Re: what jcm 900 model is this? - any good?
greendy123 said:
Hey ErikH I have a jcm900 sl-x 2100, it's made in 8/93. Do you know if those came stock with el-34's? I have currently have el-34's in it, that were already there when I bought it on ebay. I know that they switched to making 5881's stock in the late 1993, so I'm not sure if mine is one of the better sl-x amps.
Yours is right at the tail end of the stock EL34 equipped models. That's a good one.
The differences between the 5881 models and the EL34 models, from what I can see on the schematics, is the screen resistor, one metal film resistor in the fuse circuit (R30), a resistor in the phase inverter (R23), and of course the bias voltage.
The 5881 models have a 470 ohm screen resistor, bias to -50 volts approx (-42 volts for U.S.A.).
The EL34 models have a 10k screen resistor, bias to -48 volts approx.
The other resistors are small changes and have very little affect on the tone considering the tolerences used. However, 10k to 470 ohms on the screens is a big change, greater than 10%. That's quite a swing.
Changing one that has 5881's wouldn't be hard. Just those screen resistors being changed (4 in the 2100, 2 in the 2500) and a rebias would probably do it. However, I'm not an amp tech so talk to a qualified amp tech first and pay them to do it.
I know this is more than what you asked for, greendy, but this is for anyone else as well.