Having worked on and modded one of Eric Johnsons 100W Plexi Marshalls, I can tell you they are all fitted with 6L6 Power tubes, not EL34 or KT66. They are the shared cathode variety , which is the same configuration as a Super Bass....they also have the same tonestack as the Super Bass (for the technical: 56K/250pF). Eric runs them flat out...period.
To make an EL34 Marshall run with 6L6's, it requires a small mod. to the bias supply (6L6's require about -10 or so volts
more than EL34's). They tend to have a wider sound stage and because the 6L6 requires more drive voltage, tend to stay cleaner higher on the volume control. They will slowly morph into overdrive at around 5-6 on the volume. Even at 10, they are not mega overdriven but a very huge and dynamic crunch tone.
The earlier Superleads (circa '66/67) had the same shared cathode arrangement and tonestack as the one Joe has, but just had an extra couple of bypass caps that let a little more high end through on CH1........although this was partially offset by the fact they originally came with KT66's and had a higher OT primary impedance. Even with 6L6's these amps tend not to be quite as loud as the later amps, but are a bit smoother sounding. These are the amps that Hendrix used a lot !!
The very early ones had two output transformers in, each dealing with a pair of tubes.
Another famous player that used a 100W Superbass for a lot of years is Gary Moore....think his was a '73. He used it with a Boss DS-1 or SD-1.
I owned more Marshalls than I could ever use for a while.....though I've offloaded a '67, a '68, a '72, two '73's and a '76 in the past couple of years. I don't use them much now. I kept my fave which is a 50W(!) '73 shared cathode amp...oh and I have a '66 JTM-45 Replica that I built for myself based on Angus Youngs favourite amp.
Alan Phillips,
Carol-Ann Custom Amplifiers