Princeton Reverb Tremolo Problem...

[Bob M.]

The amp in question is a Silverface Fender Princeton Reverb made in the early 70s. Fender has done an admirable job of confusing Tremolo and Vibrato in the public's mind but were talking about tremolo here, amplitude modulation. The circuit in question is the stock tremolo circuit used at the time the amp was built. The only mod was to lessen a 1 meg resistor to about 470K to intensify and deepen the tremolo. It's a very common mod for this tremolo circuit. This is not an opto-isolator circuit as seen in other Fender amps of this time (and before and after). It's not my amp but my friend's. I was repairing, updating a few small things. The amp is very close to stock; I may have tweaked a tone stack cap or two and added screen grid resistors and a 3-prong AC cord to the amp. None of these thing will affect the tremolo problem I'm experiencing.

Here's the problem: The tremolo is barely but perceptibly 'on' when the intensity pot is on zero (fully counter-clockwise). The Princeton/Princeton Reverb tremolo circuit is always active, unless turned off by a footswitch or the use of a dummy plug, unlike the Fender opto-isolator circuit which works opposite (always off, unless…). But normally, you can defeat and eliminate the tremolo by keeping the Intensity pot on zero.

So what have I done so far to attempt to eliminate this feint tremolo always on problem? I checked all grounds in and around the tremolo circuit. Then I double-checked them. Then I measured the intensity pot of the tremolo circuit, both in and out of the circuit. I checked the fully counter-clockwise resistance to ground to make sure it wasn't too high or differing at all from other pots of the same value/taper. It wasn't. Still, no change. Then I subbed in a new pot. No change. I measured every resistor in the circuit and changed out any that weren't a close tolerance. I re-installed the 1 Meg resistor that controls the tremolo depth. No change. I changed out every capacitor in the tremolo circuit. Again, no change. So I'm a bit stuck on this one. Yes, I can get rid of the very feint tremolo by using the dummy plug on the rear panel RCA jack but that's not what my friend wants. This isn't a problem at all in the other 5~6 Princeton/Princeton Reverb amps that I own, only this amp.

My questions are: has anyone else experienced this problem before and if so, how did you solve it. And what should I try next?

Thanks for reading this and hopefully contributing,

Bob M.

[52 Bill]

If you disconnect the oscillator output from the depth pot do you still hear the tremolo?

It seems that you have checked everything that should fix this problem. The only additional thing that I would check is the bias supply filter cap. What about the main filters?

Worst case is to change the depth control to one with a switch that can shut off the oscillator when turned down.

[Bob M.]

Thanks for your reply.

I will disconnect the oscillator output and see what happens.

Bias cap = Brand new

Filters are discreet, individual and new (no multi-cap can).

Yes, I have formulated an in/out switch that works fine (at the rear panel RCA jack) but that is circumventing the problem as opposed to actually solving it. I will look for a 250-Lin pot with an on/off switch for the Intensity control.

I agree; I thought I was pretty thorough with my troubleshooting but I still have the problem.

Bob M.

[Tom Phillips]

In my experience all the amps with this tremolo circuit exhibit the same behavior. Sometimes, the bleed through of the Tremolo effect is so small that you can't actually hear it but you can always "see" when you scope the output. The only way to completely eliminate it is to turn the tremolo oscillator off as originally intended. The fact that you did a mod to intensify the tremolo probably made the problem worse in your amp. The setup of the circuit is a fine balance between tremolo intensity range and output tube idle bias setting. The optimum performance for an individual player is also dependent upon how hard the amp is driven. If the tremolo intensity is set too high then the bias will keep getting driven too far toward tube saturation if the amp is over-driven.

The bottom line is that I think you are trying to fix something that is just the way the circuit is. Made harder because of the "mod."

[Chuck H]

Second here on Tom's observation. Bias wiggle trems are always on a little unless you stop the oscillator. That may or may not be perceptible depending on how 'not off' the tremolo is and how sensitive to it the user is. Anything you do short of switching off the oscillator to lessen the problem will reduce tremolo intensity.

[cbarrow7625]

Thanks for your reply.
Bias cap = Brand new

Brand new doesn't necessarily mean it is perfect. I don't have very much experience with these circuits so I'm just spit-balling here, but clearly the bias cap should create an AC ground for the tremolo oscillation on the low end of the intensity pot. the first thing I would try is to parallel the bis cap with the largest, lowest DCR poly-type cap that I have; the bigger the better. I imagine that would make a difference. If it does, I would then try out a number of bias caps, possibly from different manufacturers and/or just buy the more expensive low DCR electrolytics for that circuit and try those out.

I can appreciate that the circuit will always hove some tremolo going on, the AC ground for the oscillator cannot be perfect, but it sounds like it is worse than it needs to be. I don't see any other possible solution other than improving the AC ground performance of that bias cap. You may also want to play with jumpering the ground end of the bias cap to other ground nodes in the amp to see if that helps create a better AC ground path to get rid of the oscillation. But that would only be secondary to evaluating the performance of the cap itself.

[pontiacpete]

If you are going to go with switching route you could add one of these to the pot.
Radio Daze sells them.

[Bob M.]

PR Trem Problem...

Thanks to everyone for your thoughtful replies.

As you've read above, one of the things I tried was to 'un-mod' the tremolo and go back to the stock 1 meg resistor (which goes from the plate to the .1 cap then onto the intensity pot). There was no change at all in the amount of tremolo 'bleed thru' with the stock 1 Meg resistor back in place or the 'modded' 470K resistor.

I agree new doesn't always mean the part is perfect, or even good enough. I will try your cap and grounding ideas.

I realize this circuit is a fine tuning balancing act between the achieving proper bias and getting a good tremolo sound as well.

I used the Gerald Weber 'Shotgun approach' - change out absolutely everything (in the tremolo circuit and in the bias circuit as well) but as I've reported, that still didn't get it done; the problem persists. Even though it may be only seen on a scope and not heard at all, I don't have this problem on any of my other Princeton/Princeton Reverb amps. It might be there but not to the degree I'm hearing it from the amp in question.

I will suggest to my friend that he reconsider using the footswitch/dummy plug or the on/off pot idea to turn off the tremolo oscillator.

I appreciate everyone taking the time to contribute.

Bob M.

[Chuck H]

I don't have this problem on any of my other Princeton/Princeton Reverb amps. It might be there but not to the degree I'm hearing it from the amp in question.

Well... It's a bit of work, but...

You could open up your own PR and measure the voltages, bias current, measure/check part values, etc. to see if anything jumps out. I've never heard a Fender with bias wiggle trem that had objectionably audible trem occurring with the intensity set to zero. As you mention, yes, you can see it with a scope. It's virtually inaudible when playing though. I would dig just out of curiosity.

[Alex R]

The bias setting itself will change the trem effect quite radically. I have often ended up with quite a cold bias as a compromise on these to tame an always-on thump in time with the trem, etc. I suspect a slightly colder bias might remove the always-on trem. I guess there's a reason why Fender used opto trem on later models.

[Chuck H]

Especially troublesome is the use of bias vary trem in high Vp amps with big bottles. I didn't know this when I built my latest amp. Compounding the problem is that this amp changes/switches between two designs where one employs NFB and the other doesn't!!! I didn't find a cold bias helpful in my case. It usually made the problems worse. I did find that different sets of power tubes, each adjusted for similar current relative to their dissipation rating, performed better or worse. I didn't check but I suspect that a little imbalance in the power tubes helps eliminate unwanted crossover garble and such from the trem function. Of course this reduces max possible output power, but if it's only a few watts it really doesn't matter much for a guitar amp.