AB763 Fender Alternate Reverb Arrangement

[Matador]

The stock 60's Fender Reverb topology has the reverb signal being injected in parallel with the 'dry' signal across a 3M3 || 10pF LPF network that is placed in series with the dry signal. This arrangement works since the dry signal path and the reverb signal path both undergo 360 degrees of phase shift thus can be summed across this network.

It's also pretty noisy: shorting across the 3M3 || 10pF network causes the background hiss to drop considerably (and yes, you lose the reverb), which got me to thinking: if I were building a single channel version of the circuit, the normal and vibrato channel are mixed together at the input to the PI through 220K mix resistors. If there is no dry channel, one just shorts one of the 220K resistors to ground. But what if we were to use it to mix the reverb in with the dry signal?

Schem is here: https://robrobinette.com/images/Guitar/Deluxe_Models/AB763_Deluxe_Reverb_Annotated_Schematic.jpg

One thing I notice is that injecting the reverb at the 220k mix resistor places the reverb signal out of phase with the dry signal, causing cancellation. However since we pass through the reverb transformer, there's an opportunity to add 180 degrees more phase shift by reversing the leads of the reverb transformer, and which point we align back with the polarity of the vibrato channel, and in theory this should work and be far less noisy.

Has anyone tried this? I've attached the 'stock' topology and my 'proposed' topology.

 

[CJ]

There is no phase with the reverb signal , the springs turn it into one big blob.

 

[Matador]

I only ask because I input a 400Hz sine wave, and scoped the phase at various points, and the reverb is definitely out of phase with respect to where the normal and vibrato channels are mixed.

But are you saying in reality, it can be mixed in 'anywhere' along the signal chain where the gain staging makes sense?

 

[CJ]

You might notice a difference if you flip the phase with the transformer on a steady signal of 400 hz, but when you play a guitar signal I don't think it will make much difference, try and see.

Since those springs have a delay time, your guitar signal phase will come out of the pan at different angles depending on what note you hit.

But I get wanting to avoid the 3.3 meg / 10 pf network. Almost all of your signal is going through that 10 of cap as Xc is only 40 k ohms compared to 3 meg.

Dang it I had the best sounding amp, it was an epiphone tube amp chassis with a diy circuit, I wanted to add reverb so j spliced something in there, the problem was that when you turned the reverb knob up, the gain went up, but there was a point there where the overdrive and the verb sounded awesome. So what do I do? Tear it out to try something different and I lose the magic tone and I fic not bother to write a schemo so the tonf is gone forever.

 

[merlin]

If there is no dry channel, one just shorts one of the 220K resistors to ground. But what if we were to use it to mix the reverb in with the dry signal?

You can do that. It's not really different from the stock arrangement except you have an extra tube stage in the dry path before mixing, which is kinda putting the cart before the horse. The reason Fender had to use that noisy 3M3 resistor is that the wet signal is very small compared to the dry, so he had to knock the dry signal level down a lot. If you put the extra gain stage in the wet recovery path instead, you can use smaller mix resistors and hence less noise. Some amps even use the two grids of the phase inverter for mixing wet and dry.

As CJ said, phase is irrelevant. Sinewave testing is misleading because signals appear coincident when they're actually being delayed.