[AMPS] SB-220 standby switch

[Rich Measures]

>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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