Here's a few shots of the completed amp plus the shot of the guts I took a while back (re-post). PSX is supposed to stand for Power Scaled Xpress but I've ended up changing it enough that it's merely 'Express based' at best. I had to increase the PI tail resistor to 22K and change the 91K PI plate resistor to 82K to get the PI distorting nicely...then I did some stuff to the preamp. It's a mean sumbitch...don't ask it to get pretty. Scumback H75s in the over-sized 2x12.
What a beauty! Congratulations on a very well done job!
How about some short clips demonstrating the Power Scaling (say, a single lined riff throughout the PS range, ditto for some chords).
Thanks. Clips are forthcoming, maybe next weekend. It takes some playing around with to get the power scaling dialed in but luckily my post-PI MV (required to use power scaling the way I'm using it) 'tracks' pretty well with the power scaling control. So if the power scaling is at say, 1/2; then the MV should be set at just below half for clean power tubes and just above half for dirty power tubes. In an Express (and even my modded amp) the PI clips before anything, way before, so the power tubes clipping is just icing on the cake...it's not really that much of a difference. Feels different though...chewier.
So, what exactly is that thing? The huge white rectangular ceramic resistor looking device on the board that sits behind the power/standby switches?
I was wondering if you implemented a Sag Control, that dials in this resistance to starve MOSFET current in the power scaling kit (*exactly what I do - although I don't use a resistor that big).
Also, I'm not sure why you felt a normal standby wouldn't work in this amp. I'd appreciate your thoughts on that too.
Hi Mickey,
Like I said on TGP (adding it here for completeness)...that's the 'choke' resistor. I'd love to hear about adding a Sag control.
Regarding the standby...unless I misread the documentation Kevin makes it sound like you can't so I just took it at face value. I assumed it had to do with the fact that the bias scaling circuitry 'tracks' the screen voltage (which is scaled)...so in order to have correct bias...you have to have screen voltage (i.e. standby must be ON). I thought it might be a problem but only for an instant upon hitting the standby switch.
Scumback H75s in the over-sized 2x12.
Mine has morphed over time as well. 
Congratulations on a very well done job!
