Princeton reverb 12at7 voltage is inconvenient.

[oxbow_lake]

Hi everyone. I'm thinking about building a Princeton Reverb-like amp, except I would like to use oddball tubes, partly because you can get quality NOS tubes cheap, partly because I don't mind a little less power, and mostly for the fun of doing something different.

With that in mind, I don't think most of the output tubes I'm looking at (6k6, 25l6, 6aq5, 6dy5) are going to be terribly happy with 400V on the plates. Which is actually great news for a frugally-minded fellow such as myself because it means I can use a smaller (less expensive) PT.

Where I'm getting stuck is with the reverb driver. From what I've read, I get the impression that they really do want 400+V on the 12at7 (to maximize headroom? just to drive the tank hard enough?), whereas without that tube I could get away with a B+ of like 330V.

Any bright ideas for a topologically similar but lower voltage reverb circuit that will still drive the tank with authority?

Sort of related but not really: is the filament current mismatch between a 6N2P and a 12at7 (.34A vs .3A) enough that running them in series (i.e. with a 12.6Vac heater supply dropping 6.3V over each) would cause problems? Feel free to shake your head at that question and move on. Asking for a friend.

Cheers.

[Stevem]

For the price of a reverb driver transformer and the tube you drive it you can get a SS reverb section the only needs 5 volts .

[sluckey]

Sort of related but not really: is the filament current mismatch between a 6N2P and a 12at7 (.34A vs .3A) enough that running them in series (i.e. with a 12.6Vac heater supply dropping 6.3V over each) would cause problems? Feel free to shake your head at that question and move on. Asking for a friend.

The fact that the tubes would normally operate at different currents means the resistance of each tube filament is also different and that means that in a series circuit the voltage drops will be different. The math says the 6N2P would drop 5.9V and the 12AT7 would drop 6.7V. Both of those voltages is within 10% change from the ideal 6.3V rating. I would not be afraid to do it.

May I ask why?

[Leo_Gnardo]

All those Fenders that run their reverb drive at full whack and beyond - you don't really need that. Run your 12AT7 off whatever power supply's in the amp and get on with life. Should the notion of not thrashing the reverb quite so hard appeal to you, try this. To further reduce drive to almost sane levels I often use a 2200 ohm cathode resistor and no bypass cap. Still plenty of drive, a less stressed reverb sound, and an easier life for your drive tube. Not everybody needs to be Dick Dale.

[didit]

Hello --

A pair of mil-spec quality 6AQ5/6005 ($10-15 at most) will work durably in a Princeton circuit with a 330VDC B+. Keep plate current within max dissipation using a sensible bias.

Data sheet value for 6V6 "max voltage" spec is 315VDC, and yet Fender's Deluxe Reverb run between 410-450VDC works so well it's still in production.

Best ..

[lonote]

I get the impression that they really do want 400+V on the 12at7 (to maximize headroom? just to drive the tank hard enough?)

I recently built a Champ with a full 2-tube Fender reverb circuit + a dwell control. I am running +357V on the plates of the 12AT7 with more reverb than I can use.

[oxbow_lake]

May I ask why?

Sure. If using 25L6 output tubes then my plan was to use a 24V filament transformer, wire the 25L6s in parallel and then stack three 6N2Ps and one 12AT7 in series.

[oxbow_lake]

For the price of a reverb driver transformer and the tube you drive it you can get a SS reverb section the only needs 5 volts .

I'm not opposed to this approach, but I have a lot of questions and it looks somewhat more complicated than just a 12AT7 and a transformer. I'm always game to learn some new stuff!

I looked at this page, which is pleasantly in depth, but the driver circuits all seem a little more complicated than I'd like, requiring at least one op amp and 2 transistors, and of course a power supply.
https://sound-au.com/articles/reverb.htm
That page recommends not using any of the official Accutronics driver circuits, which is fine since the simplest one appears to requires a $15 opamp anyway.

I have a mid-70s discrete SS amp (Univox U65RN) which has a 2 transistor reverb driver running off of 24V, (circuit attached), which is more my speed and style. Could that be used? Am I right that for any of the simpler/lower-power drivers, a lower impedance tank (i.e. 8 ohm) will work better? The reverb tank in that amp was some bizarre early accutronics with 3" springs and it sounded terrible; when I hooked up a tank with 600 ohm input impedance it worked but sounds very dark.

Screenshot from 2024-03-12 06-08-24.png

Then I also checked out the schematic for an Princeton 112+ SS amp I used to have which drives the tank with just half of a TL072, which seems to fly in the face of all of the above, but as far as I can recall the reverb didn't sound terrible.

If you have a simple and effective circuit that you like, or references that I can check out for myself, I'd be quite interested.

[oxbow_lake]

Thanks everyone for your responses. Sounds like if nothing else I can run the 12AT7 at lower plate voltage and it will sound just fine.

[Stevem]

This is what I had in mind that I posted about.

[oxbow_lake]

Good heavens! Scandalous!