Super Reverb with Issues

Discussion in 'Amp Central Station' started by ArcticWhite, May 6, 2021.

  1. ArcticWhite

    ArcticWhite Tele-Afflicted

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    So I'm helping a buddy out with his 1976 Silverface Super Reverb.

    The amp has been gone through by a previous tech. All electrolytic caps replaced, and the screen resistors on the power tubes. But oddly, the previous guy left the death cap in place, and wired the auxiliary power outlet backward - with hot on the neutral side :mad:.

    I fixed that mess already, and now I'm using
    @robrob 's calculator to set the bias. Here are my numbers:

    Voltage on both power tube plates is 460v.
    Voltage drops between the plates and the OT center tap is 1.55 and 1.50.
    Resistance from both plates to center tap is 50 ohms.

    So bias current is 1.55v/50ohm = 0.031 amps.
    460v x 0.031amps = 14.26 watts.

    Is all that done correctly?
    Entered in the calculator:

    Screenshot_20210506-160922_Chrome.jpg

    I'm not sure how to read these results. Anyone?
    Please advise.

    Second, the tremolo is barely audible, and makes a ticking sound which speeds up along with the Speed pot.
    Two of the three tremolo caps have been replaced already.
    I've tried a couple of known good 12AX7s in the tremolo socket.
    Reverb works well.
    I have a working footswitch for the tremolo.
    What's next?

    20210506_153948.jpg 20210506_154006.jpg 20210506_154012.jpg
     
    Last edited: May 6, 2021
  2. corliss1

    corliss1 Poster Extraordinaire Platinum Supporter

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    It's almost certainly the trem bug itself if it's working, and the speed works, and it's just kinda weak even with a known-good tube. They do get old and wear out. I mean, check the voltages there of course, but it's likely bug replacement time.

    If it still ticks after that, the Fender published fix of a cap has been around forever, and usually does the trick.
     
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  3. Paul G.

    Paul G. Friend of Leo's

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    Um, you're not measuring 1.5V between plate and cathode, you're measuring 1.5V between center tap and plate.

    You need to divide 1.5V by 50 ohms to calculate current. So 1.5/50=.03A or 30mA. 1.55/50=.031A

    .03A x 460V = 13.8W. .031A x 460V = 14.25W.

    13.8W / 30W max plate dissipation of 6L6GC = 46%, 14.25 / 30W = 47.5% of max plate dissipation.
    Your bias is low. Adjust it and measure everything again -- the current will come up, but the voltage will come down a bit.
     
    Last edited: May 6, 2021
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  4. ArcticWhite

    ArcticWhite Tele-Afflicted

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    Got it. D'oh.
    My calculations were correct, then. I was just using Rob's app wrong.
    Thanks.
    I will adjust bias as suggested.
     
  5. LudwigvonBirk

    LudwigvonBirk Tele-Holic

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    Great amp and year.

    Roaches can slowly die and/or be unpredictable, seen it. It’s a trivial cheap replacement job. (Keep the old one just in case).

    Also since you’re in there, there’s a whole bunch of older resistors (20?) that might be due for replacement with quieter carbon-films.
     
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  6. ArcticWhite

    ArcticWhite Tele-Afflicted

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    Using the bias pot, I can't adjust this thing any hotter than 55% of max plate dissipation.
    The previous guy modified the bias adjustment as shown - in an attempt to bias it hotter?
    What's next?

    IMG_20210506_201912.jpg
     
  7. ArcticWhite

    ArcticWhite Tele-Afflicted

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    I couldn't move the bias to 70% with the amp as it was. Had to change the resistor on the bias pot to 6k in order to get it there.
    Does this indicate some other problem? (The 220k grid leak resistors are in spec, as are the stoppers.)


    IMG_20210507_113104.jpg
     
  8. Paul G.

    Paul G. Friend of Leo's

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    First of all, 55% mpd is probably fine.

    Now -- check the 470 ohm resistor on the bias. Also check for DC coming through the phase inverter coupling caps.
     
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