How can I get the tremolo on my Fender Princeton to sound more tremolo-y?

April 7, 2007 9:49 PM

Fender Gear Geeks: The tremolo on my late 70s Fender Princeton isn't as tremolo-y as I want. Is there anything I can do?

My Fender Princeton is a wonderful amp and I was just playing with the tremolo last night and noticed that it isn't as full as I'd like it. You can hear the effect when the depth is all the way up, but the regular tone still comes through, which doesn't create the desired effect. When I took it in a year ago to have the tubes changed, the person said that the reverb and tremolo tubes were reversed. The reverb now works better than it did before, but I'm wondering if switching the tubes caused a problem with the tremolo (I'm not totally convinced the amp dude knows what he's talking about). Is my problem normal for the Princeton amplifier? Does this model of amplifier just not have the tremolo power that the 60s Bassman did (I had a '64 for years and loved it)?

posted by sleepy pete to Grab Bag (6 answers total)

Best answer: Silverface Princeton Reverb, am I right?

The 12AT7 is for the reverb. All the other little tubes are 12AX7's. The Fender Amp Field Guide notes that "biases vary" on the 1/2 of the 12AX7 that is used to drive the trem circuit; I found this page that seems to suggest that the capacitances of the simple RC circuits can vary too.

And finally here is a Surf Bug that replaces the stock Fender widget in the trem to make the trem deeper. I don't know if they were still using these in the post-CBS era, though.

Long story short: I'm sure this can be done; I don't know how to do it; I bet if you fired off some emails to the folks behind the webpages I linked to, you'd find out how to do it in short order.
posted by ikkyu2 at 10:12 PM on April 7, 2007

Are you competent with electronics? If so you then you can track down the appropriate schematic and layout diagrams on the net. I would start with ampwares.com. The second of ikkyu2's links suggests several things to check out. In an amp of this vintage it is pretty safe to say that any electrolytic capacitors no longer function properly, and perhaps even at all. A regular capacitance meter won't show this though. You need an ESR meter. Most people don't have one. Just replace all electrolytics. Check all the voltages against the circuit diagram and replace resistors as appropriate. If you do not feel competent in this area just take it in for service. There are lethal voltages inside of a tube amp, and there can be even after it has been turned off and unplugged for hours or even days.
posted by caddis at 7:39 AM on April 8, 2007

Response by poster: Thank you both for your answers. Although I have the schematics, I'm not competent enough to mess around with a tube amp. Knowing that it's possibly not just the model helps a lot. Thanks again.
posted by sleepy pete at 7:53 AM on April 8, 2007

This is something a competent tech should be able to do fix or improve fairly easily. Ask around at your local music shops for the Fender guy, or go to a Fender BB and ask around there.

But the first step is switch the reverb and tremolo tubes since it sort of sounds like you were happy with it before. If the tubes are marked on the chassis like on the schematic try swapping V2 (12AT7) with V4 (12AX7). If that does the trick then simply buy another 12AT7 so you have that tube in V2 and V4.
posted by 6550 at 12:07 PM on April 8, 2007

I'm pretty certain if you do what 6550 advised, you'll see some minor loss of gain in your preamp stage. Other than that I'm not aware of any problems it could cause, but that doesn't mean that it won't cause any.
posted by ikkyu2 at 4:54 PM on April 8, 2007

Best answer: ikkyu2, generally it's perfectly safe to swap any of the common 12A*7 family around, though the sound and/or gain may suffer by running the tube at a non-optimum bias. There are certain conditions that might have adverse effects, such as a specific type being biased at the limits of dissipation but, as a rule, those conditions aren't found in standard commercial guitar amplifiers like Fenders. Since the tubes are pin compatible the designers really can't assume people won't get tubes mixed up.

If anyone is interested the family ranges in gain from most to least: 12AX7/7025, 5751, 12AT7, 12AY7, 12AV7, 12AU7. So if an amp has too much gain you could try putting a 5751 or 12AT7 in place of a 12AX7. If the amp has too little gain try the opposite.

The only caution I'd make a note is the 12AX7 and 12AU7 are so very different in gain that they don't tend to make good substitutes for one another, unless the amp is opened up and that stage is rebiased (change the cathode resistor).
posted by 6550 at 8:00 PM on April 8, 2007