Need pix of original 20W Marshall PTP boards

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.

 

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

 

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.

 

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?

 

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).

 

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.