Silverface princeton vibrato issue

[rokplo]

Hi !

I just picked up a silverface princeton (non reverb)

The Fender Logo was missing and the grillcloth has been changed, so it's difficult to determine its exact year of building.

I also have a silverface vibrochamp.

My problem is that the vibrato on the princeton is very very light. I put the depth on 10 and speed around 5, and the effect is very light. My vibrochamp, wich has the same functionalities, has a deep vibrato. If i set it up to 10, it's very very strong.


So, is it normal on a Princeton ? does it have a "lighter" vibrato than the vibrochamp ? Or is it something i could repair ? (a bad cap to change ?)

All the tubes are new, and the problem was also there with the old tubes.


Thanx !!!!!

[theunrulychef]

Seems like the PR tremolo is notorious for being a little weak. It's a little different than the Vibro Champ circuit if I remember correctly.

It's a bit sensitive to the output tube bias & the volume you're playing at. Mine is good & strong lower on the dial, but when cranked, it gets a little weak. I found this a while back & was planning on doing some testing with mine the next time I have the chassis out. They talk about reducing the 1M coming off of pin 1 of the vibrato tube, but I haven't tried it out for myself. I'm just learning this stuff as well, so I'm trying to get my brain around what's supposed to happen electrically BEFORE I pull out the soldering iron . My tremolo works pretty well already though, it's more of that "could it be better" thing in the back of my mind.

First I'd check the resistors & look for dc on the caps in the vibrato section to make sure the components are still good. Also, take a look at your output tube bias to see if it's running on the hot side.

[Don Moose]

The princeton (just like the champ) uses a bias modulator trem. I don't know about notorious weakness - the trem in the one I killed was almost scary extreme when dimed. As mentioned, though, that particular set of 6V6s at that particular bias could have a problem.

I'd also look at the PI coupling caps for a small leakage - that would push the bias toward the hot end and may mess with the range the trem is able to swing.

Schematics are available if needed.

[rokplo]

I found some informations on the net confirming that biasing the princeton to hot would affect the trem ....

some people say that lowering the value of a resistor (1M -> 330 K) that connects to pin 1 of the second 12AX7 could deeper the trem ....


i'm not very good with schematics .... could someone explain me clearly wich resistor i should replace ?

[Mandopicker]

I would replace the old ceramic disk caps in the Vibrato circuit with new silver mica's or something else of similar "to spec" value. That , in addition to a new tube, will help make sure your vibrato is in good working order.

[rokplo]

all tubes are new, i told it before .... i prefer leaving a maximum components original (i like the sound of this amp), so i'll try changing the 1M resistor to a lower value .....

[Chris / CMW amps]

Change the 1M resistor (between plate trem.-triode-stage to 0.022 and 0.1 caps) to something like 470k-680k (from top of my head).

Checking e-caps/etc. is also recommend but the 1M does attenuate the vibrato-"signal" very much so more "swing" can be get by using a smaller value for that resistor.

Hope this helps, enjoy your amp!

[theunrulychef]

Checking e-caps/etc. is also recommend but the 1M does attenuate the vibrato-"signal" very much so more "swing" can be get by using a smaller value for that resistor.

This is what I was trying to figure out. I thought lowering the 1M value would attenuate the signal going into the tube, which didn't make sense to me if I wanted deeper tremolo. How does that create a larger swing? Is that because having a larger "signal" clips before you get maximum "off" level on the output tubes?

BTW, instead of a straight swap for that 1M, you might just clip in another 1M in parallel first to see how it sounds. Would be quicker for testing purposes before you start pulling stuff - much easier to undo if it doesn't fix your problem.

Darn it, all this talk is gonna make my want to pull my chassis to see for myself.

[rokplo]

he he ... pull your chassis out right now ;-)

I just swapped the 1M resistor for a 220K 2W i had in stock and it's ....................perfect !

same tremolo range than in my vibrochamp !

.... bang bang ... my baby shot me down ... it's the sound i can get now ;-))

thanx to everyone !!

[theunrulychef]

I just had to do it. Been putting it off for a while. I paralleled another 1M in there with some jumpers so I could go back & forth & listen to the difference. It certainly made the vibrato stronger. I would have to say it sounds closer to the roach trems in the bigger BF amps - much more of an off / on sort of thing going on. One of the things I like about the Princeton trem is that it's more subtle & smooth. This is good too though, stays nice & strong even when cranked.

I'll leave it with the paralleled 1M's & play it in a band situation before I make a final verdict. I have a feeling it'll work really well for that sort of setting. For sitting around the house, lower volume stuff, I'd probably keep the stock circuit; it's just smoother.

This is a good mod. I wish I could figure out WHY it works.

[Chris / CMW amps]

Ok, glad to see it works for you both!

The bias-tremolo does work because of putting a "wave" (from that tube-halve-oscilator-output) on the bias-voltage.

AS EXAMPLE:

normally that voltage is -35 Volt. When adding that tremolo-wiggel-signal to it, it will vary between (LET'S SAY) -30 and -40 Volts. So 10 Volts swing. If that isn't enough (in the case of lots of princetons) than that wiggle-signal-voltage needs to be somewhat larger, LET'S SAY 15 Volts.

Between the oscillator and the intensity-pot there's a resistor to weaken that signal somewhat to get a proper intensity-pot-adjustment-range but in this case that resistor is attenuating too much signal. It's in series with the int.-pot so using a smaller resistor value will increase the amount of wiggle-voltage.

Hope you can follow this, I'm not a native uk/usa-speaker and I'm very tired because of work and saturdaynight

[Bruce / Mission Amps]

....

Hope you can follow this, I'm not a native uk/usa-speaker and I'm very tired because of work and saturdaynight

Oh oh... nothing quite like a hard night's work in Amsterdam....

[Chris / CMW amps]

Oh oh... nothing quite like a hard night's work in Amsterdam....

(and I even didn't go to Amsterdam...)

(can you check my mails, about A'dam/etc. was a joke!!!! I hope I didn't offend you or anyone in anyway!!!!)