Reverb problems on a Princeton BF

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by piloto117, Sep 5, 2019.

  1. piloto117

    piloto117 Member

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    Hey guys. The other day I was rehearsing with this amp which I don't really crank (I keep the volume around 3-4). There were some power surges during the rehearsal and my reverb went off. I've already checked the usual, the reverb tank, cables, jacks and tubes. From my diagnosis, the problem is in the reverb driver part of the circuit. So I opened it up and saw no obvious signs of a failed component. I checked the voltage on pin 6 where the transformers primary is connected and get no reading at all. Could the reverb tranny be fried?
    I also did notice a frayed speaker connector which wasn't fully connected and soldered that back up. Lastly, the bias is a bit off whack and had to readjust (by ear) because I was getting some nasty distortion.
    I want to retube the power section (rebias) and possibly order a tranny from mojotone. Would this be the way to go? Should I do some further readings?
    Thanks in advance, this forum has always been very helpful!
     
  2. Deric

    Deric Member

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    First....what's a Princeton BF? Sounds like you have a Blackface Princeton Reverb?

    Did you pull tubes when checking voltage? If it's a Princeton Reverb - pull V2 then check for voltage on both the red and blue reverb transformer wires. If you have proper voltage on the Red wire and nothing on the Blue wire (pin 6 of V2), disconnect both and measure resistance from the Red to Blue. If you get no reading then transformer is bad for sure.
     
  3. piloto117

    piloto117 Member

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    Yeah, sorry about the confusion. It's a 1964 pre-CBS Fender Blackface Princeton Reverb.
    I'll check the primary again this weekend. I appreciate your input Derec!
     
  4. Darkness

    Darkness Member

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    Has amp been serviced?
     
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  5. zenas

    zenas Member

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    You said you checked the tubes. Does that mean swapping in a known good one? Particularly the 12at7 reverb driver V2.

    Did you try another reverb tank? Often if you set another Fender Amp close enough back to back the cables will reach.
    Speaking of those cables to the tank. Any chance they got reversed?
     
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  6. piloto117

    piloto117 Member

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    Yes, last year I did a cap job and replaced failing tubes, put in a nicely matched power tubes. Everything was working great.
    I know I can discard tubes, cables, reverb tank and bad connections, because I have a fully functional vibrolux reverb so I swapped all of those parts around.
    I did check again the primary in that reverb driver and I think it is fried because I get no voltage nor any resistance reading. There is some resistance on the secondary part of the tranny though. I went ahead and ordered a new one. I'll report back my findings.
     
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  7. bob-i

    bob-i Member

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    No voltage on the primary doesn’t sound like a bad xformer to me. I’d check the supply rail. The reverb gets its B+ from the same node as the screens so check the voltage at the screens.
     
  8. HotBluePlates

    HotBluePlates Member

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    I didn't know what "No voltage" meant. 0v from Blue wire to Red wire? 0v at plate of 12AT7?

    Likewise for "No resistance." Blue wire to Red wire = 0Ω? Blue wire to Red wire = ∞Ω? Or "OL"?
     
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