Date: Sat, 7 Jan 2017 09:01:08 -0800
From: Catherine James <catherine.james@att.net>
To: Amps Group <amps@contesting.com>
Subject: [Amps] Glow, gettering, and voltage
I've noticed an interesting effect with the 3-500Z tubes in my SB-220.
When I run a kilowatt SSB on the low voltage setting on low HF bands, there is
very little glow. When I run the same power on the high voltage setting (using
less exciter drive), there is a noticeable dull orange glow.
I cannot think of a reason why higher voltage operation would be less
efficient. Thoughts?
73,
Cathy
N5WVR
### two reasons.
1- with the B+ being 41 % higher, the idle current will also be higher.
Higher idle current x higher B+ = lotsa more idle power.
2- IF you run the same po in both settings, the plate current will be a lot
less in the higher B+ setting. Your plate load Z will have skyrocketed.
With higher plate load Z, the tank coil has to have twice the uh. The tune +
load caps have to be reduced to one half their values. Since that is not
done, your resulting tank circuit Q will have doubled..... like from 12 to
24. Sky high loaded Q = lousy efficiency. Lousy eff = higher plate
dissipation. Combined with higher idle power = bad news.
## actually the higher idle current is not the issue, only when keyed, and
not driven.
## The SB-220 and other similar amps like the TL-922, drake L4B etc, all had
2 x B+ positions. Back then the rules were 1 kw dc input on cw mode,
and 2 kw pep input on ssb. Back then the only legal way to tune up the amp to
1200-1300 w out was either into a dummy load, or use the lower B+ position,
less drive and less plate current. Then when the B+ was switched to the
higher position, and drive increased a bit, the amp was still tuned correctly,
with no further
tune and load adjustments. Reason is, the plate load Z did not change.
Tank Q remains the same in both cases.
## EG: per the manual, the Drake amp is tuned to 565 ma @ 1770 vdc....
under load. = 1 kw dc input. Switch to ssb position, increase drive a
bit, and now its 800 ma @ 2500 vdc = 2 kw dc input.
2500 / 1770 = 41 % increase. IOW square root of 2 = 1.414 You will
notice that in the case of all those old amps that had a CW / SSB position,
that the higher B+
was always 41% higher than the CW position.
## On any of those amps, running low power, like 1 kw dc input, but using the
SSB / higher B+ position always resulted in lousy eff..... tank Q has
doubled.
Try this. If you have a known reasonably accurate wattmeter, measure PO and
also loaded plate current and loaded B+ using both ur low + high B+
settings,
but running the same PO in both cases. Then calculate the dc input. Then
calculate the tank eff using PO / dc input. Run a brief cxr to do the
tests... just long enough to
take some steady state loaded B+ / plate current / PO / grid current
readings.
Jim VE7RF
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