R.G. Keen Swell Pedal

[anyuser00]

Hello, I can't seem to find the page on RG's site that talks about the swell pedal that will work with some tube amps.  I think it did some kind of DC offset or something.  I am trying to figure out if it will work with my amp.

[Arno van der Heijden]

It's the Prescription Electronics Experience pedal. Do a search in the archives for 'swell' and 'experience' and you'll get a lot of hits.

From R.G. Keen:

I should have saved my comments on this when I originally posted them. The "swell" circuit on the Prescription Electronics pedal is exactly the same circuit as a 1968(about) article in Popular Electronics by Anthony Leo about building an entire distortion circuit, but it's modestly mis-used to take the output at a place that has a DC level that starts high and drops to about zero over a bit of time. It's also DC coupled out, so the DC level is transferred to the input of the amp that you plug it into. This DC level pushes the input bias of amps that are DC coupled at the input into not carrying signal. Since it's a one-size-fits-all kind of thing, it only works right for amps which have

(a) DC input coupling
(b) about a 2V range before being pushed into saturation or cutoff on the input
(c) NO DC coupling to the speakers!!! Do this on a solid state DC coupled and and you'll fry a speaker.

Many tube amps have no input coupling cap, about a 2V range, and are AC coupled, so they work OK. SOME solid state amps are close enough, and can work OK. Some don't work well at all. Oh, yeah. The "swell" circuit only works right when you have very fresh batteries. Older batteries don't make the happy-accident level changing work right. When it works, the volume on each note starts quiet and ramps up to full level, kind of an attack delay. The Experience Pedal is just the Foxx Tone Machine circuit followed by the A. Leo Fuzz circuit, and some very, very complex switching.

[anyuser00]

I did a search that's how I found it again, but I remember reading I thought on GEOFEX but can't find the article now.  I remembered it explaining it very clearly that's why I'm looking for it.