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[Amps] SB220

To: "Roy Koeppe" <royanjoy@ncn.net>,"Amps Reflector" <amps@contesting.com>
Subject: [Amps] SB220
From: "Tom W8JI" <w8ji@w8ji.com>
Date: Sun, 16 Jul 2006 18:07:04 -0400
List-post: <mailto:amps@contesting.com>
> 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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