That filament choke is very cool...So simple....Are those ferrites any special mixture? How did you test it?...Thanks for the pics..
Hi Steve,
Filament Choke Test:
The choke test is done by running a single wire thru the ferrite cores and then putting a resistor in series with this wire to ground. The resistor value is the estimated input impedance to the GG amp. Mine is about 12 ohms. (4 tubes)
Connect the RF signal generator to the wire on one side of the "choke" and to gnd. Set the gen to the lowest desired freq, like 1.8mhz.
With a scope, measure across the resistor and then across the choke wire. (Or across the whole circuit and subtract the resistor voltage) You want at least 90% of the RF voltage to fall across the choke. 10:1 ratio minimum. The LOWER the input impedance of your amplifier, the LESS inductance the choke needs to be. When operating the amplifier, you want the majority of RF to go thru the tube fil/grid and not thru the choke, so a minimum 10:1 impedance ratio is needed as a rule of thumb.
You can take out some ferrite beads and watch the voltage divide more across the resistor, as a double check. Add beads until diminishing returns.
I compared this technique to a standard coil-wound bifilar filament choke and could see no difference in performance, ONCE you get enuff ferrite core in there. Mine took only three cores.
I used standard 43 ferrite material with the big cores, about the size of small lemons. But you can use smaller cores too, as long as the wires can fit thru. I don't know if saturation is a factor, but since there is filament bi-directional canceling alternating current, maybe there is no threat. Anyone know?
I used this choke method in my other linear amps and it has worked out well for years. Easy to drive the amps on 160M in GG.
T