Need pix of original 20W Marshall PTP boards

[loverocker]

Anyone able to post photos of PTP boards from the original Marshall 20Wer?

I just picked up a '72 PA version and it's had some work done (e.g. changed to 220K mixer resistors) and seems to have the "other" PI arrangement (the PI variant that's drawn in on the schematic on p212 of the Doyle book).

I'd quite like to compare the circuit with other amps.

 

[saros141]

can't post an image in this thread, don't know why... anyway, here's a link

 

[loverocker]

Thanks - that's perfect. Interesting - yours also has the 220K mixers. Maybe it's a factory variation? What year is yours?

 

[saros141]

Sorry, it's not my amp, just a picture I saved. Not one of the earliest ones, with the bulgin on the front... it says JMP on the front and P.A. 20 on the back. Can't quite make out the serial number. Let me know if you'd like to see the other pictures in the series.

There was a first version 20W (that the store persisted in dating at 1965) here in Toronto for the better part of a year, it made its way down to $2kCAD and the consignor just gave up and took it back. I was itching to give it a try but never did, maybe someday I'll get a chance.

 

[fullerplast]

I believe the original PA20 had 680K mixer resistors. FWIW.

Just for kicks, here's the reissue 20W (non PA), pretty similar, but notice the extra cap...

20W reissue

 

[loverocker]

No probs, saros. Yup - I think that's a later one, too. I've just had a bunch of pix emailed to me and most have 220K mixers and the 220K resistors around the PI (whereas most of the schematics in Doyle show the single 1M instead). Maybe Marshall changed the circuit pretty early?

Thanks fullerplast That's an interesting comparison. The original Lead & Bass also seemed a very minor variation from the PA model. I see it still has the shared cathode cap on V1, too.

 

[Swarty]

There is very little diference between any of the various models...just a cap added here or there to make them brighter. I have a PA and a bass model one is a '69 and one is a '70 One has one 1 cap on top of the chassis and one has 2. ...they are great little amps.

 

[loverocker]

I just had some interesting email from a friend who's had/seen several early ones, plus I looked up my Mullard circuit book. It seems there was a lot more variation over time than there is between the PA/Lead/Bass versions:
* early ones had that cathode-coupled PI (with the 1M resistor - as shown in Doyle). The Mullard book says that this has about half the gain of a LTP - I think it's the only time Marshall used this type of PI
* they have screen resistors and higher value grid stoppers. I'll have to re-open miune, but I don't think there were any screen resistors at all!)
* several cap value changes

This guy's visiting soon and I'll ask him to bring over his '67 for an A/B test with my '72. The reissue looks as if it has the same circuit design as the later models (thinks... does it have a plexi front panel or metal panel...?)

Swarty - so your '69 has two fat filter caps on the PTP board? Does it also have two 100ohm screen resistors wired directly on the EL84 sockets?

 

[Swarty]

It has the 2 fat filters on the board, but I don't recall any screen resistors hanging on the sockets.

 

[clay_finley]

Here's a '71 PA 20:

 

[loverocker]

Thanks Clay. I'm guessing that the selector on the back is for mains voltage? Mine and quite a few others I've seen don't have them (sold for the UK, I guess).

 

[Blue Strat]

I had one of these on my bench a few weeks ago. This one had 220K mixer resistors as well. I'm beginning to wonder if any of these were built to the published schematics.

 

[loverocker]

Me too. I've been emailed a pic of an early model with 100K mixer resistors (plus the 1M cathode-coupled PI and a few other cap value differences). It's very early, though - a plexi with 4-digit serial number.