12AU7 -> small PT as SE OT -> spring reverb?

[Bergheim]

First, thanks for updating with results & revised drawings. I find lots of dead-end threads frustrating and it is nice to know how things turned out.

I think the clips sound really good, a result I would be pretty happy with.

I would encourage you to fool around with the send/return settings & play the dwell against the output & explore those changes. I have added a dwell to my last few amps & you can get any number of usable settings between the two. So far I have used pretty much stock Fender circuits, so these alternates are pretty intriguing.

Your drawing lists the tank # which denotes an 8 Ohm input. Is that what you used in the end? (first post mentioned a 150 Ohm tank) EDIT: Nuke beat me to it..

Thanks, yeah I think it turned out pretty sweet in the end. The dwell pot changes the reverb character quite a lot, mainly taking the surf splatter and sustain out of it. Nice when you want a more subtle trail effect, but the added noise due to less signal from the tank and therefore having to turn up the reverb level keeps me setting the dwell as high as possible and adjusting the level accordingly.
The recovery circuit is considerably noisier than the dry channel, even with the recovery stage input grid grounded, although the reverb tank adds a bit of noise as well. The noise within the circuit is mainly rectified hum and hiss. Adding plate bypass caps and cap to ground don't really change a lot, other than tone.
Makes me wonder if an OP-amp recovery stage is the way to go, for lower noise at least.

[lonote]

Thanks for adding that, I wondered about the recovery noise & was hoping to hear that that your 2-stage recovery circuit might have mitigated some of it.

I have found the reverb recovery noise to be the perpetual problem child in the amps I have built. Very typical amounts, but it would be nice if they were quieter.

And you are right about having the dwell lower & the recovery higher, it does get noisier quickly. Always a trade-off I suppose.

[Bergheim]

I wasn't satisfied with the noise level in the recovery circuit, so I did some changes:
- Fully bypassed (22uF/2.7k) the cathode on both stages. Better power supply noise/hum rejection.
- Removed the 68k input grid stopper on the first recovery stage. Less series resistance in the signal path, less noise.
- Switched the 330pF grid-to-ground cap on the first recovery stage to a 1nF plate-to-ground cap. A grid-to-ground cap after an inductor creates a resonant peak that increases the oscillation risk. Also, it doesn't remove any noise created in the gain stage itself. A plate-to-ground cap solves both these issues.
- Lowered the plate resistor values on both gain stages to compensate for the increased gain from bypassing the cathode on the second stage, as swell as being able to get rid of the 330k/330k interstage attenuator. Less series resistance, less noise. As for noise in the plate resistors themselves, I used a resistor noise calculator to compare the noise level versus gain with different resistor values. As far as I can see, the noise and gain are proportional to each other, so reducing the plate resistor value doesn't inherently improve the S/N ratio.
- Changed the 1M reverb level pot to a 25k(!) pot. Yep, 25k. Despite halving the plate resistors, I still had too much gain after removing the interstage attenuator and bypassing the second stage's cathode. A 25k pot dropped the gain of the second stage just enough to get to the same output level as before.
- Changed the coupling cap on the second stage to 33nF to retain the same ish LF roll-of frequency as before. A 22nF would be perfect, but I was out of stock.

I've played the amp for a couple of days, and so far I'm really pleased.
Overall, the recovery circuit sounds a bit cleaner and definitively less noisy than before. If I should guess, with both channels dimed, I'd say the reverb circuit is 3dB noisier than the dry channel. The output level and tone is pretty close to what it was before.

Bergheim Rødampen 2025-04-26.png

[lonote]

Thanks for updating.

I have a new-to-me reverb circuit I am about to try out, these ideas will be useful in tweaking for recovery noise.