The 1966 Marshall JTM45 MKII Plexi 50-watt head is one of the most historically significant amplifiers ever built. This is the amp that defined British rock tone, the one Clapton, Townshend, and Page plugged into before the world knew what a Marshall even was.

The JTM45 began as Jim Marshall's interpretation of the 1959 Fender Bassman circuit, but the substitution of ECC83 preamp tubes and KT66 output valves gave it a character distinctly its own: thicker in the midrange, more compressed at the top end, with a harmonic richness that the original Bassman never quite achieved. By 1966, the final year of JTM45 production, Marshall had refined the design to what many consider its peak form — the transition to aluminum-faced Plexiglas panels was underway, and this amp sits right at that historically loaded moment. At 50 watts through a pair of KT66s, it has considerably more headroom than its name implies, but push it into breakup and you get that unmistakable bloom and sag that no modern amplifier has fully replicated.

This example presents in Very Good condition — cosmetically solid with original tolex, original knobs, handle, and pots intact. The amp has been professionally serviced, with filter caps replaced as part of that work, and it runs a grounded power cord for safe modern use. Tube complement currently installed includes a Mesa 5U4GB rectifier, a pair of Marshall-branded KT66 power tubes, and a mixed preamp set of Sylvania, unknown, and JJ ECC83/12AX7 types.

Dating is confirmed through internal codes rather than a visible label — consistent with how Marshall documented production in this period. Transformer details are documented in the photographs. The cabinet measures 25.76" wide by 9" tall by 8.25" deep and weighs in at 32 lbs 14 oz — compact by modern standards, substantial in hand. Extension speaker outputs are present. 

This amp is built for the player or collector - roll back the volume and it delivers a warm, touch-sensitive clean that responds to pick attack in a way that modern circuits simply don't. Open it up and it does blues, rock, and everything in between with a directness and physicality that is deeply addictive. If you've been chasing this sound through pedals and modern clones, this is the source material.

The output transformer is replaced, a pretty common change on these older Marshalls.  The back panel is missing.  

A genuine 1966 JTM45 MKII in this condition is not something that comes through often. These amps shaped the sound of rock music more than almost any other single piece of equipment, and the window to own one at anything resembling a reasonable price continues to narrow. If this is the amp you've been looking for, act accordingly.

Mahar's Vintage Guitars is a brick-and-mortar and web-based retailer. We are happy to talk with you before you make a purchase.   Please reach out if you feel so inclined! We cross market our items here and on our own site, so items are subject to sale elsewhere at any time (however we will never cancel a sale if it happens here first). See our profile here, our Google reviews as well as our 100% positive Reverb feedback. Also, Reverb limits us to only 25 pictures of our items. There is a good chance we have more. Contact us if you’d like to see them.

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Specifications:

Brand: Marshall
Model: JTM45 MKII Plexi 50-Watt Head
Year: 1966
Date Noted: Internal codes
Watts: 50
Color: Black
Dimensions: W 25.76" x H 9" x D 8.25"
Weight: 32 lbs 14 oz
Rectifier Tube: 5U4GB (Mesa)
Power Tubes: 2 x KT66 (Marshall-branded)
Preamp Tubes: 12AX7 (Sylvania), 12AX7 (?), ECC83 (JJ)
Power Transformer: See photos
Choke Transformer: See photos
Output Transformer: See photos
Filter Caps: Replaced (serviced)
Pots/Codes: Original
Tolex: Original
Knobs: Original
Handle: Original
Power Cord: Grounded
Extension Speaker Out: Yes
Reverb/Effects: No
Pedal: No
Case/Cover: No
Condition: Very Good
Serviced: Yes

Excerpts from real online end-user reviews:

"Electronically, these early Marshalls are basically clones of the 1959 Fender Bassman amp, but the use of different preamp (ECC83) and power amp (KT66) tubes in the JTM45 gave them a raucous character all their own." - Reverb.com
"Roll off the volume and you get a pretty nice clean out of it. Add just a touch more volume and it will do blues all day. They take OD pedals great." - Reddit, r/GuitarAmps
"Soon you could hear Marshalls on nearly every record coming from England, and subsequently in the recordings of generations of guitarists around the world that they inspired." - Premier Guitar

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Shipping Policy
By completing your purchase you are agreeing to this shipping policy. Shipments require signature on delivery, no exceptions. Less than 1% of our shipments are damaged in transit, and we are committed to making sure your item arrives safely. If there is damage, retain all packing materials intact, photograph them immediately, and send the photos to .

Amplifiers: We take additional precautions to prevent common shipping damage. Depending on the amp, this may include removing tubes, pulling the chassis from the cabinet if heavy transformers present a drop risk, or removing heavy speakers that could crack baffles in transit. Reassembly is required on delivery — we label everything clearly. These measures add to shipping costs but reflect hard-won experience. All amps are tested in our shop upon intake and again before shipping. We cover shipping damage but cannot be responsible for the failure of vintage components in instruments that are 20 to 50-plus years old.