Support HammondWiki. Donate!
PayPal

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

TopicModification

The content of this page is Copyright (C) 2000, 2001, 2002 Geoffrey T. Dairiki and the other authors of the content, whoever they may be.
This is free information and you are welcome redistribute it under certain conditions; see http://www.dairiki.org/HammondWiki/opl.html for details.
Absolutely no warrantee is made as to the correctness of the information on this page.