take the V76 for instance, it has 2 chokes, about 250H and 400H,
first off, when you have current flowing in a wire, you have a magnetic field that extends into outer space, a steel core pull these field lines in from outer space and concentrates them inside the core, more concentrated lines of flux means more inductance which is what we want to load a tube with, we want to develop our signal across this choke so we can transmit it to the next tube without losing it to the pwr supply caps, so the choke is like an AC resistor, but has a lower DC resistance than a plate resistor, which means we get more of the pwr supply voltage to the plate of the tube, we can swing this voltage up and down with the tube and get more output,
400H is used in stage 2 of the 4 stage amp, it gives you more gain and less pwr supply hum at the critical first stages of mic amplification,
the second choke, 250H, allows you to use nickel alloy in the output transformer,
why? because without the choke, you have to run the B+ plate supply thru the OPT(output xfmr), a nickel core OPT does not like DC, it saturates the core, nickel saturates at 5-8 k gauss(measurement of flux density) , steel at 18 k gauss, DC=more gauss, so you have to lace the lams of the core in a way that lowers he inductance, all E lams going one way, all I lams stacked together to form an air gap between the E and I lams, this lowers the permeabillity of the core which means it will not concentrate the magnetic lines of flux as easily, you are increasing the resistance of the core, just like adding more resistance to a circuit, only instead of decreasing current like in a circuit, you are decreasing the ability of the core to pull in lines of magnetic flux, the core is more reluctant to be magnetized by the DC current of the plate supply, so to maintain inductance to load the tube, you have to increase the core size and use steel lams that do not saturate as easy as nickel,
now with the output tube plate choke of 250 Henries, (lower Henries because there is more current flowing in the output tube, which means bigger wire and thus, less room for turns and thus less inductance), also, the output tube has lower plate resistance which means we can use a lower value of inductance and still match the load to the tube, so now all the DC for the tube goes thru the gapped choke and not the OPT, so now we can stack the lams like this, one E lam going one way, the next E lam going the opposite way, this gives the core more perm, we can use less steel as we do not need to worry about saturation from the DC, so we can also use better steel, like a nickel allow, which will provide more inductance per turn than M6 steel, this means a smaller core, with less turns, this also means smaller wire in the OPT as it now only carries the AC signal, so less turns and smaller wire and smaller turn diameter due to the small core means less capacitance which means better high frequency response, and we have way more Henries with the ungapped nickel core, so low end improves also, so now we can build a V76 OPT that runs flat from 5 Hz to 120K Hz! so the purpose of the two chokes in the V76 is more gain, less noise, and better frequency response, now the output stage puts out more AC voltage than the inner stage 2, so we need to use an ally that will take the extra voltage, so we mix a little steel in with the nickel, that second stage core can be an 80% mu metal alloy, but we need to water that perm down a bit for the output stage,
but now we have a new problem, we have to use a lot of turns in the choke to get more inductance with the gapped choke core, about 21,000 turns for the 400H and about 17,000 turns for the 250H choke, that is a lot of turns, and a lot of capacitance, so we break the plate chokes into 3 sections which lowers stray C by a factor of 3 quadratic, or 3^2 which = 9 times less stray C, otherwise the stray C shorts out the hi end into the pwr supply cap, thus defeating the purpose of the choke in the first place,
