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Are these voltages too high on a 12AX7 in an 5F1?

Discussion in 'Shock Brother's DIY Amps' started by metropolis74, Apr 29, 2009.

  1. metropolis74

    metropolis74 Tele-Holic

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    This thread is lind of related to my Did I measure B+ correctly thread http://www.tdpri.com/forum/shock-brothers-diy-amps/159217-did-i-measure-b-correctly.html
    I got some advice about measuring voltages in a tube amp and last night I was comfortable enough to measure B+ at the key areas of my used Marsh built 5F1. However I think that the voltages at the preamp tube are high. The reason I started poking around in the amp is because it has developed a low 60 cycle hum after the first few days I've had it (or my ears are just more critical of it now and the hum has always been there). It's not a terrible hum, but it's bad enough to drive me crazy in a small room and the hum will prevent close mic recording. Volume does not affect the hum until about 9 or so on the dial. When compared side by side at idle with a healthy vintage '63 Tweed Champ, my Marsh has more hum. Otherwise the amp sounds fine.

    Without further ado:
    5Y3 Mesa-Boogie
    Pin 8 = 372V

    6V6 JJ
    Pin 3 = 360V
    Pin 4 = 342V

    12AX7 JJ
    Pin 1 = 251V
    Pin 6 = 192V
    Seems high doesn't it? Yes I triple checked the readings.


    Any advice is appreciated. Thanks.
     
  2. JohnnyCrash

    JohnnyCrash Doctor of Teleocity

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    The original schematic shows these voltages:

    5Y3:
    340v

    6V6:
    Pin 4 - 295

    12AX7:
    Pin 1 - 150
    Pin 6 - 150

    So it looks as though most of your voltages are in-line.

    Considering higher modern wall voltages (as well as the possibility the 5Y3 might not be a "proper" 5Y3), you look like you're going to be about 30-40 volts higher than the original, which is OK.

    The preamp tube does seem slightly high though. The tube can take it as 12AX7's are often given that much voltage when they're in tonestack or PI spots... but for a pream spot it may mess with the circuit's gain/headroom a bit.

    1. Check the 12AX7's filter cap (that last 8uF cap). Is the resistor feeding that cap a 22k? Measure its resistance to be sure (should be close enough to 22k give or take 5% either way).

    2. Are the 12AX7's plate resistors both 100k (the resistors feeding the 12AX7's pins 1 and 6)? Measure them to be sure they're 100k (should be close enough to 100k give or take 5% either way).
     
  3. metropolis74

    metropolis74 Tele-Holic

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    Yep, 22k.

    Double yep, 100k each.
     
  4. ThermionicScott

    ThermionicScott Poster Extraordinaire

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    What's the voltage at the last 8uF filter cap? From that, you should be able to determine the current drawn by the voltage dropped across each plate resistor. Sounds like your JJ is a little weak on the current draw, but it's not hurting anything.

    - Scott
     
  5. JohnnyCrash

    JohnnyCrash Doctor of Teleocity

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    Ah. A calculator and OHM's law :)
     
  6. metropolis74

    metropolis74 Tele-Holic

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    Oooh...looks like I have some cipherin' to do. Will do that tonight.

    I sent a message to Mike Marsh to get his opinion since he's the one that originally built this amp and he replied that the V1 voltages are higher than what he specs. There's got to be a bad component in there somewhere. He would fix it for free if I was nearby even though I'm not the original owner. What a nice guy. Too bad I'm nowhere near New Jersey. For now I'm going to keep plugging along myself and with the advice from the TDPRI. You guys rock!
     
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