Swart Space Tone - Reverb reduces when volume past 70%

[Garrett335]

Good morning!
I have a question regarding a Swart Amp (Space Tone) Single ended amp I'm working on.
The amp appears to be functioning properly except the fact that with the reverb level progressively decreases as the volume pot is raised past 70%.
Could this just be a design flaw?
This being a boutique builder there are no schems available, I may draw one out as it's a pretty simple S.E. circuit. (Attached photo of the guts)
Thank you!

[sluckey]

The amp appears to be functioning properly except the fact that with the reverb level progressively decreases as the volume pot is raised past 70%.
Could this just be a design flaw?

Impossible to say without a schematic.

[Garrett335]

Okay I did my best to draw out the preamp and reverb circuits, does this help?

[pdf64]

From the schematic, I think what you’re describing is normal behaviour for that circuit. It’s not really a design flaw, more just the way it works.
Kinda akin to the interaction between the volume controls in a 5E3.

[sluckey]

I don't think the volume and tone circuit is drawn quite right. But regardless, the drive signal for the reverb splits off ***BEFORE*** the volume pot, so as you increase the volume of the dry signal, it becomes greater, but the reverb signal is still the same. So the dry to wet signal ratio becomes greater, causing the wet reverb to sound weaker.

[pullshocks]

That’s some coooooold bias on V1A

[roarshock]

I've fussed with a few similar circuits. For the feeling of a little more reverb, you can experiment with what I believe to be a 100k mixing resistor right after the tone and volume. You might enjoy a 220k or a 470k to tamp down the dry signal, or it might mess with the magic of Swart sound. Your ears will tell you!

[Garrett335]

Yeah I messed up on the tone stack (Whoops ha)
Thank you for your insight on the circuit Sluckey, PDF64, Roarshock.
Just curious, what would happen if you moved the reverb input V2A Grid from before the tone stack, to after it?
Thanks again!

[Bieworm]

Okay I did my best to draw out the preamp and reverb circuits, does this help?

those cathode resistors on the preamp are 1k5, not 150k of 150R (I can't read what you wrote)
What interests me more is the reverb circuit. It's incomplete... but those Swart amps have a nice 1-tube reverb and I wonder why. They are capacitor driven I think?

[pdf64]

… what would happen if you moved the reverb input V2A Grid from before the tone stack, to after it?

The max reverb level that you currently get with the volume up full (not much?) would become about the max reverb level available at any volume setting.

[Garrett335]

… what would happen if you moved the reverb input V2A Grid from before the tone stack, to after it?

The max reverb level that you currently get with the volume up full (not much?) would become about the max reverb level available at any volume setting.

Thanks so much PDF!

[Greenwichpaul]

This is a known phenomenon. Kinda a feature rather than a flaw. Some folks like the fact you can turn the volume right down and have the reverb only. I am guessing your circuit actually couples tp the reverb tank via capacitor?

This article - an add-on reverb based on Swart - mentions how the Swart reverb approach is basically the Ampeg approach

https://guitar.com/guides/diy-workshop/build-tube-spring-reverb-unit-amplifier/

[pdf64]

… I am guessing your circuit actually couples tp the reverb tank via capacitor?
…

The photo in the OP shows a ‘red drop’ cap wired between the V2 valve socket ((presumably the anode of the reverb driver triode) and an adjacent RCA socket (presumably the reverb send).

[PRR]

...the reverb level progressively decreases as the volume pot is raised past 70%....

At mid-resistance the VOL pot wiper impedance is like 250k. At full-up, less than 60k looking back into the first tube's plate.

The wet signal comes through >220k.

So at midway on VOL the wet signal is like 1/2 of the recovered signal, at full-up VOL it is like 1/5th. Enuff drop to notice. Ya, 70% rotation could be the 'edge of the drop".

I say it is what it is and it isn't musically wrong. If you need Total Malibu Surf at high VOL settings, you probably want Dick Dale's BIG Fender. The Swart may be more of an all-arounder.

[Garrett335]

… I am guessing your circuit actually couples tp the reverb tank via capacitor?
…

The photo in the OP shows a ‘red drop’ cap wired between the V2 valve socket ((presumably the anode of the reverb driver triode) and an adjacent RCA socket (presumably the reverb send).

You are correct sir, that cap is connected to the send, thank you for all the input and help with this. The owner of the amp was cool with the explanation.