PR-20 Converted To PR-40

I have a PR-20 that I've converted to a PR-40.

The main difference between the two is the power supply. The PR-40 has two rectifier tubes, while the PR-20 has only one. Several of the resistors in the power supply are different as well. There are a few other differences as well. I can go into more detail if someone expresses interest.

I've heard that many of the "Lounge"-type organists of the sixties used to swear by the combination of a PR-40 and a Leslie.

While, personally, I certainly wouldn't trade my Leslie for a PR-, I think the combination of the two is quite nice. The PR-40 seems to have a much deeper bass response than a 122. Though this depends quite a bit on the placement of the open-backed cabinet! I've found that placing the PR's back against the wall with a 1 or 2 inch gap between wall and cabinet is best. The cabinet response seems quite dependent on the wall-to-cabinet gap.

--JeffDairiki

The Differences

For those who are considering undertaking this task, here's a copy of my (terse) notes regarding the differences in components and values between the two models. --JeffDairiki

1. Add V14 5U4GB to power supply.

2. Adjust gain of treble channel.

   R11, R14: 220K -> 150K
   R12, R13: 150K -> 180K

3. Reverb recovery changes:

   R56: 8.2K -> gone

   More gain:
   R54: 180K -> 220K
   R59: 470 -> 390 (more gain)

   Less gain for bass than treble:
   R55:   33K -> 15K
   R55A:  n/a -> 18K

4. More gain for reverb channel when treble reverb off:

   R68: 180K -> 150K

5. For stability?

   C37A: n/a -> 150pf

6. Bass gain & cutoff?

   C13: 2200pf -> 1500pf
   R30: 560K -> n/a
   C14: 1200pf -> n/a

7. Room size switch

   R32, R39: 390K -> 270K
   R33, R38: 680K -> 220K
   R34, R37: n/a  -> 150K
   R35, R36: n/a  -> 330K
   Switch: DPDT -> DP4T.

8. Whole feedback scheme of bass final stage changed:

   R45, R46: 330K -> 47K
   R47: 10K, 1W -> n/a
   R48: 130, 3W -> n/a
   C23: 2200pf -> n/a
   C23A: 100/25 -> n/a
   R47, R48: n/a -> 270K
   C20, C22: 100pf -> 1000pf
   R86, R89: n/a -> 47K
   C51, C54: n/a -> 1000pf
   R87, R88: n/a -> 270
   C52, C53: n/a -> 100/25

9. Power Supply

   R77: 500, 5W -> 100, 5W
   R79: 1K -> 5.6K 1W

 HammondModifications

 ToneCabinets