>> From: daveb <daveb@buffnet.net>
>
>Hi Dave,
>
>> an anode transformer primary (2.5Kw level) ? I've noticed that
>> 25 Amp versions are available that switch on at zero crossing.
>> Sounds like a nice way to eliminate the mechanical relay with current
>> limit resistor for surge protection.
>
>That's a total misconception Dave, and you are not alone. One
>amplifier manufacturer I know of also sells an amp with a zero
>crossing turn on "thinking" and claiming it limits inrush.
>
>What is forgotten is this. Inrush occurs over several AC
>cycles, not just a fraction of one cycle.
>
Hear, hear. In my homebrew amplifiers, the step-start process takes
around 40 cycles, or 2/3 second.
>Starting at zero almost guaranties maximum inrush damage to
>components, because the entire first quarter cycle of ramp up is
>available every time for surge, as well as the start of the next
>half cycle, and so on.
>
True, but each subsequent quarter cycle would result in progressively
less and less current as the filament resistance decreases and the HV
filter C charges.
>What you really want to do is slowly start the supplies, and remove
>limiting after all the components have had a change to charge (or
>filaments to reach higher temperatures.
Well put, Mr. Rauch.
>
>Inrush is a duration problem over a few seconds, it is absolutely not
>a 1/240th of a second spike problem cured by zero crossing turn on.
>Zero crossing is like using a fuse to protect a grid...
>
OTOH, a frangible resistor or an instrument fuse is more likely to blow
during a glitch than is the 20w grid in a 3-500Z. So far, I know of no
one who has had a 3-500Z filament to grid short when the grid was
protected by a 30 ohm 1/2w carbon film resistor. However, there have
been many such resistors that went kaput. This is why we supply spares
in the suppressor retrofit kits.
Rich---
R. L. Measures, 805-386-3734, AG6K
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