1967 JMP PA 50 Watt Model 1963

[neikeel]

Looking at the board it looks like channel 1&2 bypass the tone stack!
With both 1&2 mixers bypassed you will find 3&4 relatively flat, although I see 3 has a volume pot bypass cap.

 

[whatdisay]

Looking at the board it looks like channel 1&2 bypass the tone stack!
With both 1&2 mixers bypassed you will find 3&4 relatively flat, although I see 3 has a volume pot bypass cap.

Interesting. The tone pots are definitely active for Channels I & II. But (unlike my ‘73 Park 50W Bass), changing the volume pots of non-engaged channels does nothing to alter the channel I’m plugged into.

 

[whatdisay]

Finally getting a chance to take this 50 W PA to my tech, and I think I’m going to have him set it up to where two of the channels are 1986 specs and the other two 1987.

 

[Matthews Guitars]

Nice score! Bet it wasn't cheap!

I'm just going to point this out, not suggesting you do it...but PA heads were popular with modders because you've already got extra controls and a whole extra preamp tube and board to go along with it, just sitting right there waiting for you to do something with it. If you're looking for a high gain mod, you're staring at an ideal candidate for it. And with care, any such mod can be made 100 percent reversible.

That's assuming you don't actually have need for all four channels....which is typically the case.

Sure, many people would wish to preserve it as close to original as possible. But its purpose is to make a guitar sound GOOD, right? So...particularly given that it's already had a transformer replaced, true originality is already gone.

I'm not saying to mod it. I'm not saying not to. I'm saying it's an ideal platform for some mods if that's what you want to do.

 

[Matthews Guitars]

I'd like to ask the owner to please go to amparchives.com and register and then upload these photos (at least...maybe more) to the site. They'd be a very good and welcome addition to the Marshall amp photo database.

 

[Pete Farrington]

I think I’m going to have him set it up to where two of the channels are 1986 specs and the other two 1987.

Those models have different arrangements for the input stage’s cathodes, yeah.
But they’re also different at V2 cathode, tonestack treble cap and slope resistor values, NFB and coupling caps to the output valves’ control grids.

 

[neikeel]

I did this to my 1963 Super PA with KT66s:

V1 is set up as a 1987, V2 is set up as 1986, but as you see different caps on the pots for different flavours.

 

[whatdisay]

I appreciate everyone’s feedback!

Still pondering how to set it up. I was certain I wanted it to be half 1986 and half 1987, but as I already have a ‘78 Park 50 watt Bass, it might make sense to go with a 1987 & 2204 flavor setup.

 

[Matthews Guitars]

Why not use half of that extra preamp tube to turn one channel into a 2204? That totally leave the 1987 circuit alone. You just add the cascaded gain of the 2204 in one channel. I'm thinking of ways to do that to one of my own amps. But it doesn't have the luxury of eight input jacks to play with. Just the regular four. I'll repurpose channel 2's low gain jack to be the cascaded input.

 

[whatdisay]

Why not use half of that extra preamp tube to turn one channel into a 2204? That totally leave the 1987 circuit alone. You just add the cascaded gain of the 2204 in one channel. I'm thinking of ways to do that to one of my own amps. But it doesn't have the luxury of eight input jacks to play with. Just the regular four. I'll repurpose channel 2's low gain jack to be the cascaded input.

Ha!

 

[Matthews Guitars]

When I started to write that reply, your recent reply wasn't visible yet. I got distracted by other business and hit the POST REPLY button after you'd posted yours. You don't have to be jerky about it.

 

[playloud]

I don't think he was being a jerk about it. More of a "great minds think alike" situation!

 

[Matthews Guitars]

Funny, I was thinking kind of the same thing...but covering all the bases, too.

 

[whatdisay]

When I started to write that reply, your recent reply wasn't visible yet. I got distracted by other business and hit the POST REPLY button after you'd posted yours. You don't have to be jerky about it.

My “ha” comment was only my own laugh that you and I apparently came to the same idea at the same time. Zero offense meant.

 

[Matthews Guitars]

Gotcha.

It's a cool amp. I'd feel like it would be no sin to reversibly mod it to make it a more useful amplifier.

 

[whatdisay]

So I finally got to drop the ‘67 Super PA off at my tech’s, and after he addresses some the items you fine folks mentioned in this thread, he’s going to set it up so the channels I & II will be 1987 specs and channel III will be 2204 specs.

