Slowing down Tremolo and reducing Reverb depth in Princeton Reverb

[Martin Beer]

I've just finished an AA1164 PR build, and while I'm really pleased with the sound, I'd like to tweak a couple of things about it. I'd like to reduce the speed of the tremolo when the pot is at zero. Is this a common issue with these? I gather that I could change one of the caps attached to V4a, but can anyone suggest which cap and which value, to save me some experimentation?
Also, the reverb has more depth than I'll ever need. With the control at 10:00 it's already into surf territory (I'm using a MOD 9AB2C1B tank). Would I be best to work on reducing the gain of the driver stage or the recovery stage? Or just try a different pot value?
This was my first scratch build, so I'm pleased that everything is working without too many hitches.

[Enzo]

I routinely slow down Fender trems. Stock, they adjust from too fast to silly vibration. I prefer the low end to be a slow sexy throb, and the fast end is still way faster than we need.

See the two 0.01uf feedback caps? I generally make them 0.02uf.

Experimanting is simple. To increase a cap, just parallel another along side it. Have the amp on with trem running. Set it up so you can hear the speed of trem. With the knob at slowest, touch another cap in parallel with one of the feedback caps. Hear the difference. If 0.01 to 0.02 is not enough, try 0.03uf, or increase the one that was already 0.02 to 0.047. You can go too far, but within reason, such changes just move the speed range.

[Martin Beer]

Thanks for that. I'm glad I used large eyelets, as it shouldn't be too much trouble to add another cap in parallel with them.

[Enzo]

Two in parallel is the same as one larger one to the amp. But I usually replace the old part instead of adding a neighbor, just to keep clutter down. I touch-parallel to experiment for value, then install one of whatever I decide.

[Tom Phillips]

...Also, the reverb has more depth than I'll ever need. With the control at 10:00 it's already into surf territory (I'm using a MOD 9AB2C1B tank). Would I be best to work on reducing the gain of the driver stage or the recovery stage? Or just try a different pot value?...

There are three things that come to mind. And each will reduce the amount of reverb in different ways. Each will also affect the character of the reverb and you will need to decide via listening test which one by itself or which combination of 2 or 3 of them you like best.

#1 First check the reverb Pot to verify that it is a linear taper part. If it is audio taper then the effect will be as you describe. Too much reverb early in the rotation. If that’s not the problem then a lower value pot could be used with a “makeup” resistor on top. However, this is not a common complaint with Princeton Reverb amps so I suspect something else is causing your problem.

#2 Try a 4AB3C1B model tank. You could borrow one from another amp if you don’t want to commit to buying one. Now that I’m thinking about this, you also need to verify that the existing tank is OK. If the dampers in the transducer assembly are bad then the decay time can be excessive and result in too much reverb.

#3 You could temporarily substitute a 1Meg Ω pot for the stock 1MΩ resistor that is connected between the 12AT7 grids and ground. This will be a drive control. It will work much like the dwell control on a three knob reverb unit. Adjust to taste and listen. If you like the effect you could pick a sweet spot and install permanent splitter resistors or add the pot the the rear panel or install an internal trim pot for future tweaking.

Cheers,
Tom

[Martin Beer]

Thanks Tom. I've just checked the pot value in the amp to confirm that I put a 100k linear in there, and I did. It's already surfy sounding at 10 o'clock, but it keeps increasing steadily beyond that. Setting the reverb halfway is getting overwhelming already, and at full it is absolutely outrageous. The reverb sounds good, if I keep it below 10 o'clock. I wonder if this is a difference between the MOD reverb tanks and an Accutronics? I'll try to figure out a way of testing the tank, but unfortunately I don't have another reverb amp to swap tanks with. I guess that as this is a home built amp, it would also be worth checking all my resistor values again in the reverb circuit.

[Tom Phillips]

The important thing is that you can get the sound you like. If you just need to set the reverb control low to achieve that then you are good to go. I find that most players set the reverb to ~3 or less. My reference is to the blackface style knobs marked 1 through 10 on the skirt. Since you are using the 10 o'clock reference I'm guessing that you are using chicken head pointer knobs or something similar. Your PR home built will have a little more reverb at any given setting because you are using a three spring tank vice the 2 spring unit that Fender originally used. I think there is a good chance that you amp is operating normally. Reverb settings of higher than 3 (10 o'clock) on all of the BF fender designs are in surf territory and anything over half way up is very intense and rarely used.

If you don't find anything wrong with your circuit then my suggestion #3 to add a dwell control would be a fun mod. Especially since there are no issues about modding a vintage amp.

Regards,
Tom

[Martin Beer]

Cool, it sounds like my amp is working normally then. I'm in the UK and haven't played through many blackface or silverface fenders, so I don't have much of a reference point as to how they would usually respond. I'm used to playing through B-list British amps like WEM, Selmer and Carlsbro, so the PR clone is a refreshing change.
All of my component values look OK, so I'll give the dwell control a try. I think I have a 1M pot in my parts box.

[riz]

I wanted second (third?) the notion that your circuit is working normally. On "10" it should be pretty outta hand! If it goes from zero to full-on, with a usable range somewhere in between, you are good. I also agree you might want to try a two-spring tank. But why not tweek to have an adjustment range that reflects your tastes, too? The dwell knob might be just what you are after!