Name:
         
         Scott 
  E-Mail:
         nnswartz@inlink.com
    Date:
         10/1/99 8:20 PM
 Subject:
         Reverb Circuit Alternative







All,

I posted this over on open forum, but to give everyone a chance to see it I'll
put it here also. The subject is the tone of the standard Fender reverb circuit
and the impact of that 3.3M series resistor that it uses. 

I read the qoute by Whit in a post below:

    This seems like a better approach, since you don't end up trying
    to stuff your carefully tweaked tone thru the 3.3M resistor on
    it's way to the PI.

and I am in complete agreement.

Several years ago I developed a alternative circuit to combat this. I should
start by saying it is based upon on a circuit in Dan Torres book, Inside
Guitar Amps, for an effects loop that splits the signal using a phase splitter
biased by a tap on the cathode resistor (like a Princeton PI). It uses 2 1/2
tube positions as follows:

first tube: 12AU7
1/2 tube is phase splitter
1/2 tube is gain stage for pan driver

second tube: 6K6 or EL84, could be 12AT7 like Fender
This tube is the pan driver

1/2 tube: 12AX7
This is the recovery amp from reverb pan

The output of the phase splitter plate couples to the gain stage which
couples to the pan driver.

The signal from the recovery amp and the in phase signal from the cathode
of the phase splitter are routed to a 1M mixer pot and the output of the mixer
goes to the PI.

The dry signal therefore sees only the resistance of the pot, or 500K when
the pot is at its midpoint.

A pot is placed before the pan driver tube, giving a "dwell" control.

The complete circuit is approximately unity gain.

This circuit gives very good control over the reverb tone and the dry signal
is unchanged due to the massive headroom of the phase splitter (it can
easily swing 30 Vrms).

Feel free to experiment with this circuit concept as you wish. A
contribution to the discussion for the good ideas I have gotten.