Princeton Reverb circuit refinements

[tristanc]

The Princeton Reverb is a classic circuit. It’s fine as is.

But, my understanding is that these were built to as-cheap-as-possible-yet-passible-tone requirements and, thus, BoMs. They struck lucky and their popularity shows that. However, there are mods people do to make them better, and components added in the reissue to make them safer / more suitable for the modern era.

Inspired by Merlin's new book, given cost is no barrier to a DIY clone of a Princeton Reverb, what circuit refinements would you incorporate?

I don’t care about aesthetics / period correctness on the inside. I do care about safety / good engineering practice and, ultimately, the classic PR sound.

Given the constraint of using a ‘traditional’ turret board from a kit, I was thinking:

• MF resistors instead of CC
• bleeder resistor added
• reduce post-PI coupling caps to 0.022uF or 0.047uF
• add 470k stopper to PI
• adding stoppers to grid and screens on the 6V6s
• using a red LED for the tremolo oscillator bias
• add protection resistor on the speaker jack
• add variable bias (and balance?) control
• do my best to ground things appropriately, even if it means running short lengths of wire to connect the various star groupings, and connecting only to chassis at the input jack.

The PRRI schematic has a bunch of these things done already (diodes here and there for protection, fuses, e.g.). I don’t want to go overboard as that would complicate the pre-made turret board.

Have I gone overboard? Not far enough? Destroyed the mojo?

[tubeswell]

IMHO, the best mod you can do for a PR that won't affect the mojo of the original blackface amp vibe is to keep the entire circuit as-is, but swap out the filter cap can for separate filter caps at each HT rail supply node and change to a Merlin-style daisy-chained galactic ground return system. This will get rid of some/most of the hum in the noise floor.

Also, if your PT is such that the amp will run at higher voltages (i.e 430-450ish), you could also swap out the PR OT for a DR (6k6 25W) OT and go to a 12" speaker - this opens up the sound and ensures slightly improved power output. And of course, you will probably want to run JJ6V6S if your voltages are going to be that high.

Also recommend a couple of fail-safe protections, namely:
1) 1.5kV protection diodes in series with the rectifier tube plates
2) a 1k load resistor across the reverb transformer secondary and a 2-3kV 100-2000pF cap across the reverb transformer primary to help prevent the RT from failure.

[stratomaster]

Reverb HT moved to the unused node with the 18k feeding it reduced to 12k and the reverb driver resistor reduced to 1.5k will give you a nearly silent noise floor from the reverb circuit.

I like to use a large cathode cap (250μF) on the recovery for good measure like the standalone reverb units did.

I also like a 3.3μF cap on the 2nd stage cathode instead of messing with the coupling caps.

[waldner]

Rob Robinette has a nicely laid out version of the Princeton with his suggested mods here:


https://robrobinette.com/AA1164_Princeton_Reverb.htm#Suggested_Mods

[tristanc]

Rob Robinette has a nicely laid out version of the Princeton with his suggested mods here

Thanks - that's what started me on the journey. Also comparing the 'original' scheme with the reissue. There's a fine balance between copying it verbatim in a new build and making some alterations whereby you end up with a completely different amp.

This is my 2026 amp build challenge - taking my time and, hopefully, using up some of the components I have on hand. I like the idea of making a few small changes (a cap here and there, re-routing the grounding e.g.) and doing it within the existing turret board(s). ie, what's the biggest improvement you can do with the fewest components.