64 Princeton reverb clone, tremolo problem

Discussion in 'Amp Central Station' started by dankomal, Oct 9, 2016.

  1. dankomal

    dankomal Tele-Meister

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    Hi all,
    I have a 64 princeton reverb clone and the tremolo sound has almost disappeared.
    The speed knob barely makes any difference to the sound.
    I have sprayed contact cleaner into v4 and replaced a valve but this hasn't fixed my issue.
    The foot switch looks sound.
    Is it time to take it to a tech or is there anything else I can check?
    Cheers.
    Mal
     
  2. Mike Simpson

    Mike Simpson Doctor of Teleocity

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    You do not need a foot switch to make a Princeton Tremolo work. I would unplug the switch for now.
    You say you "replaced a valve"... I assume you replaced V4 ? What tube did you put in V4 ?

    What are the power tubes biased at and how long have they been in the amp?
    Princeton tremolo is a bias vary type and if the power tubes are biased too hot or too cold the tremolo will not work well.
    I have also found that when the power tubes are needing replaced the tremolo does not work as well.

    princeton reverb tubes

    V1-A: input and tone stack driver
    V1-B: tone stack recovery
    V2-A/B: these are paralleled and drive the reverb tank
    V3-A: reverb recovery
    V3-B: pre-inverter gain stage
    V4-A: tremolo oscillator
    V4-B: unity-gain (almost) phase inverter
    V5, V6: output tubes
    V7: rectifier
     
  3. dankomal

    dankomal Tele-Meister

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    Hi,
    Yes I replaced v4 12ax7 with a ecc83, I have a couple of 12ax7s too.
    I don't know what the power tubes are biased at, they are sylvanias and came with the amp, they've probably been in for around four years though the amp doesn't get gigged, just home use two or three times a week.
    The trem worked really well until I last week when I noticed the fault.
    The speed knob does virtually nothing now and the foot switch has always been plugged in and left in the on position.
     
  4. jazzguitar

    jazzguitar Tele-Afflicted

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    Does the intensity knob make a difference?

    If the speed knob does nothing, the oscillator may have a problem. If you built this, then try reading the resistances from the speed pot center lug to the others while turning the speed knob, best with an analog meter where you can see the needle move.
     
  5. jazzguitar

    jazzguitar Tele-Afflicted

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    If you mean, hoewver, the intensity knob then remember the tremolo is a bias vary tremolo and will depend on a particular bias range (not too hot). Also with worn tubes the tremolo may get weak.
     
  6. Mike Simpson

    Mike Simpson Doctor of Teleocity

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    Unplug the foot switch.
     
  7. dankomal

    dankomal Tele-Meister

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    I've unplugged the footswitch, it made no difference.
    The intensity knob does work slightly though the speed knob does nothing.
    I bought the amp new but am just a player and don't know how to troubleshoot the amp.
     
  8. dankilling

    dankilling Tele-Afflicted Ad Free Member

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    If swapping v4 for a known good tube has no effect, and the power tubes are known good, there is something inside the amp that likely needs to be looked at.

    If you aren't sure about looking inside the amp, take it to a tech and have the tech check the bias. Too high a bias can mess with the tremolo's ability to function. That said, if there is something else going on in there a tech would still be your best bet at this point.
     
  9. Thin69

    Thin69 Friend of Leo's

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    Time for a tech. It's probably something minor such as the bias being too hot. There are lethal voltages inside a tube amp that can be present long after the amp is turned off and it's easy to do further damage if you don't know what your doing.
     
  10. dankomal

    dankomal Tele-Meister

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    Thanks a lot guys, I've no intention of messing around inside the amp, it's definitely time for a tech to have a look inside.
     
  11. wabashslim

    wabashslim Friend of Leo's

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    The capacitors in the trem circuit likely need replacing. I replaced 2 or 3 of them in my '67 PR and got some of the trem back after years of having practically none. Then when I went to check the bias I found I had to replace the diode, resistor and capacitor in that circuit too (on the small board in the corner). After taking care of all that & setting the bias for the new JJ tubes the trem works great, better even than when I got the amp 40+ years ago. Tell this to whoever's going to work on it.
     
  12. dankomal

    dankomal Tele-Meister

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    Will do
     
  13. Mike Simpson

    Mike Simpson Doctor of Teleocity

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    I own 3 Princeton Reverbs, 1974, 1975 and 1980. Each time the tremolo is weak or does not work it is usually the power tubes and once it was V4 and the power tubes. The bias level affects the ability to vary the bias for the effect. In the stock Princeton the bias can be adjusted by changing a resistor in the bias supply circuit although I doubt a lot of them were ever changed from the factory resistor, in the AA1164 diagram it is a 22k . I would be inclined to replace the 6v6 power tubes with some Tung Sol reissues to see if the tremolo works with known good tubes. I have read a number of times that JJ's are not recommend for a Princeton. Having someone knowledgeable look at it is almost always good advice too.
     
    Last edited: Oct 11, 2016
  14. dankomal

    dankomal Tele-Meister

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    The amps developed more of a hiss, I can't get it in to my amp Guy for three weeks!
    I pulled the 6v6s out and they have a rainbow effect on both of them, they both glow orange when on.
    Here's a pic to show what I mean, I don't know if they are ready for changing.[​IMG]
     
  15. Mike Simpson

    Mike Simpson Doctor of Teleocity

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    Don't turn the amp on again until you get it to a tech. If the plates are glowing and not just the heaters the tubes are red plating and are biased way too hot indicating other problems.
     
  16. dankomal

    dankomal Tele-Meister

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    Cheers, I'll leave it alone., I'll post again when it's fixed.
     
  17. dankomal

    dankomal Tele-Meister

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    Ok, the amps working fine now, it wasn't redplating, just the heaters. The bias had drifted upwards and needed sorting, the speed knob got changed to a cts as it wasn't working and a few caps got changed.
    Sounds great now with the same output tubes in.
    Thanks all for your input.
     
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