@modulator So I've been doing some digging.
I've found this page which has some good explanations:
Especially this video (which is linked on that page):
I also found this official schematic from Fender to be the clearest and it doesn't leave out any details.
So on to your questions:
modulator 1.Looking at many circuit diagrams (mostly similar) I see the normal diagrams show the filament heaters to be earthed on the one side. Looking at many layout diagrams, the heater wires are a twisted pair from the power transformer to the relevant heaters. No earth.
One layout diagram (Tube Depo) shows two 100 Ohm resistors, one to each leg of the last heater filament, coupled together, to earth. This earths both wires?
I don't think they NEED to be earthed for the valves to operate, but it is a good idea to have at least some kind of chassis reference even if just for safety's sake. I like the two 100 Ohm resistors trick, this provides some minimum load so the 6V doesn't run away even if some of the valves are unplugged and/or the pilot light is gone.
modulator The kit is supplied with two glass bead diodes, I would guess this is only used if one does not want to use the rectifier valve? Rather small, anyway.
Yeah, its going to be either the two diodes or the 5Y3.
modulator Some circuits show a 22 microFard electrolytic cap parallel to the input resistor (earth side, pre-amp valve pin 3), as per the 5F2-A circuit where it apparently makes up for losses due to the tone circuit. Would it make any difference if I use this cap or not?
My understanding is that the cap will provide a stronger bias to the valve for higher frequencies, thus boosting the highs. As the frequency goes up, the impedance of the capacitor goes down (1/jwC) which effectively reduces the size of the cathode bias resistor. The 3dB point of the 1K5 resistor and the 22uF cap is 4.8Hz (1/2piR*C), which means that everything above 4.8Hz will get progressively louder (at ~20dB per decade) as the frequency goes up.
You could potentially wire this capacitor via a switch and get a "bright" switch.
modulator NB Somebody, please tell me how a diode valve works? I see the two HT leads (+ / - 375V) from the power transformer going to two plates, good. I see a heated filament, with 5V supply from the power transformer, supplying electrons, good. I see the center tap going off to the low tension/earth side of the circuit, making the voltage across the whole thing high, ... OK? I see a wire to the HT side of the turret board, running from the heater supply, uhm? How does this make the HT +375 or whatever V? My limited logic says this is Negative territory?
Electrons accelerate from the heated filament side to the +/ - 375 V leads side. Can only rush there if the relevant plate is positive, and then to the other plate, etc. So, the filaments are negative? HELP!!
I've never used or seen a rectifier valve before, but it looks like is a single package containing two "Fleming Valves".
I understand that part of the circuit as follows:
The first thing to recognize is that the centre tap on the high-voltage transformer cause the two HV transformer wires to be 180deg out of phase with each other relative to earth:
Excuse my paint:
So during the positive half-cycle (on the primary) the flow is like so:
And during the negative half-cycle, it is like so:
It doesn't really matter which side of the heater (also the cathode) is more negative, as long as it is more negative than the plate the electrons will want to jump across.
So this is all theory of course --- I haven't tried it, but that is how I understand it. I'm keen to know how it works out