I'm alive! And happy to report a robust 560 volts on B+ and a healthy -166 on bias. However my power switch is bad, those Marshall rockers can't take the heat from soldering, they melt... the standby switch survived but the on-off switch went belly up - which is okay because I forgot to get the lighted one last time.
So far so good, next step is to turn on the standby switch and see if we have juice on the other side of the choke. And, build out the bias circuit. Then pop the big tubes in there and see if they behave. The fuses should arrive Monday, and the switch on Tuesday.
Meanwhile all the tube sockets are wired and ready for terminal strips.
This is going to be an oddball amp. Looks like a Marshall, but it ain't. The trick on this amp is cutting DOWN the gain in the right places.
The first thing that happens in this amp, before even the gain control, is your guitar signal goes to 8v. Which coincidentally, is exactly the voltage the PI needs to drive the output tubes to full power. Those first two stages are perfectly clean, no grit whatsoever. If you connect the 8v clean directly to the power amp you get a loud clean amp. Which you can then put pedals in front of and etc
And, this amp will replicate the gain structure of the mother amp, except I'm going to move the 3rd (clean) gain stage to the back, behind the cathode follower. In this location it will be in Fender's "reverb recovery" position, but more importantly it adds another different type of crunch to the cold clipper and the cathode follower. Bogner uses this trick, so does Mesa and some others.
What I'm after with this amp is a seamless transition from clean to extreme gain. In other words "all you have to do is turn up the gain control". This is why precision gain is important in this amp. The fun starts at 50%, when the cold clipper kicks in (at 4v, which is half of 8v), and that'll be "by itself" until the cathode follower starts overdriving at about 65%, and the last gain stage only crunches around 90%.
The signal level has to go DOWN at each stage to make this work. No more than 8-10 volts of signal is useful at the input of the power amp, unless we want to abuse the output tubes. The last gain stage (the one in the reverb position) could in theory boost the signal by as much as 80x, but in this amp we're going to cut that way down with judicious resistors at the input and output. And we'll cut the outputs of the cold clipper and the cathode follower the same way. (My version of the cathode follower is insanely gainy, if I drive it with 8v there's so much grit it starts blocking).
Since there's no room for 24 knobs like in the mother amp, all the essential functions have to be compressed into six pull-knobs, and only the most essential functions can be represented. The tone stack takes up four of those knobs, and the master volume is essential which leaves only enough room for a single gain control. So these gain ratios have to be "hard coded", so to speak. The preamp gain control can only determine how much of the 8v signal gets applied to the subsequent gain stages. When the cold clipper (for example) is "in" we can do 100%, and in that case the first 50% of the range of the gain knob will be clean, and the second 50% will be clipped. However the cold clipper does have some gain, so when it gets an 8v input we have to limit its output to the approximately 8v range. Same story for the cathode follower, so we can standardize on an 8v signal level through the gain stages, but then this has to drop again, in advance of the "reverb recovery" stage.
The tone shaping then becomes pretty easy, you no longer have to worry about signal level because you "want" some reduction, so you just put the tone circuitry in all the same places the level circuitry is. This way tone shaping can be done "per stage" and the result is lots of versatility and flexibility.
Compared to a JCM-800, you get all this for the price of one extra tube and one extra can capacitor. (And some terminal strips).
