MODS to the Marshall JTM600/JCM600 series amps

JCM600 range from Marshall website

JCM600 range from Marshall website

We are now implementing mods & upgrades to a broad selection of guitar & bass amplification, primarily valve (tube) but occasionally solid-state as well. The most common examples are the Fender Pro-Junior & Blues Junior models, however we have been selectively modding Marshall amps since the 1980’s. In this blog we have a quick look at the Marshall JTM600 & JCM600 series amps – it would appear that the differences between the two are limited to cosmetics & choice of speakers, the electronics remains the same for both series.

JCM600 combo

JCM600 combo

These amps remain among the more obscure of Marshall’s amp offerings of the last 20 years or so, very few guitarists would have played through one of them, let alone owned one. We do have a few Central Coast (NSW) customers that quite enjoy these amps, including “Gazebo”, the man who inspired this blog by commissioning us to mod his JTM60 amp head (we modded the “clean” channel only).

JCM601 model

JCM601 model

These amps don’t have a great reputation for reliability. We have replaced several transformers over the years, for example. The first batch of JTM600’s were blowing the main high voltage filter capacitors during the warranty period, although this problem has obviously been resolved. They are not particularly easy to work on – getting the circuit boards out and re-installing them is quite time consuming. Operating the EL34 power output valves (tubes) in the horizontal plane rather than the more conventional vertical plane is just asking for trouble. You have to choose your EL34’s carefully – with some brands there is the potential for an internal short-circuit to occur, if the heater filament sags for example. Please note, this is not normally a problem with the 12AX7 preamp valves. Regardless of your choice of valves, most of the heat from the power output valves goes straight up into the amp.

the JTM30 combo

the JTM30 combo

Over the years, we have tried to come up with ways to “bullet proof” these amps. Not all EL34’s like high voltages on the screen grids, for example. Installing a small DC powered cooling fan may assist the issue of heat build up within the chassis. The JTM30 model has presented less problems, but has one stupid design mistake – a non adjustable bias supply voltage, which complicates replacing the 5881 output valves unnecessarily.

Stock, the 60 watt amps sound a bit thin and a bit sterile. The “lead” channel develops a type of distortion that sounds like it came from a stompbox, rather than from a valve amp. The main focus of our mods is to convert the “clean” channel to a circuit sounding as close as possible to a Marshall model 1987, ie a blues/crunchy tone with more fat bottom end and crisp high end. This is achieved by substituting selected components (resistors/capacitors) with more traditional “plexi” circuit values. This is what we achieved for “Gazebo” just this week. Once the job was completed, the “clean” channel was suddenly very useable for many styles.

With the “lead” channel you could easily go overboard, spending hours rebuilding the whole circuit. The simplest approach would be to remove the clipping diodes for a more natural valve (tube) medium distorted tone, although having done this it may be necessary to make some changes to balance the circuit for the now much higher signal levels.

Modding the clean channel as described, plus limited mods to the lead channel is a cost effective way to significantly improve the performance of these amps. We need to allow a minimum of 2 hours labour for this work, plus materials. We would have to quote re specific improvements to the power amp, eg adding a cooling fan, etc. Regards – I.R.

3 Responses to “MODS to the Marshall JTM600/JCM600 series amps”

  1. rakkit1 Says:

    The internal 500ma fuse in my JCM 600 1×12 combo will blow once in a while. From reading forums on the subject it seem to be a common issue. Have you a fix for this issue? Otherwise, I was wondering if converting that internal fuse into an external fuse might be the best way to go.

    • ivanrichards Says:

      hello: operating the EL34 output valves horizontally rather than vertically will almost inevitably lead to problems as it is possible for the heater filament to sag and cause a short-circuit; the physical design was a dumb idea ! shifting the HT fuse to an accessible location on the rear panel would make life easier, and such a MOD should be implemented by someone who is suitably qualified to work on high voltage electronic equipment; IR.

      • rakkit1 Says:

        Thanks for your reply. I guess the root cause fix then is simply to lay the amp on its back making the EL34’s vertical. : )
        Seriously though, I appreciate your response. Dave

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