Phase I cont'd:
With the Artiste's power section and power supply left pretty much as-is, there's less room to make SNAFUs that can kill you or the amp, but it pays to be cautious. I took out the HT fuse and powered up to check:
Testing and power up
* mains power input - on both sides of the On/Off switch?
* heater supply - still OK?
* presence of AC across the Standby switch - approx 700V?
Next came the power up with valves in place; as a precaution I used no-name valves to start. Everything was A-OK, except for some blue glow on the power valves and - after about 5 minutes - some red-plating on one valve.
Time to rebias...
If you're using the same power valves, I'm not sure that rebiasing is really necessary. The change to the circuit is quite small and there's won't be much less current drawn through the new preamp, so voltages shouldn't change much. But I've got a bias probe so it's not too difficult to rebias anyway. The process is a bit cumbersome but I didn't feel inclined to add the 1ohm cathode resistors needed for the alternative bias measurement technique (there was very little space around the power valve sockets anyway). Anyway, I found that the bias was waaaay hot (remember - I had chosen no-name valves to test the amp). So I rebiased down to 40mA, and with 440V on the plates, this was about 17.5W dissipation. The blue glow was gone, as was the red-plating.
I did all of the usual measurements across the pins on V1, V3, V4, V5 and V6 (V2 is currently empty - see previous page). Everything was what you'd expect from the 440V plate measurement. So, for instance, the V1 plates had 180V and 144V. What this means is that there was no need to tweak resistor values in the power supply to alter voltages. Good news - I wasn't looking forward to that.
Time to plug in
Plugging in a Les Paul and connecting to a 4x12... and it works perfectly. Phew! Immediately it sounds like a Marshall should, and nothing like it did as an Artiste. Played it quietly at about 2-3 on the Volume pots and got the chance to crank it the following day.
It is very well-behaved. There's no excessive noise or hum nor any oscillation at any setting of the controls. Of course, none of this is down to me :) - it's the big advantage
of using a layout that Marshall had already debugged.
Plus it's a low-gain preamp and there's all that space on the turret board. With so few wires crossing, it's easier to avoid oscillation.
What does it sound like?
I've got a '67 Plexi JMP and some early '70s metal panel JMPs, and the plexified Artiste is closer to the '67 in tone (although my '67 is one of the last to have a valve rectifier). I think this is because there's no cap on the cathode of the second stage (V3), so there's quite a bit less gain than in a typical '70s amp.
I'd quite like the option of having a bit more gain on tap, but there's no hurry (and there's still that unused 12AX7 wired up and ready for action), but for now I'll leave it as it is.
I will get around to making sound clips. And I'll add them here ASAP.
This ultra-simple conversion leaves several things open for further mods, if you're so inclined. The first one for me is the addition of a master volume, and it has to be a post-PI master volume to be effective (see the Phase II links at top and bottom of the page).
There's also a reverb transformer and the reverb tank both sitting idle. The transformer - just behind the filter cap in the above photo - is tricky to remove because its fixing nuts are right underneath the turret board. I've left mine in place, and I may return to it in the future. Not for reverb though - you can use it as a low-power output transformer with a dummy load. This would get you a signal you can tap via a voltage divider to re-amplify through the amp's 50W power stage (rather like a low-cost version of the Guytron GT-100).
The extra valve, V2, is more interesting, and allows for the addition of a lot more gain in the preamp. It's possible to use it for Marshall 2203/2204-style
cascaded gain, or even further, to Soldano SLO levels. I'll be experimenting in Phase III, but for now, I'll be playing the amp as-is. :)
Next - Phase II: Post-PI master volume