Introduction | Phase I: Simple Plexi conversion | Phase III: TBA...!



Phase II:
Adding a post-PI master volume

Well, the plexified Artiste probably doesn't put out any more power than it did before the modifications, but the typical Marshall crunch only comes at some pretty high sound levels - the most common criticism of the Plexi circuit. The only type of master volume that makes sense if you want to keep this crunch at all volume levels is one that comes between the phase inverter (V4 on our plexified Artiste) and the power valves. This allows you to dime the front end of the amp, get crunch from all valves up to and including the PI, and then feed as much - or as little - of that crunchy signal as you like to the power valves.
        There are several versions of the Post-PI master volume around, all based on a pot across the two outputs of the phase inverter (i.e. after the coupling caps - 0.1uF in the Artiste). The simplest is a 1M audio pot across this point, but there are also versions that use a dual-gang pot and extra capacitors. I'll try the simplest first and see how that works, but before switching on the soldering iron, there's the thorny but important question of where to put the pot.


Siting the master volume pot

When most people add this type of master volume to a vintage Marshall, they don't want to drill a hole in the chassis - it ruins the amp's value. So it's common to put the pot on the back panel (typically removing one of the speaker sockets and using that socket's hole), or on the front panel (removing one of the four input sockets and using that socket's hole). But with the Artiste chassis, there are more options available.

        First, the Artiste has eight pots on the front panel anyway, and the Plexi circuit only uses six of them. So one approach is to shuffle the pots up and use the last one - the one originally used for the Presence (see photo above). This is probably the easiest, especially if you'll be undoing the pots to fit a new control panel: you can add the master volume at the same time. But I've got plans to use all of the currently unemployed pots in the middle of the control panel, so I decided on a different approach.
        I saw that the steel chassis in my Artiste has an extra hole pre-drilled by Marshall, originally for a 'polarity' switch (see photo below). Yours might have this hole, or even a polarity switch that you don't need. The brass-coloured aluminium control panel covers this hole up on my UK amp. I can use this hole, and shift along the On/Off switch, Standby switch and indicator lamp one hole towards the end of the chassis. This then frees up the lamp hole for the master volume pot.

        Conveniently, this hole is the right distance away from the Presence knob for the control spacing to look correct. Even better: the new position of the On/Off switch is symmetrical with the position of the input jacks on the other end of the chassis. This symmetry is what Marshall would have had in mind when they designed the control layout in the first place.


Don't touch that drill, Eugene!

So, first of all, I need to drill through the thin aluminium control panel at the polarity switch position, right? Well, no. I'll eventually get a new control panel made with correct labels, but there's no need to drill the Artiste's original panel. You can never undrill a hole, so I removed that thin aluminium panel instead. I'll make do with the blank chassis face until I finalize my other controls (to come in Phase III).
        Here's another no-drilling tip: if you use a modern pot for the master volume, you won't have to enlarge the hole at the lamp's original position - it's just big enough to accept the smaller diameter mounting shaft of modern pots. You will need to find a way of making an indicator lamp fit the Standby switch's original hole, though. I'm deferring that decision for the moment.


Which post-PI master volume to use?

There are several versions of the post-PI master volume. I started with the simplest - a cross-line master of the type used in some Matchless amps and some of Kevin O'Connor's designs. It's just a 1M audio pot across the bias resistors. But I thought that it sounded a bit 'congested' on crunchy chords on this amp. Especially with a Les Paul (not so bad with a Tele).
        I decided to go for the most complicated version - the one that Marshall used in several amps (4140, 2150, Studio 15, Park 1210, etc). This version uses a dual 1M audio pot and a pair of extra coupling caps. Here's a schematic, with the extra stuff shown in red:

It's pretty simple, but adding it does require moving some components on the Artiste's turret board - basically, you have to split the connection of the PI's coupling caps and the EL34s' grid bias resistors - shown in this photo, as the two points of the 'W'-shape made up of 100nF cap - 220k resistor - 220k resistor - 100nF cap. It's not very difficult but it does need a bit of thinking about how to secure the existing capacitors and where to put the extra capacitors.

        Well the amp-tinkering gods up in tone heaven must have been smiling when the Artiste was born: right between the two bias resistors is a mounting screw for the turret board, and there is enough thread on this to secure a 5-way tag-strip. It fits perfectly, and makes secure connections easy: the existing coupling caps go to two lugs on the tag-strip and you put the new capacitors between the bias resistors and two other lugs on the tag-strip. Hard to describe, but the photo should make it clear.

        Then you just wire these four tag-strip lugs to the dual pot. At the risk of stating the 'bleeding obvious' - if you use this type of tag-strip, don't use the middle lug for any of these connections. It'll be earthed via that mounting screw. Finally, the bottom end of the pots connects to ground (I extended the ground buss wire that Marshall used along the back of the pots).



The master volume at work

This was waaaaay better than the cross-line master volume I tried first. There was no 'congestion' at all, and it was as transparent as I'd have hoped for. I've read that some people have found that these master volumes can trim some of the top end, but that isn't the case here. It works perfectly, and the extra caps block DC from the pot, so there's no noise as you dial the pot to the listening level you want.


Whatever next...?

I'm done modding for a few weeks. With just a few pennies spent on common parts (most out of the parts bin, too), the results have been well worth the effort. In fact, I like the sound of this so much that I'm rethinking my plans for using the Artiste's combo cab for my 18W project. It feels wrong somehow to split the amp up now that it sounds so right.


Coming soon - Phase III: TBA...