Replacing cathode resistors of Aikido 1st stage (or other SRPP circuit) with chokes

[snoopyma]

Hi,

As you are aware, the 1st stage of Aikido circuit consists of a top triode with cathode resistor R and a bottom triode with another cathode resistor R.

I am just thinking of replacing these two resistor R with the dual windings (340 ohm and 20H for each winding) of a Lundahl LL1668 plate choke. Any pros and cons?

Should I connect the dual windings in phase or out of phase?

If it is connected out of phase, will the parasitic capacitance between windings works quite poorly in the inverted phase setup, as the capacitance degrades the high-frequency bandwidth?

Any comments and suggestions?

Thanks.

Best Regards,
T.C. MA

 

[kevinkr]

I use choke loaded SRPP in some of my power amplifier designs, technically this makes it a "choke assisted" mu follower.

Note that you do not want to use a choke in the cathode circuit of the lower tube, your gain will be very low to non-existant and more importantly not in common with the one in the top section as this may produce an oscillator if the coils are in phase.

To minimize distortion and maximize low frequency response you should either use fixed bias on the lower tube or use a cathode bypass cap across a cathode resistor, you don't want to increase RP significantly when operating into an inductor as this will move the -3dB response point up in frequency and also increase distortion and phase shift at low frequencies. It may also adversely affect high frequency response depending on the stray/lumped capacitance the choke exhibits.