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.

[burchyk]

I have the reverb driver power feed moved to the unused node (what stratomaster has also suggested) and can confirm it did drastically reduce hum.

One more from the Merlin's book that I liked is reducing V1a grid stoppers. Mine is effectively 10k (down from 34k?) on high input, which reduced hiss when the volume is up.

[tristanc]

Thanks - that's good to know. I'll also try the "ultrapath" thing Merlin talks about in his new book.

Attached is where I am currently. It's a bit difficult to see, but components marked are altered or additions. I've modified the power nodes and grounding - whether this will destroy the mojo... 

And good point on the grid stopper. I'll likely get rid of the 2nd input (I've never used it when there) and re-purpose the hole for a 'boosted' input (a FET stage in front of the 1st stage). That means I can used a 10k.

[stratomaster]

Thanks - that's good to know. I'll also try the "ultrapath" thing Merlin talks about in his new book.

Attached is where I am currently. It's a bit difficult to see, but components marked are altered or additions. I've modified the power nodes and grounding - whether this will destroy the mojo... 

And good point on the grid stopper. I'll likely get rid of the 2nd input (I've never used it when there) and re-purpose the hole for a 'boosted' input (a FET stage in front of the 1st stage). That means I can used a 10k.

You'll likely find that the ripple is so low after moving the HT node for the reverb transformer over that the ultrapath is redundant with minimal to non-existent benefits.  There was no audible reduction in hum nor visible change on a scope with the ultrapath in or out with my PR.

On the reverb recovery a large cathode bypass cap (100+μF) is beneficial in reducing H-K hum, a log pot is useful for dialing in subtle reverb while maintaining access to Surf City, USA.

Bonus: If you study the same section of Merlin's book that covers the ultrapath you'll come across a frequency response chart of the spring reverb.  Use a plate resistor bypass cap on the recovery stage to aggressively roll off above the upper range.  This kills hiss while maintaining the full response of the reverb stage.  I think I used a 470pF, but I'd have to go back and look.

[tristanc]

You'll likely find that the ripple is so low after moving the HT node for the reverb transformer over that the ultrapath is redundant with minimal to non-existent benefits.  There was no audible reduction in hum nor visible change on a scope with the ultrapath in or out with my PR.

Thanks - good to hear that you’ve tried both.

On the reverb recovery a large cathode bypass cap (100+μF) is beneficial in reducing H-K hum, a log pot is useful for dialing in subtle reverb while maintaining access to Surf City, USA.

I’ve been wondering why LED biasing wouldn’t be appropriate for that stage?
And yes, a 100k log pot is on its way - surprisingly difficult to source.

Bonus: If you study the same section of Merlin's book that covers the ultrapath you'll come across a frequency response chart of the spring reverb.  Use a plate resistor bypass cap on the recovery stage to aggressively roll off above the upper range.  This kills hiss while maintaining the full response of the reverb stage.  I think I used a 470pF, but I'd have to go back and look.

Thanks for this - I see he mentions a 100pF grid to ground to kill oscillations, but also refers to hiss just before this. I don’t see any harm rolling off the top end here given its way above where you’d be wanting reverberations?

[pdf64]

...

Attached is where I am currently. It's a bit difficult to see, but components marked are altered or additions. I've modified the power nodes and grounding - whether this will destroy the mojo... 
...

Nevermind mojo, the bias supply won't work without any negative voltage; R37 needs moving to the AC side of D3 

[tristanc]

Nevermind mojo, the bias supply won't work without any negative voltage; R37 needs moving to the AC side of D3 

Thanks! Good catch!

So it seems re-doing the power nodes and grounding is likely to make the biggest impact. Smaller things like LED bias are, well, small and nice-to-haves.

Oh, one question I have (being really picky) is where the reverb transformer secondary should be grounded. I've assumed it's acting like a coupling cap so the ground should go to the following (recovery) stage node.

The fun challenge for me is making the changes within the confines of the existing turret boards / chassis. I think I'll move much of the tone stack to the pots, and maybe use some tag strips next to the valve sockets (using existing mounting screw holes) to free up space. I'd like to avoid drilling holes for turrets or in the chassis if I can avoid it.

[stratomaster]

I’ve been wondering why LED biasing wouldn’t be appropriate for that stage?

I haven't put pen to paper, but I think you'd end up in a different spot. The LED results in a fixed cathode voltage regardless of current through the triode, whereas the cathode voltage "falls out" from the cathode current and resistor value. I think the former is useful for non-audio stages but may alter the behavior of the stage noticably on an audio stage.

