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  • P.I input cap in Fenders

    Hi!!
    Just trying to learn something...

    Why the Bandmaster Blackface use a 500pf in Phase Inverter while most of AB763 amps use a .001?



    best regards from spain!!

  • #2
    Smaller coupling caps pass less low end. Several amps from around this period used 500pf PI coupling caps (brown Vibroverb, BF Tremolux), later bandmasters reverted to .001. Perhaps they had reliability problems with 2x10" at the time (10" speakers back then didn't allow too much headroom wattage-wise) and were trying to limit low end?

    I'd look at .001 as stock, you can go up a little if you want a thicker tone, Fender later used as much as .01, I wouldn't go higher than .02...going too high migh cause motorboating or other unwanted symptoms.

    Comment


    • #3
      thanks a lot!!
      In my amp, the pre-owner put a TAD silvermica who said:

      100J


      I asume it means 0.001uf. isnīt?

      Comment


      • #4
        Usually 100J would signify a 10pf cap of 5% tolerance.

        The third number is usually a multiplier, so I would expect it to read "102J".

        101J = 100pf
        102J = 1000pf/.001uf

        ..and so on.

        Comment


        • #5
          The letters are confused, so i could be 102J....

          I,ll check it.

          thanks!!!

          Comment


          • #6
            Most of the Fender amps with larger .01uF or .022uF couping caps also have smaller value grid load resistors on the PI tube. Look at silver face Fender Reverb amps.
            If you want to boost the amp up a bit and the amp has 500pF now, try a .0022uF.

            My personal opinion is to NEVER use any more then .0022uF to .0047uF with any fender amp that has a PI driver with 1M grid load resistors and 100nF coupling caps to the power tube grids.

            I think Marshall got away with .022uF cap into the PI because of other "built in" bass limiting factors in the amp, such as .022uF coupling caps to the PI... etc.
            Bruce

            Mission Amps
            Denver, CO. 80022
            www.missionamps.com
            303-955-2412

            Comment


            • #7
              Originally posted by MWJB View Post
              Smaller coupling caps pass less low end. Several amps from around this period used 500pf PI coupling caps (brown Vibroverb, BF Tremolux), later bandmasters reverted to .001. Perhaps they had reliability problems with 2x10" at the time (10" speakers back then didn't allow too much headroom wattage-wise) and were trying to limit low end?

              I'd look at .001 as stock, you can go up a little if you want a thicker tone, Fender later used as much as .01, I wouldn't go higher than .02...going too high migh cause motorboating or other unwanted symptoms.
              MWJB is cool.

              Comment

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