[ARC5] Receiver Voltages.

[AKLDGUY .]

Ken W7EKB wrote:

> I also find it interesting that the screen voltage on the RF and IF amps is
> about 125 VDC normally, whereas in the RCA tube manuals, that voltage is
> normally 100 VDC for a Class A RF amp.

It is (or should be) somewhat lower than 125 VDC. As I pointed out in
a posting elsewhere (perhaps you missed it), the voltage at the center
tap of the 7K/7K divider is not half the B+. It's a bit lower due to
all the screen currents that flow through the "upper" 7K.

Furthermore, screen voltage is the voltage from screen to CATHODE. If
a tube is self-biased with a cathode resistor, the voltage across that
resistor must be deducted. In the case of the 12SK7, this is several
volts.

In the case of the 12A6, rated grid bias is 12.5 volts for an output
of 3.4W. In a 250 VDC command receiver, that implies a plate voltage
of 237.5 V. The 12A6 cathode resistor fitted is actually very high,
presumably because there's tons of audio available for headphones and
a high value prolongs tube life. But that's another story. The point
is that the 12A6 plate voltage is actually somewhat less than 250.

73 de Neil ZL1ANM