No announcement yet.

recapping a 100w 75 silverface twin reverb with push pull master...Need Advice please

  • Filter
  • Time
  • Show
Clear All
new posts

  • recapping a 100w 75 silverface twin reverb with push pull master...Need Advice please

    i'm recapping a push pull MV twin and i noticed that many of the resistors are far from there spec values. for instance the power tube grid resistors are all at about 500K ohms when there supposed to be at 1500K ohm according to the schematics. the tube are more blues than i usually see. most of the other ones that i tested where all at about 1/2 of the schematic value.

    are any of these resistors crucial to replace or just the electrolytic filter caps?

    also the brown and blue coupling caps all look a bit rough, kind of lumpy. is this normal? they're not cracking, or leaking anything.

    with all of this said the amps sounds really good, no hums or buzz, has plenty of clean head room, responsive eq. very clean signal just not as loud as im pretty sure it should be. i play it at about 5 or 6 on the master and about 4 on the channel volume to play with a fairly mild drummer. shouldn't i be reaching for the ear plugs about at that level....?

    any advice or opinions would be greatly appreciated

  • #2
    Are you sure you're not measuring the screen grid resistors? These should be 470ohm, but 500 ohms is real close. In most older Fenders I've seen, the CC resistors drift up in value.
    "In theory, there is no difference between theory and practice. In practice there is."
    - Yogi Berra


    • #3
      Did you mean 500 ohms or 500 kilo ohms?

      I wouldn't sweat it if after you change ALL the electrolytic caps (and I like to change power supply resistors for reliability) and the voltages look good, bias is set and it sounds good. Minor drift is what gives these amps individual character and if nothing is too far gone then don't worry. If the pre tubes are all idling fine then no worries. Check all your voltages with no tubes first (remember they will be higher from no load/tubes). Put in your pre tubes and check voltages again. Make sure there is bias voltage on the grid pins of each power tube socket and plate and screen voltage and put your power tubes in. Set the bias and you should be good to go.

      If the coupling caps aren't leaking, leave em alone.

      Don't worry about the blue tube glow - completely normal and different for every tube.

      If those are grid resistors you may get oscillation if they go to low.


      • #4
        thanks for the responses... so the i was mixing up my readings and the resistors are all actually pretty close to their stated value. i'm gonna leave them alone

        i replaced the electrolytics yesterday and the amp sounds about the same, a little more defined low end. either way the large cardboard filter caps were developing growths where things leaked. i feel more confidant in the amp now

        thanks again


        • #5
          so i went back in and took a few voltages coming from the power transformer. the two red wires that were supposed to read 340 VAC read around 335 VDC and another blue wire that is speced at 48 VAC was about 60 VAC. taking the voltages to ground with amp on stand by and no power tubes. should i try again with the tubes in and ON? If those are right should i be concerned?

          here is the schematic i am using....

          Master Volume Chassis Layout

          Also i noticed that all of the old mallory caps had increased in capacitance, by about 50% most of the time


          • #6
            If your real complaint here is that it seems to be weak, then the correct way to test it is to run a signal into the amp with a dummy load and scope the output to measure the actual power output of the amp. It's really hard to rate the loudness from the numbers on the control knobs. No two amps will be the same.

            Test it and then if it's only putting out 40 watts look for the cause. Old tubes, bad cathode caps, bad or wrong speakers, etc.