princeton "vibrato"

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by mbratch, Mar 8, 2006.

  1. mbratch

    mbratch Member

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    The vibrato (tremolo) on the Fender Princeton is a bit weak. Anyone here tried anything to make it more pronounced?
     
  2. Dave C

    Dave C Gold Supporting Member

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    Have you tried replacing the 3 caps (.01,.01 &.02) of the oscillator circuit and checked values of resisters in tremolo circuit for drift ? Also check bias for no more than 20ma ea. You can also change the first .01 cap to a .02 to slow down the "speed" even more.
    Dave C
     
  3. mbratch

    mbratch Member

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    Thanks for the reply Dave. I haven't checked the caps or resistor values. I did check the cathode current, and they were 27ma and 23ma (using the 1ohm resistor method). And thanks for the tip on the speed, I'm interested in that, too.
     
  4. Blue Strat

    Blue Strat Member

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    The bias of the power tubes has more to do with intensity than the cap values which mostly affect the speed and operation of the oscillator.
     
  5. mbratch

    mbratch Member

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    Thanks Mike. The tremolo method in the Princeton is odd as compared to the Vibro Champ or the BF Deluxe. I can crank the Princeton intensity to 10, and it's still subtle. I'm thinking the bias may be able to come down a little and perhaps that will help (?).

    Here's a clip illustrating the difference. The first sequence is the '64 BF (5-knob) Princeton (speed 1, intensity 10) and the second is the '68 Vibro Champ (same settings).
     
  6. mbratch

    mbratch Member

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    Any additional thoughts on this? There's someone who advertises making a mod to the Princeton to significantly enhance the tremolo. The mod is claimed to be reversible, although it is "proprietary".
     
  7. WailinGuy

    WailinGuy Member

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    Try replacing the 12AX7 tube in the V4 slot (the one closest to the 6V6 power tubes). One half of that tube is used for the cathodyne (or "split load") phase inverter (used to drive the power tubes), and the other half is used for the tremelo oscillator. If that side of the tube is weak, it may reduce the maximum intensity of the tremelo effect.
     
  8. WailinGuy

    WailinGuy Member

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    Correction to the above if your amp is the non-reverb Princeton (as opposed to the Princeton Reverb): try replacing the 12AX7 in V2 (also the one closest to the power tubes).

    Another thing to try if the swapping tubes doesn't help: try replacing (especially if it's original) the 25uF (or possibly 5uF) 25V electrolytic cathode bypass cap on V2A/V4A. It's connected to pin 3 of that last preamp tube socket (if you follow the wire to the circuit board) and is soldered in parallel with a 3.3K resistor. If that cap went dead, it would weaken the tremelo effect. Also, be sure to maintain proper polarity (positive end connects to pin 3).
     
  9. mbratch

    mbratch Member

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    OK I swapped tubes and I replaced the resistor on that particular tube because it was 2.7K, but the schematic says 3.3K (so I replaced it with a 3.3K). Didn't make much of a difference (maybe slightly better?).

    The 25uF electolytic is the original, but I didn't have a spare to try in its place. I'll have to fetch one and try it. My stupid RadioShack DMM doesn't seem to understand electrolytics, so I couldn't test it.
     
  10. Wakarusa

    Wakarusa Member

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    When you replace the 25uF cap try a 50uF part, particularly if you are also thinking of the mod to slow the trem down. The larger value will increase very low frequency gain in the oscillator.
     
  11. mbratch

    mbratch Member

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    Did this .01 to .02 mod and it worked nicely. :)
     
  12. Dave C

    Dave C Gold Supporting Member

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    Glad to hear you like the speed reduction , I'd try the 25uf-50uf change too that Wakarusa mentioned if you haven't already.
    Dave C
     
  13. mbratch

    mbratch Member

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    I'll try that next, Dave. Also, I noticed that the resistor (coupled with the 25uF cap) in my amp was 2.7k, but the schematic says 3.3k. So I replaced it. The 2.7k looked original, which is strange, unless the schematics aren't up to date. This amp is a 1964.
     
  14. Dave C

    Dave C Gold Supporting Member

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    The difference in resister value could have been a running change in production but not enough to warrant a circuit revision number or suffix......or it could have simply been a mistake.
    Dave C
     
  15. Nitroburst

    Nitroburst Member

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    I have not tried this it yet on my Princeton. But when I did the stokes and Paul C. mod there was a foot note on the sheet of the layout for the Paul C. mod that suggested changing one resistor to increase the speed. It's the 1 meg resistor just to the left of the .02 .01.01 package. It's in Parrnell w/t a.1 cap and connects to the .02 PI coupling cap on top and a 220k on bottom. Find it yet?:D It was suggested on this sheet to change the 1 meg there to a 220k. Like I said , I haven't done this mod and I don't know what the results are? So please take it with a grain of salt. I have the layout for the Paul C. I can email you if you like? Just PM me.
     

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