One day I dug through my pile of salvaged transformers and came across an interesting little piece. It’s an inter-stage percussion transformer from a Hammond M3 organ, part A0-24113-0. It is listed as T5 in the AO67, M100, and other circuits. It’s about 1.5″ x 2″, not very big. Hammond used it as an inter-stage signal transformer that also coupled in a low-impedance signal on a third winding. Everyone who has ever scrapped a Hammond organ has one of these sitting around, so there are plenty of them out there for cheap or free.
I fed it a little AC signal and took some measurements. It has roughly a 55:1 winding ratio between the low-impedance Winding 3 and the first two. It also has a fairly narrow frequency response, rolling off frequencies below 100hz and above 5khz. Searching around I found another popular transformer that has a 55:1 winding and the same frequency response – a tube reverb transformer! So I started thinking… Could the same triode drive a reverb tank AND act as an intermediate gain stage? If so it might be possible to build a 1-tube add-on reverb that would be easy to retrofit into nearly any amp.
The basic idea is that this circuit would get dropped into something like a Champ, Princeton, etc preamp just after the tone/volume and before the second gain stage. The reverb and mixing network mimics the Fender AB763 vibrato channel and should provide just about unity gain, so no major change to the amp’s character.
I built a quick-and-dirty proof-of-concept circuit with what scrap I had lying around and stuck it in my 6G2ish Princeton. The circuit works surprisingly well – it made no discernible difference in volume, gain or character. The amount of midrange in the reverb frequency was a little more than I usually like, but that could be fixed by changing the .0047uf coupling cap for something smaller, or reducing or removing the 25uf cathode bypass cap on the reverb recovery stage.
This little circuit would be pretty easy to apply to most tube guitar amps and wouldn’t take up much room at all.