-02-07
    Location : California

    Re: Filament voltage mismatch

    Post by j beede on Fri Feb 18, 2011 8:17 pm

    By filament color I meant the temperature of the glowing filaments... One amp's filaments (all tubes) are visibly "hotter" than the other's. Since the PT filament taps are just integer numbers of turns, the 6.3VAC should be locked in for both amps. If the filment load in the "dimmer" amplifier is causing a voltage drop I guess the PT is assumed to be leaky or bad. I may try to use my clamp-on ammeter on a green 6.3VAC wire to compare the filiment current draw in the two amps. Is there a standard practice for measuring the health of the 6.3V tap?
    ...j

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