The JTM45 was first-born of the Marshall breed and the earliest examples of this amp, featuring an Art Deco-style ‘Coffin’ logo, are very scarce. So when vintage-amp restoration specialist Neil Perry was tasked with restoring a long-dormant ’64 example to life, he had to tread very carefully indeed. Join us as we put all his hard work to the test by cranking this fully fettled vintage beauty all the way up.
It’s the kind of find we all dream about: a historic vintage amp, bought in the years before such things became sought-after, that has been quietly dwelling in a cupboard for years and has now come to light once again, in timemachine condition.
Overall, however, the amp - a Second Series JTM45 with aluminium panel and Vynair white cloth front - was in remarkably good, near-complete original condition.
Neil replaced a couple of iffy resistors with era-appropriate NOS replacements, carefully balancing practicality with the need to keep the character of the amp as original as possible.
When working on vintage amps, this involves a careful assessment of whether to replace parts that are functioning safely but not necessarily to ‘textbook’ values.
“This is the rectifier valve, a GZ34,” Neil Perry explains.
“So that’s converting alternating current power from the wall supply to DC or direct current power. And that’s important from the point of view of how the amp responds - the amount of compression and sag it has.”
“This capacitor is designed to get rid of the jagged waveform that comes out of the rectifier.
“It comes out as a big triangular waveform and so this cap starts to clean it up. It’s a reservoir of charge and works in conjunction with the inductor  further down the amp to become a filter as well. On these early amps, these will be quite small and that again really affects the way the amp feels, the way it reacts as much as the sound.”
“These are KT66s, the output valves, which were later exchanged for the better-known EL34s in Marshall’s amps.
“In a way, you could say the KT66 was the British counterpart of the American 6L6, and it’s certainly more comparable to a 6L6 than an EL34. KT means ‘kinkless tetrode’, and any amp fitted with KT66s would have different harmonic distortion characteristics to one fitted with EL34s. Generally, you’d say 66s are harmonically richer.”
“Moving on to the preamp valves, it’s ECC83s all the way down. The first valve in line is what you call the phase-splitter, which is the valve that would be splitting the signal to drive the KT66s.
“The next one along from that powers the EQ buffering and everything else. The next one is the first actual preamp valve, which is the main gain-forming valve.”
“This is where the mains come in and gets transformed into three sets of voltages: high voltage, which goes to the rectifier, while the rectifier itself has a five-volt heater, the bit that glows; and then you’ve got your 6.3-volt heaters for the preamp and power-amp stuff.”
“This is an RS Deluxe output transformer. These were used from the very early Marshall amps and also early Vox and Wallace amps, as well.
“This handles the signal from the big output valves, very high voltages and impedances on one side, and on the other [loudspeaker] side it’s producing very low voltages to drive the speakers in the cabinet and so on.”
“This is what we call an inductor - they used to call it a choke in the old days - and that works in conjunction with the smoothing capacitors to smooth the nasty waveform that comes out of the rectifier.”
So-called ‘mustard’ capacitors fitted to the amp help date this amp to around early ’64.
An inscription on the back of the badge indicates it was made on Vittoria Street, Birmingham by funeral suppliers Butler, hence the moniker ‘coffin badge’ given to this style of badge.
The quad of inputs are more widely spaced on this MkII JTM45 than on the very earliest examples.
The pitched-top Bulgin knobs shown here are original, although this amp also sports some later, retro-fitted knobs.
Crudely hand etched ‘Bass’ probably indicates this left the Marshall line optimised for bass, not lead, although the spec difference is quite small.