Watching vids on Cathode followers now.
Here's a helpful link on DC coupled cathode followers.
So now let me introduce you to the other personality of this amp, as a high gain monster. In the basic Tweed configuration, everything is "out", it's all bypassed. But normally, unless you pull knobs, everything is "in". You have three gain stages, followed by a cold clipper, and then two DC coupled cathode followers in a row. The reverb is between the two cathode followers, and the effects loop is before the reverb.
As a player, you want your amp to be responsive, but we also know you're going to set it up for a basic sound then put pedals in front of it. If you're a high gain player you have two essential ranges of expression, which you'd like to be able to control with the volume knob on your guitar. These are clean-to-crunch, and dirty-to-searing-lead. You'll rarely use both modes in the same song (although it does happen, and when it does the transition should involve no more than a single switch, or a single foot switch).
The idea with high gain amps is to carefully control (tonally) what goes into the overdrive and what comes out, as well as the character of the distortion. The problem is you can't do it all at once, you have to do it in stages. Each stage will create some fizz that you'll want to get rid of, because high gain is a lot about the mids. And you can't be too brutal in any given stage because of the pedals, some pedals will literally blow up poorly designed high gain amps.
In this amp, you get three ordinary 12ax7 gain stages in a row. Each has a gain of about 80, so they'll take your 5 mV guitar signal up to about 200 volts. Why would we want to do this? Well, it's because of what comes next. There's a cold clipper and two cathode followers, and those three things together constitute the "high gain" section of the amp. You have to drive them to high levels to get the good behavior and the good sounding distortion. For instance the cold clipper in maximum clipping mode has a gain of less than 1, it's around .9 or so - and we want it to clip robustly so we have to give it a lot of signal to push it up to that point. Figure with a 300 volt supply you'll get a 100 volt swing before clipping, and if the gain is less than 1 that means you'll need to drive it with more than 100 volts!
This is why there are 3 gain stages before the cold clipper, we want to make sure there's enough signal to clip. However there are level controls between each stage, because the front part of each cathode follower will have a gain of about 60 so we don't need as much signal to push them up into overdrive.
The second cathode follower is also the reverb recovery amp, so you can get crunch on your reverb if you want - which is a different sound from overdriving the springs, and again you can mix and match to find out what works.
The levels are set up so switching in Stage 3 takes you into high-gain land. That's all you have to do, flip that one switch. In low gain mode you get a completely clean sound when the volume control on your guitar is about at 3, and a respectable crunchy southern rock sound at 10. When you flip the switch to engage Stage 3, you start at Lynyrd Skynyrd, Black Sabbath is about halfway, and as you keep raising the level the extreme frequencies get cut and the output becomes more mid-centric.
