72 Princeton overhaul?

[sgmarshall]

Hi All,

I just picked up a 1972 fender princeton (no reverb). I'm looking to beef it up (goal is to get more volume & headroom) and am considering the following:

Replace non stock unknown 10" speaker with a weber 12"
Replace stock output transformer with a fender deluxe output transformer (do I need to run a different rectifier if I do this?)

I heard there's something you can do with the phase inverter to raise the plate voltage to increase the headroom? (stokes mod or something?)

and also, this amp is very dark toned. I like it with my strat ok, but its too dark with a les paul or sg. Are there different value resistors I can swap out to change the range of treble vs bass in this amp?

thanks very much!

SG

[MWJB]

Changing the speaker for a 12" is a good move.

Changing the OT for a deluxe type - no you don't need a different rectifier, it's just a straight swap apart from the mounting holes (Hoffmanamps.com has a chart with all the mounting centres for stock Fender type trannies, go to the transformers page, then wiring diagrams for transformers)?

There are several mods for the phase inverter, from basic blue printing to Gerald Weber's conversion to a long tail pair (see "Tube Talk for Guitarist & Tech"), additionally GregH has some Princeton mods on the Weber VST amps board that you might like to try?

The cathode bypass caps (on pins 3 & 8 of the preamp tube) can help tweak the amp's frequency response (smaller = brighter) but as a stock Princeton has relatively little gain, I'd check out some mods to increase gain before tweaking these.

I assume that you've replaced the electrolytic caps?

[Rob Mercure]

In addition to replacing the electrolytics you might make sure that the circuit voltages are within spec. And installing a larger speaker - while probably giving you a richer sound - might just deepen your "darkness." If your voltages are within spec and your preamp tubes in good shape - worn out tubes, while not as common as you might think, will both limit headroom and darken the tone - you could increase your headroom by lowering the second 18K resistor in the B+ string to around 12-15K which will increase your B+ to the first three stages not just the PI. Changing the blocking caps between the PI's plate/cathode and the 6V6 grids to 0.05uf would significantly brighten the amp but these thangs aren't known for being particularly "dark" so I suspect that your issue is really the non-stock speaker and perhaps worn preamp tubes and/or components out of spec.

Rob