The YT Video you linked to does not show crossover distortion at all but something which scares unexperienced guys a lot:
* tube amps have a *smooth* and continuous non stop variation from clean to distorted, imagine you are rolling a wheeled cart along a long ramp, you will go from a point to a somewhat higher or lower one with no hitch, going from, say, Volume 5 to 6 or 4 may mean going from 5% distortion to 6% or 4%, no big change (since you are never *really* clean to begin with) , you can´t find a "frontier" since there´s not a defined one, and your ear accepts it.
* SS amps (*all* of them by the way) are very very very clean before clipping (say 0.5% or less ) to harsh distorted (10/20/30/50%) as soon as they clip, easy to see on a scope where you see a sharp edge at the clipping point.
Going from clean to dirty now is like hitting a staircase step: very definite, very sharp edge, "your cart bumps" on that edge, it is very annoying.
What the video guy does is play basically clean , rise volume until just the peaks distort, the resulting mix is as harsh as putting mustard on your candy, you are jumping up down all the time between the top and the bottom of the vertical wall stair step.
* experienced SS amp Guitar players simply do NOT play over the step, they just roll their cart either in the clean area or in the fully distorted one.
Here´s one example of an experienced player testing the Marshall Lead 12, your exact preamp circuit, same gain, EQ, distortion mechanism (which does NOT use diodes of any kind but slams Op Amp against the rails, like a mini SS Powwr Amp driven to 11) .
In fact yours is the improved version, with higher power (same circuit but higher rail voltage, larger transformer and a 12" speaker) plus 2 Factory "Mods" : they added a single transistor buffer to better drive the power amp and an adjustable presence control.
Of course yours may have some improperly changed parts values , but we have lots of doubts about the circuit.
I am certain circuit is the same (they used it on a ton of same era amps, why would they make a different one for this particular model?) BUT parts designation in the drawing and on the actual PCB might not match, so I ask you to print schematic on a large sheet (even better on 2 sheets and tape them together) and follow it part by part, following tracks where they go, then write actual (PCB) labels and actual part value on the paper one.
Then take a nice sharp picture of corrected schematic and post it here.
If in doubt with some part value write what´s printed on its body, don´t guess (as when you called pF what was actually uF).
That is a killer amp.
Power amp *might* be different, I´m puzzled at the "MosFet" label on front panel because those used TO3 metallic Darlingtons (MJ2501/3001).
They did also make a MosFet one, but those are VERY expensive transistors, justified only on the 100W one, definitely not in a way cheaper 30W one.
This guy ONLY plays amps full blast, with a high output humbucker equipped guitar and if not a combo, plugs them into a Marshall 4 x 12" cabinet.
https://www.youtube.com/watch?v=L-5IaYOvmjQ
To make it clear that even the cheapest cheesiest amps can sound GOOD when properly used (without any silly Mods) here´s:
https://www.youtube.com/watch?v=xwKZd-Vc3os
* tube amps have a *smooth* and continuous non stop variation from clean to distorted, imagine you are rolling a wheeled cart along a long ramp, you will go from a point to a somewhat higher or lower one with no hitch, going from, say, Volume 5 to 6 or 4 may mean going from 5% distortion to 6% or 4%, no big change (since you are never *really* clean to begin with) , you can´t find a "frontier" since there´s not a defined one, and your ear accepts it.
* SS amps (*all* of them by the way) are very very very clean before clipping (say 0.5% or less ) to harsh distorted (10/20/30/50%) as soon as they clip, easy to see on a scope where you see a sharp edge at the clipping point.
Going from clean to dirty now is like hitting a staircase step: very definite, very sharp edge, "your cart bumps" on that edge, it is very annoying.
What the video guy does is play basically clean , rise volume until just the peaks distort, the resulting mix is as harsh as putting mustard on your candy, you are jumping up down all the time between the top and the bottom of the vertical wall stair step.
* experienced SS amp Guitar players simply do NOT play over the step, they just roll their cart either in the clean area or in the fully distorted one.
Here´s one example of an experienced player testing the Marshall Lead 12, your exact preamp circuit, same gain, EQ, distortion mechanism (which does NOT use diodes of any kind but slams Op Amp against the rails, like a mini SS Powwr Amp driven to 11) .
In fact yours is the improved version, with higher power (same circuit but higher rail voltage, larger transformer and a 12" speaker) plus 2 Factory "Mods" : they added a single transistor buffer to better drive the power amp and an adjustable presence control.
Of course yours may have some improperly changed parts values , but we have lots of doubts about the circuit.
I am certain circuit is the same (they used it on a ton of same era amps, why would they make a different one for this particular model?) BUT parts designation in the drawing and on the actual PCB might not match, so I ask you to print schematic on a large sheet (even better on 2 sheets and tape them together) and follow it part by part, following tracks where they go, then write actual (PCB) labels and actual part value on the paper one.
Then take a nice sharp picture of corrected schematic and post it here.
If in doubt with some part value write what´s printed on its body, don´t guess (as when you called pF what was actually uF).
That is a killer amp.
Power amp *might* be different, I´m puzzled at the "MosFet" label on front panel because those used TO3 metallic Darlingtons (MJ2501/3001).
They did also make a MosFet one, but those are VERY expensive transistors, justified only on the 100W one, definitely not in a way cheaper 30W one.
This guy ONLY plays amps full blast, with a high output humbucker equipped guitar and if not a combo, plugs them into a Marshall 4 x 12" cabinet.
https://www.youtube.com/watch?v=L-5IaYOvmjQ
To make it clear that even the cheapest cheesiest amps can sound GOOD when properly used (without any silly Mods) here´s:
https://www.youtube.com/watch?v=xwKZd-Vc3os