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Princeton Reverb Hum...stumped

Discussion in 'Amp Tech Center' started by tube-works, Feb 20, 2014.

  1. oldpkr

    oldpkr Tele-Meister

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    my 73 was humming pretty good and drove me crazy too until I found a loose nut under one of the control knobs. tightened them all and hum disappeared. worth trying maybe?
     
  2. CoolBlueGlow

    CoolBlueGlow Tele-Afflicted

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    O.K. maybe some hints in the OP's last two posts, quoting:

    "Update, I just remeasured the heater filaments....
    Filament 1 to ground + 3.0VAC
    Filament 2 to ground + 5.1VAC
    "

    So that 2 volt imbalance is bad wrong. Something is off, either in your measuring approach or equipment, or in the circuit.

    and

    "when I press down on the pilot light assembly the hum changes intensity"

    That's a tip too...

    So, QUESTION to the OP.

    (I didn't see where you answered the question, so I'll ask again.) ARE THERE 2x100 Ohm resistors to ground on the filament leads there at at the jewel assembly location in your amp? Schematic shows there SHOULD BE.


    If "yes" double check the values on those resistors and the solder joints on both ends. In particular, examine them with a magnifying loupe and a bright light. look for burn marks and especially hairline cracks at the center of the resistors. They may be cracked and partially failed.

    If you have "no resistors" then double check proper chassis grounding of transformer housing then install the 2x100 ohm resistors in the correct location.

    If that doesn't help, then double check that the jewel assembly wiring leads are free of any even tiny amount of contact to chassis on either filament lead. This can happen at the jewel lamp assemblies, if they get twisted around some.

    If THAT doesn't help, FLIP the two filament wires at the jewel and see if the voltage imbalance follows the wires, or stays with the circuit. That should help you narrow it down.
     
  3. jhundt

    jhundt Doctor of Teleocity

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    Does it hum when you remove all the preamp tubes?

    You might try temporarily adding a second 20/450 cap in . parallel with the first cap in the can; this will double your filtering. If that reduces hum it would show that the cause is in the power supply filtration
     
  4. tube-works

    tube-works TDPRI Member

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    Hey guys...some progress made...
    jhundt- the hum stops with the removal of v3 and or v4...wired another filter as suggested...no effect...
    cbg- the amp had no 100ks at the pilot lamp..it has a CT...i unhooked the CT and installed the 100K...This resulted in a GREAT reduction in the volume of the hum...while stil present it's definately liveable...I think that the problem ultimately lies in the PT.....
     
  5. tube-works

    tube-works TDPRI Member

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    100 ohm resistors not 100K ...duh
     
  6. jhundt

    jhundt Doctor of Teleocity

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    was there a change in the mis-matched heater voltages after installing the 100 ohm resistors?
     
  7. CoolBlueGlow

    CoolBlueGlow Tele-Afflicted

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    Yeah, great question from Mr. Hunt. Something's amiss there in that PT.

    I wonder if the windings are seriously imbalanced due to some shorting or something? OP lifted the CT, (which would eliminate the imbalance) and installed a virtual CT and the hum got better. Sure sounds like PT filament winding problems to this hillbilly at least.
     
  8. jhundt

    jhundt Doctor of Teleocity

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    that's why I was wondering (in earlier post #14 on this thread) if a hum-balance pot might be a solution.

    IF the heater voltage is OK...
    IF the transformer is working OK and not deteriorating...
    MAYBE a hum-balance - in place of the two 100-ohm resistors - would allow the hum to be dialed out and make the amp useable.

    By the way, in the earlier post I referred to a 100-ohm linear resistor, and I meant to say a 100-ohm linear potentiometer.
     
  9. CoolBlueGlow

    CoolBlueGlow Tele-Afflicted

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    Yeah, a humdinger - practically a forgotten art, and a good suggestion in this case, since the phantom CT with fixed 2x100ohm values seems to have helped. As you pointed out, a dialed in humdinger would be even better.

    Both my early Ampeg B15 have humdingers with a knob, right on the back panel. Works great.
     
  10. Achysklic

    Achysklic Tele-Afflicted

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    I read somewhere that the reverb tank on the princetons are a issue...upgrade the tank hum goes away.
     
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