Question for those with more experience in this particular area than I have:
The Eimac spec sheet sez 5.0 +/- 0.25 volts measured at the socket. Do
they really mean the socket contacts or the tube pins? I see about 0.1
volt total drop across the pin to socket interfaces using the Eimac
SK-410 recommended sockets. How critical is the lower voltage limit? I
have always managed to get 5.0 at the tube pins in the past but this
time around I'm having fits just getting to 4.8 on the tube pins. If
this were a real tube like a 3CX6000 or the like, I'd have more control
by design but it really seems like more than it's worth to put in a
boost transformer at this point in the 3-500Z project. Comments?
Eimac cautions against inrush currents exceeding 2X the normal filament
current. I have a box full of tubes with grid to filament shorts that
attests to the accuracy of that statement. In the past I have always
used the typical resistor in series with the filament transformer with a
time delayed relay to short it out after a couple seconds. I used a
resistor value suggested by others and never actually checked the inrush
current. I have never had a tube failure after installing such inrush
limiting in all my other amps. This time around I made the resistor
adjustable and measured the current. I use a clamp on ammeter with an
analog meter and am under no illusions about it's ability to accurately
capture the peak current. My goal was to set up the resistor to minimize
the inrush current at turn on while also keeping it as low as possible
when the resistor was shorted out.
Turns out that the best I could do was 45 amps (two tubes) shown on the
ammeter at both turn on and when the resistor shorts out 2 seconds
later. I'd guess that the 45 amps I see on the meter is really at least
60. If I increase the resistor value, the initial turn on current is
lower, but the peak current is higher when it is shorted out. Like wise,
I can decrease the resistor value and increase the initial turn on
current and the shorted out peak current will drop. If I had known it
was this close, I would have made the step start a three step process
instead of a two step. Too late now, as the controller board is already
built and installed. Experience would suggest that this is good enough
and I guess I could verify if I had a current transformer in order to
capture the actual peak on the scope, but I don't, so can't. Any comments?
73, Larry
--
Larry - W7IUV
DN07dg
http://w7iuv.com
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