Fender Ab763 reverb circuit vs 78-80 Silverface reverb circuit

[Toxophilite]

I was noticing some differences between these two reverb circuits:
Mainly (ignoring the Master volume switch stuff)
- 680 ohm cathode resistor for the 12AT7
- no cathode bypass cap but a 560 pF cap from the cathode to the plate of the 12AT7 going to the reverb transformer


- a .002 cap in parallel with 220K resistor going to ground of the 12ax7 recovery stage grid
- After the 470K resistor coming off the wiper of the reverb pot,,instead of a 220K to ground on the wiper of the there's a 470K


Can someone explain how this affects the reverb ?


I really like reverb on my master volume super (especially on 10 for a faux sitar sound I use for a solo in a middle eastern-esque song)


but I also enjoy the AB763 reverb, maybe not as much cranked but at low levels just as much


thanks!

[sluckey]

Those changes may be more about the "Pull Boost" function than the actual reverb. Tweaking levels, hiding hiss, etc.

[pdf64]

See http://el34world.com/Forum/index.php?topic=23084.0
I think that the unbypassed cathode helps to mitigate unpleasant clipping in the dry signal path that otherwise occurs at the V3 grids; lower settings of the master volume in many SFs makes this characteristic apparent.
The small 'plate to cathode' cap probably adds some local NFB, cutting the high frequencies a bit, and so yes, possibly making the pull boost a bit sweeter.

Re the 2nF cap to 0V at V2 pin2, I seem to remember that the old accutronics site recommended a bit of capacitive loading on the tank output, to provide a more linear freq response.

Re the 470k grid (compared to 220k) grid leak resistor at V4 pin7, I think it will ~double the max available channel gain, which, by increasing the preamp overdrive available, may be beneficial in a master volume preamp.

[2deaf]

- a .002 cap in parallel with 220K resistor going to ground of the 12ax7 recovery stage grid

Fender started doing that in the sixties, long before the ill-advised boost thing.  Other manufacturers were also using these capacitors decades ago.

The tank output coil and the capacitor in question have a resonant frequency.  At the resonant frequency, the coil puts out its maximum signal.  In this case the resonant frequency is between 5KHz and 6KHz depending on how they wound the coil that day and the actual capacitance versus the labeled capacitance.  The frequency response looks like a parametric equalizer with a sharp peak at the resonant frequency and trailing off on both sides of the peak.  The result is a brighter sounding reverb.