Can’t wait to try it out when he’s done! Thanks again to everyone who chimed in.

 

[whatdisay]

Update:

I need to confess right off the bat that my technical knowledge of amps and amp-speak is terrible, but I do feel a bit compelled to provide an update on what my amp tech did to this '67 PA50 in order to turn it into my favorite of all my (25+) amps.

So many of you provided super-helpful feedback in this thread regarding the condition of this amp when I got it, and I passed all of it along to Steve (tech). To make the changes I describe below, he ended up removing and preserving what I'm calling the "small board" from the amp in order to preserve some of the caps on it (I'll post a pic of it if I can remember to).

Again, I'm sure I'm not using the perfectly accurate terminology here, but what Steve basically did was: (1) take Channels I and II and turn them into a 1987 50 watt Lead and then (2) take Channels III and IV and turn them into a 2204. The result is just astonishing. I feel like I can get any tone I need, from super-clean to thick, harmonic distortion, simply by selecting one of the 3 channels (4th is blocked off to be part of the two gain stages for Channel III) and tweaking my Power Station volume a bit so I don't blow my ears out (sometimes I run Channel I or II without the Power Station at all, when not needing gainy tones).

I will eventually put together some clips, I just wanted to pop in and say I can't recommend strongly enough what a gold mine a PA50, with its 4 channels, can be, in the hands of the right tech. This has every Marshall tone I would ever want, and it's warm and punchy and perfect.

Big shout out to Steve of Farrell Amplification Just a solid, brilliant guy who knows what he's doing, takes the time to find and absorb the original schematics and thoroughly explain what he's doing/not doing and the expected outcomes of same.

 

[Matthews Guitars]

Awesome work!
Now, could I ask you for a favor?

I make reproductions of the front and rear panels for many older Marshall models. And the super PA is one type I don't have information on.

I'd like to get the dimensional information on your amp's front panel that will let me start designing replicas of it.

The simplest way to do it would be to lay an inch scale ruler alongside the front panel and malke photos, so I can see the total length of the front panel and the hole locations with a reasonable degree of accuracy.

The dimensions are almost certainly all going to be fractional inches, not metric. Marshall always used Imperial fractional measurements in the 60s and 70s and I'm not sure if they ever changed over to metric for that.

 

[Whatwhatringrang]

Cool amp there. Pa heads rule and cabs do also. I also love the pa cabs except currently putting sealed backs on them . A bit Less money ,more stuff in them win win. Very little part count to do whatever you want to them and or disconnect the extra stuff. All old parts pretty much still available to mod. They sound great as is also Win win .

Maybe even less likely to be faked or parted together like a regular model.

Here are pics of some except they are 100’s.
!

7 numbers away from 7026 to the right here which has a Merren replacement PT. Everything else stock. I have the original working original European/U.K original pt for that amp. However I like how it sounds so much that I do not see my self putting back in there in there. Maybe I will give it try as I have the old kt66 too get away with it.That amp is probably my favorite of any I have played over the years for certain stuff. There are a couple hundred number serial more recent pictured in here .This had a couple of bad carbon comps and other small things in the pre amp . That prevented it from sounding good.So when I got it sounded sick . That was a easy fix . I have a feeling the tech who replaced the filters probably did not need to . When you flip these amps so the guts are facing you I always tighten the contacts with the one fuze holders on the floor side. I noticed it sparked in the dark from being upside down to loose of contact. This tech thought it was something else with filter caps but I guarantee if was that fuze holder. I know because the I seen a video of him Magooing it .

right here you can see the little sticker marks over the 20 on the speakers. They fell off . The other two have the older style real 20 watts.

 

[whatdisay]

Awesome work!
Now, could I ask you for a favor?

I make reproductions of the front and rear panels for many older Marshall models. And the super PA is one type I don't have information on.

I'd like to get the dimensional information on your amp's front panel that will let me start designing replicas of it.

The simplest way to do it would be to lay an inch scale ruler alongside the front panel and malke photos, so I can see the total length of the front panel and the hole locations with a reasonable degree of accuracy.

The dimensions are almost certainly all going to be fractional inches, not metric. Marshall always used Imperial fractional measurements in the 60s and 70s and I'm not sure if they ever changed over to metric for that.

FYI my amp is not in the original cabinet. If you're just wanting the dimensions of the actual panels, i.e. after I pull it out of the cab, I'm happy to do that.