More importantly, however, is I think the shunting of h-K leakage hum relies on the reactance of the bypass capacitor being significantly lower than the plate-cathode impedance.  That is, the path to ground through the capacitor is more attractive to the mains frequency cathode input is more attractive than the one to the plate.  This demands the presence of a capacitor.

So while you can get the two biasing arrangements to look similar in terms of load lines, the specific behavior we're looking to exploit needs a capacitor in place. If I'm wrong on this point, I'd love a correction. 

Thanks for this - I see he mentions a 100pF grid to ground to kill oscillations, but also refers to hiss just before this. I don’t see any harm rolling off the top end here given its way above where you’d be wanting reverberations?

There was a thread on TDPRI that I can't find at the moment where they discussed that a capacitor bypassing the recovery stage grid leak doesn't behave like a simple low pass.  Instead it looks a lot more like the frequency response plot of a guitar pickup with a tone control circuit due to the presence of the inductance of the reverb tank output transducer.  Conventional wisdom is that this cap shunts hiss and high frequency to ground, but in that thread simulations showed it behaved as a treble peaking circuit before the roll off.  It's for this reason that I prefer to do the treble shaping with a plate bypass cap.  You can even add a cap across the outer lugs of the Reverb pot for a steeper roll off of the hissy frequencies in conjunction with the plate bypass cap--though I haven't tried this yet. The reverb circuit noise is plenty low on a PR with just a large bypass cap and the cleaner HT node.

[tristanc]

I haven't put pen to paper, but I think you'd end up in a different spot. The LED results in a fixed cathode voltage regardless of current through the triode, whereas the cathode voltage "falls out" from the cathode current and resistor value. I think the former is useful for non-audio stages but may alter the behavior of the stage noticably on an audio stage.

Thanks! I've learned something.

There was a thread on TDPRI that I can't find at the moment where they discussed that a capacitor bypassing the recovery stage grid leak doesn't behave like a simple low pass.  Instead it looks a lot more like the frequency response plot of a guitar pickup with a tone control circuit due to the presence of the inductance of the reverb tank output transducer.

And again - learning something new. Thanks.

[tristanc]

I'm passing over the schematic now stripping out additions that may over complicate. So far:

- removing the ultrapath cap given I'm re-doing the power nodes & grounding
- abandoned OT protection diodes (don't think I'd have room, and have never installed them...)

Where I could do with some input: should I bother with a balance pot for the power tubes?

It's a cheap part which means I don't need to pay loads for 'matched' tubes. Albeit only likely to happen maybe twice in the lifetime of this amp. Though it should be an easy addition to the turret board (famous last words).

I've put all my workings here https://github.com/tristancollins/PrincetonReverbUpdate. There's a branch called "mods" with these circuit modifications: https://github.com/tristancollins/PrincetonReverbUpdate/tree/mods.

Latest schematic here: https://github.com/tristancollins/PrincetonReverbUpdate/blob/mods/schematic/PrincetonReverbUpdate.pdf

I'll do my best to borrow Rob Robinette's layout DIYLC files and update them for this schematic. Maybe even a spice model. We'll see.

[stratomaster]

R45 is in the wrong spot. It needs to go directly to the grid. The way you have it now is a voltage divider with the 1M.

I'd also recommend wiring the bias supply closer to what is recommended by Merlin in his site.  With 25μF as you've got it, it's under filtered enough to be audible.  Instead of wiring the pot at a variable resistor, wire it as a voltage divider and take the bias off the wiper with a 100k safety resistor and a 2nd cap to ground off the wiper or the bias input lug on the Intensity pot.

[tristanc]

R45 is in the wrong spot. It needs to go directly to the grid. The way you have it now is a voltage divider with the 1M.

Thanks - another good catch!

I'd also recommend wiring the bias supply closer to what is recommended by Merlin in his site.  With 25μF as you've got it, it's under filtered enough to be audible.  Instead of wiring the pot at a variable resistor, wire it as a voltage divider and take the bias off the wiper with a 100k safety resistor and a 2nd cap to ground off the wiper or the bias input lug on the Intensity pot.

Yes, I usually follow his bias supply recommendations - the bias board (original and in the kit) has limited turrets as is. Perhaps this one will need some drilling. Or maybe a new board.

Having put the idea in my head - the extra step to actually just lay out a PCB that fits the mounting holes wouldn't be much more effort....

[stratomaster]

Yes, I usually follow his bias supply recommendations - the bias board (original and in the kit) has limited turrets as is. Perhaps this one will need some drilling. Or maybe a new board.

No need. Just a little outside the box thinking. Hang the 2nd cap off the input to the Intensity pot.