TM36: what's the difference between clean, crunch and lead channels - electronically speaking?

Hi all. New tube amp user here. I've had the TM36 with the HK matching 1x12 cab for a few weeks. Love the wide range of tones.

I'm just curious to understand what's going on inside the amp and why the three channels sound so different. Where does all the distortion comes from on the crunch and lead channels?

I guess this is a basic and naive question regarding guitar tube amps... Please forgive me if it has been discussed before.

Thanks!

The two preamp valves you have available in your TM36 are used differently.  (The third is the phase splitter/inverter stage of the power amp.)  Remember each of these has two triodes inside it. I can only speak accurately for the GM36 or the TM18 as these schematics are out in public, the TM36 is not but the principle will be exactly the same.  The Clean channel uses two of the available triodes and the Lead channel then adds in the third one for additional gain.  The Crunch could be the same as the Lead but I would think it is set up as the Clean but with more gain in at least the second stage. The GM36 uses the fourth triode as a follower for the output of the valve stages and I would expect the TM36 to do the same.  In front of the valve stages are solid state Input Buffer and Gain stages.  After them are solid state Tone Control and active Volume control stages.  Other EQ such as the Resonance and Presence controls is also active solid state.

Switching between the channels also trims the solid state Input, Gain and Tone Control stages.  Their local gain and tone shaping is altered for each channel.  It's a fairly complex setup which I spent a lot of time computer modelling in Spice looking to mod it to suit my own needs.  I gave up in the end as it's impossible to isolate the parts of the sound you want to mod, there are too many interacting factors which are altered by the switching.  This is a very involved amp not just a simple 5 valve classic job like your older Marshall/Fender/Peavey/Mesa designs.

There are a few ways of creating distortion in this amp and not all sound alike (or good).  The input buffer stage acts as a progressive Tubescreamer type circuit so it is important not to hit it too hard for more classic tones.  Don't be frightened to use the guitar or pedal volume controls to turn down the input level and keep that stage clean.  It is a mistake to just thrash it and also thrash the valve stages so they both give max distortion.  You get pretty rough tones that way.  High input levels with low Gain internal settings, or low input levels with high gain internal settings give two different types of sound.  You learn to blend these as you need.  It's very flexible and takes a bit of getting used to in a way that the classic valve amps do not.  They tend to be great one trick ponies whereas the H&Ks tend to not quite have that classic tone but are much wider in the range of tones they offer.