Posted by xcortes (A) on April 05, 2019 at 07:24:14

In Reply to: RE: Parafeed Baby O posted by xcortes on March 31, 2019 at 20:09:08

Paul Joppa helped me with the circuit adjustments so the credir goes to him.

Paul's goal was to retain the normalized load impedance so that the distortion character of the output tube remains the same with changes in the load impedance and power. At the same time he retained the plate dissipation fort the 2A3.

For the parafeed BabyO with 2A3, run the tube at 295v plate to cathode and 49.4mA current. Bias will be about 58 volts, so the cathode resistor should be 1173 ohms. Plate voltage is thus 353 volts; I did that with a 0.82 input cap instead of the original's 0.68uf and got the voltages spot on (with 115 vac supply).

Paul also suggested that the driver may have may marginal headroom, To improve that situation without changing the topology or tonal character, he recommended changing the 333-ohm cathode resistors (both sections) to 500 ohms.

I grounded the opt to the cathode of the 2A3.

I'm sure Gordon/Paul's circuit is worthy of Mike's Mexico Series masterpieces. I hope my build is too!