Al: I am building a pair of these , but this choke has been the holdup....I know the DC resistance, so I may use a couple resistors to get it going and by then I can get someone to wind me a couple of these chokes.....I could see that it was a choke but I couldn't figure how it worked in that circuit....I have all the other stuff, plus a friend is making the chassis for me.....Thanks for your help on this.........Will
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Hi Will, If you want to temporarily substitute resistors for the choke, the value should be considerably higher than the choke's dc resistance. Otherwise the gain of the stage will be reduced. But it shouldn't be too high, or the dc plate voltage (the quiescent, or zero signal plate voltage) will be reduced significantly, which will also lower the gain. Essentially the choke allows B+ to be applied to the plates through a minimal impedance (1.8K if the 3.6K you mentioned is the end-to-end resistance of the choke), while presenting a much higher impedance at audio frequencies. That maximizes gain at audio frequencies, without reducing the dc plate voltage to the extent that a resistor would. Another approach to consider, as a temporary measure, would be to use an audio transformer with a resistor connected across its secondary. You would need one transformer for each of the two 6J5's, having a dc resistance in its primary of 1.8K (assuming the choke is 1.8K from center tap to each end). You could then experiment with various resistor values on the secondary. Looking at the plate characteristic curves for the 6J5 in my older RCA tube manual, and at the operating voltages indicated on the schematic, my very rough ballpark guess would be 20K or 30K for the plate resistor. If the resistor is on the secondary side of a transformer, factor that by the square of the turns ratio, of course. Best of luck with your project! -- Al |
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