[Heath] Biasing in the SB-200

[Kenneth G. Gordon]

This is interesting: the way Heathkit gets -2 VDC (approximately) for the grids of the 572Bs in 
the SB-200 is via a simple voltage-divider across the -130VDC bias supply.

The voltage-divider is formed by the resistance of the 110VAC DPDT relay coil paralleled 
with R-18, a 2K 7 watt cement resistor, that combined resistance is in series with R-16, a 33 
ohm 1 watt resistor.

When the "bottom" end of R-16 is shorted to ground at the "Antenna Relay" RCA jack on the 
back panel, the resistances form a voltage divider across the bias supply, drawing about 64 
mA (130VDC/2033ohms=0.064), and at the same time applying about 120 VDC to the relay.

64 mA through 33 ohms equals 2.1 VDC.

Since R-18 and the relay coil are in parallel, the same voltage appears across both.

If R-16 were reduced to zero ohms, the current through the system would be 65 mA.

So, a very simple way to adjust the bias, and thus the idle current, for the two 572Bs would 
be to replace R-16 with a wirewound 5-watt 50 ohm pot or "slider" resistor.

The reason I suggest this is that I have been told by someone who knows that the modern 
Chinese and Russian-made 572Bs require less bias than the original Cetrons.

Interesting!

Ken Gordon W7EKB