> Because the SB200 was not rated at a kilowatt input power,
> as measured on the meters, on SSB.
>
>
> But this seems wrong. When we tune up for maximum output
> with resulting
> readings of 450 mA at 2200 volts on both amps, equaling
> one KW input
> power key down on both, we can't drive the SB-220 any
> higher on its
> plate meter than we can on the SB-200's plate meter during
> SSB voice
> use. The higher peaks of current would result in
> instantaneously lower
> plate Z which would not be properly matched by the pi-net
> setting
> adjustment. The only way we could achieve increased PEP
> would be to not
> tune for the 450 mA previously stated. I assume this is
> what was really
> meant.
Roy,
The law was 1 kW DC input with suitable metering. That
specification included a dampening or response factor in the
meters, and to run a full KW you had to be able to read HV
and Ip at the same time. In grounded grid, the driver power
has to be deducted from the allowable input power. If the
driver ran 50 watts, you could run 950 watts plate input
power in the GG amplifier.
The tuning instructions for the Heath SB220 were to fully
load to a kilowatt on CW (minus the driver power). You then
flip the little switch on the front panel, and plate voltage
increased about 41%. Current also, for the same drive,
increased the same amount. Anode operating impedance did
NOT change, since HV and plate current went up by the same
proportion, so there was no need to retune.
The result was the loaded DC value of 1 kW current became
1.414 times more, and voltage became 1.414 times more. This
is where the mysterious 2kW PEP INPUT comes from. 1.414
times 1.414 is 2. The plate input doubles on crests of
modulation when the amp was loaded to 1kW dc (minus driver
power) because the VOLTAGE was 1.414 times higher on SSB.
You were NOT allowed to see, by law, more than 1kW INPUT on
the highest meter swings on SSB. That being so, the SB220
was never intended or designed to be used at an indicated
plate current reading of more than that required to flick
the meters up to 1kW on SSB voice peaks. That would have
been about 300mA or so on SSB.
The SB220 transformer and other components, including tube
cooling, was designed for 1kW maximum indicated panel meter
power.
I hope this explains the real reason for the "2kW" SSB
rating of the 220, the reason the transformer was sized as
it was, and why the cooling system is as it is. It was a 1kW
DC plate input amplifier indicated by highest visable meter
reading on all modes. Not 2kW, not 1200 watts output, not
1500 watts PEP output.
And of course as we all know now, the transformer does not
saturate at xx power. Nether does the filament transformer.
The filament transformer in conjunction with the resistive
losses in filament wiring and the filament choke works to
limit inrush current to acceptable values.
By the way, the claims indirectly heated tubes are not
subject to inrush current are absolutely false. Measure the
current, and you will see an indirectly heated cathode tube
has a LONGER period of excessive current than a directly
heated tube.
73 Tom
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