[Heath] Biasing in the SB-200

[Jerry Haigwood]

Ken,
     The Chinese Shuguang 572Bs are equivalent to the Cetron.  The Svetlana
572Bs require different bias.  I am currently running a pair of Taylor 572Bs
which I believe are of Chinese manufacture - at least they have a small
"PRC" printed on them. ;-)  I did not change the biasing and they are
performing very well (600W output with 52W of drive).  The Shuguang 572Bs
are of excellent quality better than I had anticipated from the Chinese.  A
pair of tubes cost $59.95 and shipping $24.90.  They come with a 30 day
money back guarantee and a 1 year warranty.  They are available on ebay.  I
have a very nice Clipperton-L with the original Cetrons still in it.  It
still makes 1100-1200W peak on SSB.  Whenever I decide to change tubes, I am
buying 4 of the Shuguangs.  I enjoy watching your progress with the SB-200.
I think I told you that I rebuilt my SB-200 a few months ago.
73, Jerry W5JH

-----Original Message-----
From: heath-bounces at puck.nether.net [mailto:heath-bounces at puck.nether.net]
On Behalf Of Kenneth G. Gordon
Sent: Tuesday, September 27, 2011 6:41 PM
To: Heath at puck.nether.net
Subject: [Heath] Biasing in the SB-200

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
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