>I see terms like "unjustifiable" being used in connection with the
>proposed standby switch on the SB-220. However, there's a different
>angle.
>
>To say that turning a 3-500Z on and off a few thousand times has
>absolutely no impact on its lifetime, is ludicrous.
I have a made in 1967 Amperex tube in my SB-220. The filament lights.
The emission is acceptable. This tube has been turned on and off many
times. I have tested many failed 3-500Zs. I have never found a filament
that would not light. These things are hardly like incandesent lamps,
Peter. The operating temperature is a relatively cool 1800 degrees
kelvin.
Some old timers like to run the filaments for a few minutes before they
turn on the anode supply. Do you think adding a switch to perform this
function is justifyable?
...snip...
>Also, several posts have mentioned (without specific explanation
>that I noticed) that the HV must be on the low setting to ensure that
>the tube lifetime is not jeopardised.
Filament inrush I is limited by the special core design in the filament
transformer- -no matter which HV tap is selected. The jeopardized part
is the on/off mains switch. On the HV tap, there is considerably less
resistance, so the startup current is substantively higher than is the
case with the LV tap. If anyone needs a replacement switch, I have some
with higher current capability.
-Rich---
R. L. Measures, 805-386-3734, AG6K
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