Philco 41-280 question

[wa2ise]

Originally Posted by DavGoodlin

The 41 tubes do not employ a phase splitter stage, but have an odd circuit where one G2 is to B+ and the other G2 via a 3.9K resistor with feedback capacitor to the other tubes' G1.

The cost cutting went too far there. That arrangement won't sound that good. I'd install a real phase inverter tube somewhere, probably under the chassis.

[old_coot88]

Originally Posted by wa2ise

The cost cutting went too far there. That arrangement won't sound that good. I'd install a real phase inverter tube somewhere, probably under the chassis.

Short of adding a real phase inverter, another way to get p-p output on the cheap is to use a grounded-grid, common cathode setup.
That's where both tubes' cathodes are strapped together and unbypassed.
Tube 1 is driven in the totally conventional manner (ie., thru a coupling cap from the first AF stage).
Tube 2 has its control grid (G1) grounded. Since Tube 2 shares common cathode with Tube 1, it is driven 180 degrees out of phase via its grounded grid. Ergo, push-pull.

The cathode resistor's value is chosen to bias the tubes' current at (roughly) the most linear spot on the curve.

This method has a few downsides, but offers real push-pull with minimal parts.