1967 JMP PA 50 Watt Model 1963

whatdisay

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Fairly new to this forum but have been reading a lot of posts and am pretty blown away by the wealth of expertise in here.

I’ve been playing guitar for about 40 years, mostly on Fender and Gibson amps, but recently stepped into the Marshall realm with the acquisition of a ‘73 PA20 and then a ‘78 Park Bass 50 watt. Now I’m hooked.

I just pulled the trigger on a 1967 Marshall Super PA 50 W head (model 1963). The plexi PA models don’t get nearly as much discussion as the Lead and Bass models, so I thought I would take a stab at getting some input from 50W Super PA owners in here. I will post some gut shots for everyone to dissect whether modifications have been made or not, as soon as I have them. I already have a few shots from the seller, which I will post here in a minute as long as I can figure out how to do it.

Thanks in advance for any anecdotes on your experiences with this amplifier!

ETA: I know the filter caps have been replaced, as well as the power transformer (with MM Axiom P4550JT-G1). Output transformer and choke appear to be original.
 
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56tweed

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Looking forward to seeing the pics. I think there is plenty of love for the PAs. You can do a lot with them. I love how my '71 PA20 is setup currently.
 

tomsvintage

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Congratulations ! It would be nice to see some pictures :) You can also resize jpgs to 1024 / 920 pixels to load here.

Here is one of my favorites from 1964 :)



Great amps !!

Here is Joe Walsh with the James Gang in 1971 playing through one that looks like it may be similar to yours but maybe a 100 watter. One of the 2 amps here has a beer glass very close to the top vent :pops:



This video has the script logo 50 watt pa head :)

 

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playloud

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Looks like an early '67, judging by the tube rectifier and unperf'ed main board. Pre-"black flag"?

Is it just me, or is the cap on the presence knob unusually small? (Assuming it is indeed a 0.1uf - can't make it out in the pic.)
 

neikeel

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With the caveats you mention it looks very neat and well cared for.
Only odd things I see are both ch1 and ch2 mixers bypassed and that green wire typically the grid of ch1and ch2 going straight the the output of the treble cap.
I’ve modded a couple and set 1&2 up as an early lead amp (100pF on ch1 pot, bypassed mixer on ch1) that is it or the one I sold last year with 1&2 as early lead amp and 3&4 as later lead amp. If interested I’ll send links to thread in the ads section. I’m sure you will see how it sounds first!
 

kuli

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Nice amp!
Maybe the cabinet is not original to the amp, looks like a 100W cabinet, these amps came in 'small-box heads with large-box width'.
 

whatdisay

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Really appreciate everyone’s feedback. This is by far the most I’ve spent on an amp and it’s so difficult to know what’s been modified or not. I’ve got a great local tech who’s great at finding period-correct replacement parts for my other amps from the 50s and 60s so this Super PA will first be going to him to check out top to bottom. I want to make sure it gets set up to last as long as possible (and of course sound great).
 

TAZIN

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The amp is an early 1967 build and has seen some changes. The mains transformer been changed out to a Mercury Magnetics (P4550-G2 or G1 ??).
The bias caps have been changed and the circuit updated with a trimmer pot.
The two 8K2 1-watt resistors are newer Allen Bradley's and originally it would have had just one 8K2 resistor in that position.
Obviously new filter caps.
Added mixer bypass caps (silver micas).
Added Volume pot bypass cap.
New cathode bypass cap for V2a & b.
As neikeel mentioned, not sure what's up with the green wire near the mixer resistors on the main board. Normally that's the circuit tie-in point for the other two channels which utilize the other small turret board.
That long gray Dubilier signal cap is not a typical brand/style seen in these amps so it could be a replacement.
Added 1K/5w screen/grid resistors.
The Bulgin mains socket could be a replacement.
The signal cap on the Presence pot is a replacement...Looks like a incorrect value too...
Indicator lamp is a replacement.
One of the Fuse holders has been replaced and the other fuse holder has the thumb screw section replaced.
The head cabinet is not original and look like a repro based on a 100w cab due to the vents. Also notice the radii on the back panel cut out for the chassis.
 

whatdisay

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The amp is an early 1967 build and has seen some changes. The mains transformer been changed out to a Mercury Magnetics (P4550-G2 or G1 ??).
The bias caps have been changed and the circuit updated with a trimmer pot.
The two 8K2 1-watt resistors are newer Allen Bradley's and originally it would have had just one 8K2 resistor in that position.
Obviously new filter caps.
Added mixer bypass caps (silver micas).
Added Volume pot bypass cap.
New cathode bypass cap for V2a & b.
As neikeel mentioned, not sure what's up with the green wire near the mixer resistors on the main board. Normally that's the circuit tie-in point for the other two channels which utilize the other small turret board.
That long gray Dubilier signal cap is not a typical brand/style seen in these amps so it could be a replacement.
Added 1K/5w screen/grid resistors.
The Bulgin mains socket could be a replacement.
The signal cap on the Presence pot is a replacement...Looks like a incorrect value too...
Indicator lamp is a replacement.
One of the Fuse holders has been replaced and the other fuse holder has the thumb screw section replaced.
The head cabinet is not original and look like a repro based on a 100w cab due to the vents. Also notice the radii on the back panel cut out for the chassis.

Really appreciate this! Sent it all to my tech so he can go ahead and be ready to address the 8K2 resistors, green wire issue, and presence pot signal cap issues. I'd like to get it as close to original specs as possible.
 

stratburst

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This looks like one of several PA heads a dealer picked up locally in Nashville and was selling at the Amego’s guitar show last month based on the head shell and PT.
 

whatdisay

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. . . and the instant I flipped the switch from Standby to On the mains fuse blew.

Not the evening I expected!

So I got a new fuse and swapped out the EL34s and I now have glorious sound!
 
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whatdisay

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I'm happy to hear that you got it sorted !! It would be nice to hear it :)

Will try to grab some clips soon. Rolling a few different tubes today to see what I prefer.

Will be taking it to my tech soon to check out all the items mentioned above.

Channels I & II are bright and loud and punchy. Just fantastic. Channels III & IV are a bit muffled and not remotely as loud.
 


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