Originally Posted by MKB

The JCM800 model 2205 and 2210 both have diode clipping, as well as the 3203 Artist 30W. Also the model 2555 and 2550 heads (basically a Jubilee) have diode clipping. These are just heads, the corresponding combo amps have the diode clipping too.

For some reason Marshall went nuts with diode clipping as soon as they started embracing channel switching. Marshall seemed to have a desire to never put more than 3 tubes in a preamp for much of the eighties and nineties. They use the LED method in a few amps and pedals (Jubilee and Guv'nor respectively), but are also quite fond of using bridge rectifiers with the + and - pins connected with a diode. This basically gives you the effect of three rectifier diodes in series, for a 2.1V clipping level. The 2205 and 2210 heads use the bridge rectifier method.

Many of the recent Marshall tube amps do not have diode clipping however.