SFPR vibrato oddity

[krash]

Hey folks-

I have a '78 Princeton Reverb that has been subject of many mods over the past 25 years and I never was happy with it (this is the whole reason I started building amps!). Finally I got serious about making it usable for me and made some changes to it last weekend. I had previously deleted the reverb drive/recovery circuits and changed the voicing, but it was not working for me so I took the plunge and converted it to long-tail PI. This completely fixed the tone problem but had one very unfortunate side effect.

One thing I always loved about this amp was the vibrato. It was smooth and gorgeous. I also loved the vibrato in my 18watt, which was bias-vary as well but in the 18watt, it affected bias on the input stage triode while on the PR it varies bias on the 6V6s.

Now that I have converted it to LTPI, the vibrato has lost the smooth and linear tone and instead it has that "thwap-thwap-thwap" kind of sound like the crappy Line6 bias trem effect.

Anyone have an idea of how to remedy this?

[Bear]

Don't have direct experience with the circuit, but you might try rewiring the oscillator more like the 6G3 Deluxe. Pre-roach, oscillates the PT bias after a LTP PI. I don't know if that's a smooth sine-wave sound, so you might try to find sound clips before rewiring, but I've got none on a casual browse of youtube.

More of a square-wave, hard chop, you say? I might actually want to try that.

[Firestorm]

The LTP has a little less gain than the driver plus cathodyne PI you had originally, so the tremolo signal may be a bit too large for the audio signal coming out of the PI. There's a .02 cap to ground right after the 1M resistor in the Princeton. Try making that .047. If that doesn't do enough, you could try moving the position of the .047 to the intensity pot wiper like on the 6G3 as Bear says. (It might also be a good idea to make the bias adjustable).

[Cameron]

Hey folks-

I have a '78 Princeton Reverb that has been subject of many mods over the past 25 years and I never was happy with it (this is the whole reason I started building amps!). Finally I got serious about making it usable for me and made some changes to it last weekend. I had previously deleted the reverb drive/recovery circuits and changed the voicing, but it was not working for me so I took the plunge and converted it to long-tail PI. This completely fixed the tone problem but had one very unfortunate side effect.

One thing I always loved about this amp was the vibrato. It was smooth and gorgeous. I also loved the vibrato in my 18watt, which was bias-vary as well but in the 18watt, it affected bias on the input stage triode while on the PR it varies bias on the 6V6s.

Now that I have converted it to LTPI, the vibrato has lost the smooth and linear tone and instead it has that "thwap-thwap-thwap" kind of sound like the crappy Line6 bias trem effect.

Anyone have an idea of how to remedy this?

Try different values for the 220k and .1 cap. I dont remember what I used but try putting a 500k pot for the 220k to see what sounds best (WATCH OUT FOR THE VOLTAGE when you do this) and I think I used a .22 for the cap. Playing with these two things made It work great in a Marshall type circuit.

[krash]

Guys - I converted to LTPI by moving the trem circuit to V3B and using V4A+B for the LTPI. I don't use reverb so I pulled everything related to reverb recovery and drive, leaving me with an empty socket and an unused triode. I still have the stage after the make-up gain... so it's input -> tone stack -> makeup -> extra stage -> LTPI -> PT and the trem is on the PI bias (per stock Fender SFPR). Very much like a wreck with vibrato.

and it's not a square wave sort of trem. it's a phase distortion kind of thing on the front end of the trem cycle. so it's kind of like a sinusoid but the leading edge is distorted in what sounds like a phase orientation. it doesn't linearly increase with volume, it obviously passes through some intermediate position whereby it is in phase.

like this:

Code: Select all

      _
     /  \
   ==    \
  ==      \       
 /         \   / ...
/           \_/

where the "==" is a region where it sounds like it's going in phase and out of phase again.

guess I need to make a recording.


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