Questions about reverb behavior on the AA1164 vs. PRRI circuits

[Joulupukki]

I have a 2017 PRRI that I’ve done a couple of mods to and also just built a Mojotone Princeton Reverb. One of the differences I notice between the two is that when I turn up the reverb knob on the Mojotone (AA1164 circuit) there is noticeable hum whether or not the reverb foot switch has been engaged. On the PRRI the extra reverb hum goes completely silent when the foot switch is open (off).

So that makes me wonder…

1. What in the PRRI circuit makes the reverb go completely silent when the foot switch is off?
2. What is the function of the 220pF capacitors that are on the input and output RCA jacks of the PRRI? The AA1164 circuit does not have these.

 

[Trem-o-lux]

The reverb switch grounds the input of the reverb recovery stage. Are you using the same footswitch for both? Check whether it is really grounding the input, i.e. are the switch and cable 100% good?.
You might have a noisy reverb recovery tube. Also, check the bypass cap on the recovery tube - if that is bad the tube can make hum.

The 220pF caps are there to limit the bandwidth - the reverb tank only works over a certain range of frequencies, so anything above that range is just making noise.

 

[Jeff Gehring]

1. What in the PRRI circuit makes the reverb go completely silent when the foot switch is off?

The reissue footswitch grounds the signal at the plate of the recovery tube after the coupling cap (the high side of the reverb pot), thus suppressing any noise the recovery triode may have added. The original grounds the signal at the grid of the recovery tube, any spurious noise from the recovery tube at its plate will still be coupled to the next stages even when the reverb is "off".

The caps on the transducers are to alter the reverb response at higher frequencies. The input transducer appears mostly inductive, with an impedance that rises with increasing frequency. The cap alters that response. You see this cap mentioned briefly in one of the Accutronics Drive Circuit diagrams.

 

[Joulupukki]

Are you using the same footswitch for both?

No. The PRRI uses a 1/4” stereo plug. The original uses an RCA jack.

Check whether it is really grounding the input, i.e. are the switch and cable 100% good?.

It’s brand new and I believe it’s working the way it should. I was mostly just curious about the differences between the two schematics.

The 220pF caps are there to limit the bandwidth - the reverb tank only works over a certain range of frequencies, so anything above that range is just making noise.

Ah, that makes sense. Thanks!

The reissue footswitch grounds the signal at the plate of the recovery tube after the coupling cap (the high side of the reverb pot), thus suppressing any noise the recovery triode may have added. The original grounds the signal at the grid of the recovery tube, any spurious noise from the recovery tube at its plate will still be coupled to the next stages even when the reverb is "off".

Thank you for pointing that out. I’m still new at understanding the schematics fully but find it all fascinating. Do folks ever mod to re-route the reverb switch on the originals to ground the output of the recovery tube after the coupling cap to get this behavior?

The caps on the transducers are to alter the reverb response at higher frequencies. The input transducer appears mostly inductive, with an impedance that rises with increasing frequency. The cap alters that response. You see this cap mentioned briefly in one of the Accutronics Drive Circuit diagrams.

Very interesting. I doubt I would notice much difference if I were to make this particular change as the reverbs between these two sound so similar.

I appreciate you guys taking the time to answer my questions. Thank you very much!