Saturday, October 13, 2012
Transforming a Marshall JCM2000 DSL 100 watt head
Just this year Magnetic Components, a/k/a Classic Tone. began offering power transformers for these amps, and I obtained one. Of course, the wiring color codes are different, and you have to make up some of your own connectors because the Classic Tone does not come with spade connectors installed.
The first thing I did was make a map of the old connectors on the Dagnall and where they went to and on what board-main, or power inlet. Then, I took the hookup diagram that Classic Tone provides and started doing the connections. It's simple enough if you can read a schematic. If you get a little confused it's a good idea to draw it out for yourself. It's also a good idea to save some of the wiring you remove, because you're going to have to do a little splicing to wire up the bias circuit with the right size spade connectors. Some of the connectors are a bit smaller than our standard, but you can take your new ones, squeeze them a bit and re-flatten them out, making sure they're a good fit on the lug. You can see what the finished installation looks like in the photos.
The Classic Tone has more metal in the core, and it's about halfway between the original installation and the Mercury Magnetics Fat Stack.
Next, removing the old power transformer you can mount the new one so that the green and green/yellow leads point toward the filament circuit lugs on the main board. After wiring up the power input per the schematic for 120v (primaries in parallel) you can proceed to wire up the filament circuit and the B+ (ok, ok, high tension) circuit, which runs through the standby switch. At this point I would stop, and power up the amp with no tubes in it. You can then check for 6.3v on the filament circuit-I check the preamp tube sockets with an old RCA cleartop tube. At this point, make a chart and measure your filament, plate (pin 3) and screen (pin 4) voltages and note them. You should see something resembling operating voltages.
Now, you'll have to splice the small plugs onto your bias lugs on the main board. The old ones are white-black-white, and the new ones will be white-blue-white.
Once you've done that you can power up again and measure your bias voltages on pin 5 of the power tubes and you should see something around 45 volts DC-remember, it's rectified.
If this is all good, you're ready to police your wiring job, set up to check bias per the Marshall method using the Molex connector on the back of the tube pan, and adjust as required. It's always a good idea to start out low and slow until everything's had a chance to settle down and you're sure you aren't going to blow any fuses. Then you can adjust up to around 80 mv per side using the pots provided.