[Elecraft] Ameritron 240V inrush limiter: any benefit on a KPA500?

[Victor Rosenthal]

The simplest circuit, which I added to my TL922, is a relay operated 
from a DC supply with a capacitor across the coil. The circuit can be 
set up to provide from a fraction of a second to a much longer delay. 
When the relay is de-energized, a small resistance is inserted in series 
with the line to the plate and/or filament transformer. When the relay 
pulls in, the resistor is shorted out and the line is connected directly 
to the transformers.

I set mine up for about half a second, mostly to increase the life of 
the amplifier's on-off switch, and also to provide a more gentle 
start-up for the tube filaments. It has a side benefit of preventing the 
"thump" which sometimes occurs.

The cold resistance of the filaments is very, very low, and the inrush 
current can be very high. As K4TAX said, in a properly designed 
amplifier, the transformer itself limits the inrush current to the 
filaments and the initial charging current of the capacitors in the 
plate supply to a safe value for the tubes, rectifier diodes, and other 
components.

But I have had to replace on-off switches in the similar SB220, which 
means finding the right switch or it looks ugly. And I believe that the 
life of the filament will be longer if it heats up more gradually. In 
the big WE transmitter I mentioned in my previous message, there was a 
five-step switch to turn on the filaments, and I was told to wait about 
a second between steps.

It might be unnecessary, but it was just a few junkbox parts.

73,
Victor, 4X6GP
Rehovot, Israel
Formerly K2VCO
CWops no. 5
http://www.qsl.net/k2vco/