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Removal of Hum Control on SVT-II - why??

Discussion in 'Amps and Cabs [BG]' started by wonderjosh, Apr 13, 2010.

  1. wonderjosh


    Nov 19, 2009
    Fort Worth, TX
    I bought an SVT-II recently and I just now realized that someone has removed the hum control and replaced it with two resistors. Why the heck would someone do that? I should have noticed this when I tried the amp out and passed on the amp, but it's too late now. :rollno:

    The main reason I have an issue with the lack of a hum control is that the amp suffers from a bit more hum than other SVT's that I have played. I assume the modification is why, what do you guys think? The amp sounds fine otherwise, just has too much hum.

    Should I take it to a tech and have the a hum control pot reinstalled? I figure I should take it to a tech and have him go over it regardless eh? How much do shops charge to do stuff like that?

  2. johnk_10

    johnk_10 vintage bass nut Supporting Member Commercial User

    Feb 16, 2008
    Thousand Oaks, CA
    John K Custom Basses
    don't be too disappointed. its really an easy fix. the hum control on an amp is usually a 100 ohm pot and its used to balance the filament voltage to set it for the least possible hum. the two resistors in your amp now, are forming an artificial center tap on the 6.3V line.
  3. Hi.

    What John said.

    When buyng an used tube amp with a unknown history, it's a good practise to take it to a tech. Just in case.

    As for why the balance pot was replaced, the only explanation I can think of is that the potentiometer failed and the resistors were cheaper/easier to find. IIRC it's not the common 1/4W potentiometer that's used in there.

    In any case, it was an un-necessary short cut repair, but fortunately an easy one to fix.


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