Marshall Lead MV MkII 50W PT

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by kt77, Jul 28, 2006.

  1. kt77

    kt77 Member

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    Anyone know what the B+ typically was on these things. I got one on the bench that reads +360VDC when biased. '34's are drawing ~36ma each.

    That seems awful low for a 50watter, but god only knows what they put in these boat anchors......

    (6.5vac on heaters - ref, so the primary wiring is ok)

    Jusy want to know if the PT possibly has shorted HT secondaries.....

    TIA

    KT77, aka Chief Mushroom Cloud, aka tracynorton....depending on what forum yer lookin at
     
  2. John Phillips

    John Phillips Member

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    If the PT had a shorted secondary it would blow the mains fuse.

    360V is a little unusual, but these amps do have fairly low B+. Under 400 is normal, typically around 380-390V.

    Check that you don't have a partially shorted filter cap though - meter the current across the HT fuseholder (with the fuse out) and deduct the power tube currents. If the result is more than 10mA (the total draw for all the preamp tubes should be less than this), you have a problem.
     
  3. kt77

    kt77 Member

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    well, John...I get 6.1mA, so I'm surmising that the caps are ok.....
    I have a feeling this PT is a lo HT output, but it sure is a big mother....
    I'm just going to bias it up and send it out (after listening to it)
    the owner said he had measured ~500V at the rectifier at some time or another....but I can't confirm that either
    ????
    oh well
    thanks for the help
     
  4. John Phillips

    John Phillips Member

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    You can achieve a given power by either higher voltage and lower current or lower voltage and higher current - these 50W Marshalls do it the low-voltage/high-current way. That does need heavier transformers, but it's also probably one of the reasons why they have such a fat sound.

    The reading of about 500V will be AC across the input of the rectifier (both PT secondary halves in series). When rectified and filtered this does give about 360V. If the current draw discounting the power tubes is only 6mA, and all the other voltages are consistent, it sounds like everything is fine.


    If you're overhauling the amp, make sure the impedance selector is OK - the earlier pull-out type are a disaster waiting to happen... they often go intermittent as they get old and the metal contacts get stretched or corroded, and obviously if that happens under power the amp is running with no load. They simply aren't a good enough component for the job, and I always either replace them with a modern switch (or connect one tap to each output jack) if the owner wants to use more than one impedance, or simply hardwire the selector by moving the center wire to the appropriate pin if they're only going to use one setting. Even the later rotary switches aren't totally unknown to fail. IMO these selectors - particularly the earlier type - are at least partly responsible for old Marshalls having a 'reputation for weak transformers'.
     
  5. kt77

    kt77 Member

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    thanks for the info John
    in fact, this amp sounds pretty damn good (I'm NOT a Marshall fan, other than SLPs)...and this thing has 'superlead' thud to it....and you're right..it is fat sounding

    it is an 'overhaul' job...it had been butchered some time ago, with an added tube, and knobs...ugh

    just restored it w/ a metroamp board rewired to a 2204 (minus a few cathode caps)...and I really like this amp

    thanks again for the heads up on the PT and cap leak check!

    Tracy
     
  6. Swarty

    Swarty Member

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    I have 2 late '70s and 1 early '70s 50w, they are all under 400V. One of mine was around 360 and went to 390 after replacing the filters.
     

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