1965 Marshall JTM 100 Super PA Plexi Model 1968

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You might think, "A Marshall PA - hmpf, who needs it?".  I can see it easy to think that a PA head might be just for mics or keyboards, but let me tell you - this is as magical a guitar amp as anything out there.

This Marshall Super PA appears to be a Version 3A - among the earliest post-prototype 100 watt Marshall heads (late 1965/early 1966), right after Version two (mid/late 1965) with its dual output transformers.  I've come to this conclusion by reviewing Amp Archives as well as "The History of Marshall" by Michael Doyle and Nick Bowcott.  

But first, some back story.

"One of the main customers at that time was the Who, who continually asked for more and more power.  So started the attempts to make a 100-watt amp, but due to the limitations in transformer supplies, and our personal lack of experience (remember - in 1964 we were still only third-year City & Guilds students), the first units were in fact a standard preamp with 2x 50-watt output stages on the same chassis.  However, this soon changed.  All new models produced at this time were designed by tweaking a bit here, pinching a circuit there, and different cabinet configurations.  By introducing the EL34 valve and the correct balance of output transformer and feed voltages, the 100-watt amp was born."  "The History of Marshall" by Michael Doyle and Nick Bowcott

Getting to this amp - what you see first when you check out the chassis is the incredibly girthy power transformer.  This Hammertone Radio Spares unit is present from the very first 100 watt prototyes through early 1966, when a Drake 1204-43 power transformer was introduced. 

The output transformer appears to be the Drake 1202-84, as used in the earliest single-output transformer version (version 3A).  The choke could be the Drake 352-114 as used in version 3B as opposed to the Radiospares 20H from version 3A.  I'm not certain of this as the only cosmetic difference I can see between the two is the coloration and shape of the strap that holds the transformer to the chassis.  I will consult my amp tech to see if I can determine which it is by checking the impedance (the Drake is 110 ohms and the Radiospares is 690 ohms).  Measuring the screen voltages would be another indicator, as the higher impedance Radiospares choke leads to lower screen voltages.  It is likely that the choke is the 3A version's Radiospares 20h, however, and my judging what it is by looking at the color of the strap is foolhardy. 
The transformers are all undeniably original.

Either way, it seems that this is fundamentally a 3A version, which avoids the higher voltages present in the 3B version, which is known to eat modern tubes.  

Cosmetics absolutely confirm that it is an early version, particularly the two Vox style vents on the cabinet, a feature that changed to a single vent.  Furthermore, mustard cap date codes are from the third quarter of 1964.
The History of Marshall book states that Version 3A has serial numbers ranging from 6600 through 6800 and 3B's serial numbers range from 6600 through 7100.  That said, serial numbers are the worst way to date these amps as they were not applied in any discreet order.

This amp was part of the deep collection, one of the most prized pieces.  It is in truly exceptional condition compared to others you see from later years - you just don't see any from '65.  The circuit is nearly untouched with only sparse maintenance updates.  There are no unruly mods, no extra holes drilled in the chassis, nothing destructive as you see in so many vintage Marshalls.

The cabinet and tolex is similarly original and clean.  There may have been some minor work done around the logo at one point.  I will add some photos of this area.  Everything else, from the feet, to the vent grilles, to the handle - it is all original.  The screws that hold the chassis to the cabinet - they are mismatched. Two of them appear to be original.  There are only two screws holding the back panel on the cabinet.

The aluminum chassis that these early amps are built upon take some abuse from the weight of the transformers and careless handling.  This one is in great shape, showing some distortion but nothing debilitating.

In order to spare the amp from potential issues, we have used the amp only once, running it on a variac at 115v.  This is a nod to the fact that original caps remain throughout.  Regular use will require replacement of electrolytic caps and a thorough check of performance there after to identify any other maintenance needs.  I am of the mind to keep the amp as-is, at least for the time being.  Service could be arranged for the next owner if that person does not have a trusted amp tech to tackle the job.  

When we did use the amp, holy wow - it was a feast for the ears as you might imagine.  There is a character to plexi amps that is unmistakable.  This amp has IT - huge, bold cleans that are unparalleled to the sickest output distortion.  Somewhat surprisingly, the amp runs pretty quiet.  There is a variance between the tone you get from each of the four channels.  This is very likely something that is caused by drift in components in each channel, that would be ironed out when serviced.  Channel 3 is the star, and it sounds amazing.  The  tone and presence controls are effective and give a great range of variability.  

This is a good comparison of Marshall PA heads versus Lead and Bass models:

The earliest plexi panel JMP's like the Super Lead, Super Bass and Super PA where electronically the same:
- 820R/330uf shared cathode on V1
- 22nF coupling caps after V1 on both bright and normal channel (on PA's they are all normal channels)
- first use of 470K mixer resistors
- 250pF/56K in the tone stack
- 100nF coupling caps in the power amp
- 27K negative feedback resistor on the 16 Ohm tap

The minor differences on those amps were that the:
- Super Bass had an 470pF treble peak capacitor on the mixer of the bright channel
- Super Lead had an 470pF treble peak capacitor on the mixer of the bright channel plus a bright cap of 100pF on the bright channel volume pot
- Super PA had an additional preamp tube with also 2 channels; all channels of PA's are equally voiced
Super Leads and Basses are 2 channel amps with 4 inputs:
- Normal high sensitivity input
- Normal low sensitivity input
- Bright high sensitivity input
- Bright low sensitivity input

Super PA's are 4 channel amps with 8 inputs:
- 4 high sensitivity inputs
- 4 low sensitivity input
Because of the additional 2 volume pots an 2 mixer resistors into the summing amplifier of V2a in a PA version, a somewhat different voltage divider circuit is created.  

We will very carefully pack and ship this amp in two double-boxed packages.  The chassis will be bolted to a piece of plywood.  Foam will support the transformers within an inner box.  That box will be very carefully floated within a large outer box with copious amounts of appropriate packing material.  The cabinet will be similarly packed.

The amp is sold as-is, as are all vintage amps.  

Case/Cover?: No
Color: Black
Dimensions: W28-3/4" x H10-1/2" x D8"
Weight: 46 lbs
Watts: 100 Watts
Date: 1965 based on design and components/cap date codes
Filter Caps: Original
Pots/Codes: Original - codes not visible
Tolex/Tweed: Original
Grille: Original
Knobs: Original
Handle: Original
Power Cord: Original end on chassis side, replaced cable
Notes:

Shipping policy:
Shipments will require signature on delivery, no exceptions.  If there is damage, the buyer must retain all packing materials intact and picture them immediately and send them to sales@maharsvintageguitars.com.  Less than 1% of our shipments are damaged in transit, and we are committed to making sure your item arrives safely, but we are taking these additional measures to make sure you are happy with your purchase.