Pretty unusual to see a valve amp with a heat sink, and with a well thought out design capable of dissipating the power valves’ heat properly it shouldn’t be necessary. However this amp, produced by Marshall during only 1995-7, is something of a flawed gem – reviled (along with its bigger brother the JTM60) by many amp techs for some of its design traits, but nevertheless sought after for their tone, particularly the clean channel (possibly one of Marshall’s best).
This one had a totally fried power transformer, so had blown its fuses, had worn out valves, a seized speaker voice coil, and missing grill cloth. Add in the design flaws of poor heat dispersal leading to internal overheating of the main circuit board and components, and a capacitor coupled bias network zener-clamped at a way too hot -39V, and it was perhaps more basket case than gem really. However, the circuit board seemed in good shape so I guess this one had died early on, and just been left to moulder…
I replaced the marginally-rated burnt out power transformer with an upgrade one from Hammond, for greater reliability, and to allow output valves needing higher heater filament current to be used, so that EL34s and proper 6L6s are an option as well as the Sovtek 5881/6L6WGC (which are actually the Russian type 6P3C) that were stock. This enabled me to get it fired up without blowing fuses to verify all was capable of working and that there was no damage to the circuit board, main filter caps, rectifiers etc. However it did indeed get very hot, as the bias was allowing the static anodes’ dissipation to be about 30W per output valve at the 440V HT, causing them to red plate. With its design leading to most of this heat being dumped inside the chassis with no venting to allow it to escape, the heat buildup issue was the next that needed to be addressed.
So I fabricated a simple aluminium plate heat shield/sink to dissipate the heat safely outside the amp chassis, and reworked the bias circuit to remove the zener clamp, added an adjustable bias voltage divider, and uprated some capacitors and resistors for reliability. Now with the heat shield and with the bias correctly set, even with extended use the chassis above the valves only gets slightly warm to the touch. The heat shield also protects the phase splitter valve from thermal radiation, and shields the power valves from the transformers’ flux fields (important as the KT66s use beam forming plates) since it is securely grounded to the main earth bond in the amp.
In keeping with the JTM moniker, I wanted to use KT66 power valves in this amp, so I added uprated screen resistors and appropriate value grid stopper resistors to suit them. And, anode load resistors of 1/4W rating on the small signal valves was Marshall cost-cutting gone too far in my view, so they were replaced with 1/2W (and the amp is now less hissy for it). The large and bulbous KT66 valve envelopes were a tight squeeze, but relieving of the heat shield slightly gave adequate clearance, and I installed some pcb stand-offs on the chassis to allow the original guard to be used, both for bump protection for the valves, and to prevent accidental burns from them (as they reach more than 200 degrees C in operation). A Celestion Alnico Gold speaker (rated for 50W), ‘pinstripe’ grill cloth and a gold plated Marshall logo badge complete the JTM45 ‘Bluesbreaker’ mojo.
Playing through the Normal channel, it is easy to see where Marshall were going with this amp: the mid ’60s glory days of the original JTMs. It can light up any guitar, while having good headroom that is clean but with an overtones character that is more Voxy than Fenderish (like an AC30 there is no negative feedback loop around the power amp). The controls give good flexibility too, and with the master volume towards max and the channel volume juggled to suit the guitar pick ups the overdrive has a lot of authority from the KT66s’ breakup; there is plenty of touch sensitivity.
On the other hand, in common with many of Marshall’s amps of the era the Boost channel employs preamp symmetrical diode clipping as standard, and as this is a matter of taste (me, I like), I installed a three position toggle switch round the back of the amp to either bypass it with two varying levels of boost, or to leave it in. The higher boost setting has a gain structure that is then similar to the classic Vox Top Boost circuit, while the lower one has a similar gain level as with the diodes in the circuit, but without the hard diode clipping; in this mode the preamp valves themselves produce some satisfying softer clipping and compression with the channel’s gain and volume controls up. Again, as with the Normal channel, max the master volume in any of the three switch positions and it’s the Marshall power amp roar that takes you home. Yum.
Okay, so this is a decent sounding and versatile combo amp I can lend you if I have to take yours away to fix and you need one for gigs or recording dates. Together with the nice sounds there are bells and whistles like a series FX loop and direct out (speaker emulated), and it has a proper spring reverb that is good for ambience, though it doesn’t get to surf territory – no worries though, this amp takes pedals brilliantly. It’s not overly heavy (40 lbs with the Celestion), has a well proportioned cab for a decent bass response, and with the efficient alnico speaker it does go rather louder than 30W RMS might suggest, so it is entirely suitable for clubs; it sounds good too at lower volumes. For a bigger room or where higher gain is needed, then there is a Marshall JVM410 head available…