Hey WaltC...

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by DanD, Feb 24, 2015.

  1. DanD

    DanD Member

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    Feb 23, 2007
    Hi I texted with you long ago about my prosonic amp. I think I blew up something in the pre-amp.
    The problem is a very loud hum when the amp is switched to play. If I turnit up to 2-3 it would blow the speakers.
    Both channels are affected and when I turn up the volume. I get a very very loud hum.(It adds reverb to the very very loud hum.
    Everything after the fxloop is fine. The reverb circuit works , it adds reverb to the hum.
    Since it's so loud that to me indicates it's could be one of the components on V1 or V2.
    I've tried other good tubes to avail. I disconnected the input jack wire at the tube pin so it's no a bad ground or cold joint or jack and potentiometers.
    I checked continuity of all the parts and the ground from tube pins to hook-up wires through hook-up wire to the component then snaking my way through the circuit I check solder joints and have no issues.
    I know how to read the schematic.

    Next I used my Fluke multimeter in the diode check position and with the amp off to check continuity of the blue and brown pre-amp coupling caps in V1a switching the polarity of the meter leads to see if the caps charge. I repeated that with the meter set to ohms with no difference.
    During the 1st series of measurement in one position I get between 4-5meg ohms with a slight fluctuation and 0 in reverse.
    I assume that is normal because I checked the behavior of known good caps in the bias circuit with the same result.
    I tried repeating the procedure and one cap would measure 0 and fluctuating small numbers (-.0) with the (-) symbol on the display flashing.
    At that point I thought it was the light blue coupling cap (.1mfd) that parallels a .001mfd cap.
    I have not removed anything except the jack to pin lead and have that pushed aside not making contact with anything.
    I checked the dc voltage plate resistors in the 1st stage and it measure 370vdc +/-. I also checked the plate voltage at the second stage and it was 170vdc.
    That's where I stopped. Apprehensive to continue probing
    because of shaky hands due to my age.
    What's your opinion of the problem I'm experiencing?
    -DanD
     
    Last edited: Feb 24, 2015
  2. pdf64

    pdf64 Member

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    Aug 9, 2008
    Location:
    Staffordshire, UK.
    http://bmamps.com/Schematics/fender/prosonic.pdf

    Do you mean that the voltage at terminal 6 of V1 socket is 370Vdc? If so, that's way high, indicates that the tube section isn't conducting.

    Have you tried a 'known good' tube in V1?

    Does the hum change if V1 is removed?
     
  3. DanD

    DanD Member

    Messages:
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    Feb 23, 2007
    No sound when tube V1 is removed.
    Pin 6 is high voltage.

    -Dand
     
  4. WaltC

    WaltC Member

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    Location:
    Northern CA (Sacramento area)
    What PDF said. plus.... it could be lots of things, bad cathode bypass cap, bad 1st stage filter cap, what's the voltage on pin 1 of V1?

    be careful in there...
     

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