Amps can kill!
OK, let's get the usual warning out of the way: the high voltages in valve amplifiers can kill you. It's up to you to find out how to avoid getting a potentially lethal electric shock if you decide to open up and modify your amp. If you don't know what you're doing, you should pay an amp tech to modify your Artiste.
Simple Plexi conversion
With all caps discharged - I always use a digital multimeter to check - you can start to revamp the Artiste's circuit. If you've already worked on Marshall 50W/100W amps, you probably don't need an Artiste schematic (actually you probably don't need this page!), but I found it useful to have as a reference. Marshall has always been friendly and helpful when I've called them (here in England), and they supplied one pretty promptly. You may be asked for your amp's serial number.
What stays? What goes?
Thanks to the fact that Marshall used its usual 2xEL34 power section, we can keep the PT and OT, plus all of their wiring Ė heater supply, filter caps, front panel switches, etc. The sameís true of much of the circuit, too: from the PI input to the speaker jacks, the circuit is the ĎBassí version of the 50Wer. So, with the chassis upside down and with the back panel facing towards you, thereís no need to change anything at all on the left side of the turret board.
The photo at the top of this page shows the original layout of the Artiste's turret board. (Note: I'll upload the high-res versions of all of these photos eventually).
So the power supply is A-OK (although I did make some minor changes to the PT wiring for an easier life). But most of the rest of the board needs work. Almost all of the preamp circuit, most of the connections to the pots and some of the connections to the preamp valve sockets need to be revised.
Making the Plexi preamp
Starting at the input jacks, I decided to use just two of them. It would have been nice to have the plexiís four jacks, but on the Artiste the four jacks are split; one pair is in the middle of the front panel (a little like the layout on an 18Wer). Iíll be using an A/B/Y footswitch anyway, so I only need two jacks.
So out came the unwanted jack sockets in the centre of the front panel, and I rewired the other two sockets with the usual 68k/1M resistors for V1's grid. I did everything Plexi-style, and I grounded the shielded wire (which Marshall hadnít grounded on my Artiste!). I also put the 68K input resistors right on the V1 valve socket. I wanted as much tonal variety as possible, so the rest of the first stage is the same as a later Lead Plexi except that I didnít use a bright cap on the Bright channelís volume pot. Here are the Before/After photos for this section:
As you can see, all of these parts fit just fine on the first seven pairs of turrets at the far right of the board, and almost all of these components (0.68uF, 320uF, 0.002uF, etc) are available by raiding the Artisteís original preamp components. There was no need to change any other wiring on V1.
Unused turretboard, pots and jacks
The Artiste circuit is a lot more complicated than a typical Plexi. With two tone stacks and reverb, the turret board is bigger, holding many more components. I decided to remove the excess components rather than leave them in place. This is partly because I've got a plan for a Phase III of this project.
Similarly, the Plexi circuit only needs six pots and the Artiste has eight. I'll eventually make a new control panel - because the Artisteís labels are all wrong, with Reverb, no Middle, etc. But I certainly don't want to redrill the chassis for six pots. For now, Iím using the two pots at the far right for the two channel volumes, and the four pots at the far left as Presence Ė Bass Ė Middle Ė Treble. That leaves me with two pots and two input holes unused in the middle of the control panel.
Once again, I plan to use these control panel positions for knobs and jacks in Phase III. There are plenty of ways you can use them with the now-unemployed V2 (see below) - an FX Send and Return, Tremolo controls, etc. Meanwhile back to the Plexi conversion...
Wiring for V2... well, V3 actually
In a Plexi, V2 is the second amplification stage, with a cathode follower to drive the tone stack. There is no cathode follower in the Artiste, but the good news is that most of the components are available from the Artisteís original complement. Plus thereís a very convenient 100K resistor hanging off the correct power supply node in the centre of the turret board. Now, this is located closer to V3 - the Artisteís third preamp valve socket - and I hate wires crossing unnecessarily, so thatís the one I used to create this part of the Plexi circuit.
Thereís quite a bit of wiring to change on V3 Ė including the removal of the wiring to the reverb transformer. For this first phase, I added a temporary wire (the pink one on pin 2 in the photo below) to connect to the sum of the mixer resistors after V1. [NOTE: This all leaves V2 unused Ė I left the wiring to the valve socket in place but made sure there were no components on any of the turrets that these wires are connected to.]
Add a tone stack
All you need to make the tone stack is a 25k pot for the Middle control and some caps out of the bits box (the Artiste doesnít have enough of the traditional Marshall 0.022uF value). It didnít take long to find a layout that worked with the turrets available, and it just fits perfectly into the space just to the left of the two 10k power supply resistors.
The tone stack I put in uses standard Plexi values, except for
a 47k slope resistor. I chose this value because Iíve got this in another homebrewed amp and I quite like it. Iíll see what it sounds like, and maybe revert to a more typical Plexi value later.
Odds and ends
There was some tidying to do, too. First I wanted to hardwire those pesky selectors that Marshall used and also clean up the rat's nest of wiring around the power transformer and switches. I hardwired the PT for 240V, but left the leads for all of the other taps at their full length: taped off, coiled up and cable tied in place. My Artiste had an extra fuse added between the mains supply fuse and the HT fuse. It sat in the circuit between the HT centre tap of the PT and ground. Not sure why it was added, but I left it in place.
At the same time, I hardwired the OT secondary directly to the speaker sockets. I cut the original parallel connection between the two speaker sockets so that I could solder the grey 16ohm output to one socket and the green 8ohm output to the other (who runs a 50W Marshall into a 4ohm load anyway?). And NFB Ė from the orange 4ohm tap - is spliced directly onto the purple wire from the Presence control (I may reconnect the NFB via the 8ohm tap later).
Next - Phase I cont'd: Testing and power up