65 Princeton reissue trem circuit

[Randall]

Can someone explain how the trem circuit works in this amp? I see that it is a bias wiggle circuit, but I'm not sure how it does it. A friend visiting from home for a couple of days wants me to adjust it so it "goes deeper" (more intensity), but I really don't know where to begin.

http://www.thetubestore.com/lib/thet...-Schematic.pdf

[The Dude]

The tremolo tube is a simple oscillator that modulates the output tube bias. I'll let the builders tell you how to get a wider swing since I'm not sure of the best way to achieve that goal (maybe larger C22, reconfigure cathode for more gain?). You'll want more oscillator signal at the intensity control. Just a word of caution: It's possible to overdo it and cause tubes to red plate, so keep an eye on things.

[Jazz P Bass]

Read this: How to improve tremolo intensity?

[Chuck H]

That's a terribly colluded schematic for the trem circuit! As usual, Fender is using a different oscillator than any circuit we already know. I might try lowering the value of R9 to increase the gain of the oscillator. Being a single triode it may not have as much influence on the bias as two triode models. As Dude mentioned, there's a risk of the oscillator going too wonky ( that's the technical term ) and skewing the bias too far in one direction or both.

[Enzo]

Looks like a common trem circuit to me. Basic triode oscillator with the three feedback caps. Some amps use two triodes, but the second one is not the oscillator there. The difference is whether the circuit needs a tube to drive a trem bug or not. No bug here.

I don't worry about biasing the tubes. The trem voltage at a given instant might make the power tubes hotter or colder than desired, but it is only for an instant, followed by an equal instant where the bias goes the other way. It averages out for the tubes, and they don't know the difference.

TP28 is where the LFO signal appears. How much signal you have there? That is fed to the intensity pot through blocking cap C22. SO how much signal is on the left end of that pot? That is what you have to work with. How does that compare to the bias voltage? If the trem peaks exceed the bias voltage, you are going to drive the power tube grids into conduction on those peaks and it will be heard as a noise. There is a limit to how far you can wiggle the bias.

[Chuck H]

Yes, it's a common oscillator circuit. I was referring to how it is coupled from the plate of that single triode. I didn't realize this is typical of the small bottle amps. I'm more familiar with what Fender used for 6L6 amps where a CF is added.

[pdf64]

What are the plate / cathode idle currents of of the power tubes (with trem off)?
If excessive the trem intensity tends to become diminished.

[soundmasterg]

To get more intensity, you can add a triode stage after the oscillator stage as a CF, or add a MOSFET source follower after the oscillator stage, but with both of these there are issues. There isn't room to add another triode without cutting up the amp, and the MOSFET source follower needs a stable low voltage, moderate current supply, which means adding a small transformer, and again no room. The easiest way to add a little more intensity is to bias the oscillator with an LED in the cathode instead of the resistor and cap.

Greg

[Randall]

Thanks guys, this confirms my suspicion that the amp isn't broken, it is working the way it was designed. In essence it is what it is, and it's not realistic to think I am going to get it where he wants to be with it during a single overnight visit with him over my shoulder the entire time. I declined.

[potatofarmer]

Easy fix - LED bias the oscillator tube, and put a diode across the intensity pot to prevent it from pushing the bias hotter. You could probably also decrease R14 if need be.
Reference:
Is your tremolo too weak?


edit: Jazz P Bass beat me to it, sorry for the repeat