>On Thu, 26 Jul 2001 18:00:06 -0700, Curt Terwilliger wrote:
>>because otherwise one day in the heat of battle I will
>>forget to reduce the drive power manually and probably cook the 3CX800s
>Is it even possible to cook the tubes in an 87A? With my 91b, the
>overdrive protection circuit would trip in an instant if I ever did
>that. I'd bet the 87A is similarly protected.
>Of course, if you just want to add the protection circuit for peace of
>mind, that's ok too.
/\ A major danger to 8874s, 3CX800A7s, et cerera from higher drive is
not - as is popularly thought - from excessive grid-current. Instead, it
is destruction of the emissive (strontium and barium oxide) coating on
the cathode. An autopsy photo of this fatal malady in an 8874 appears on
p.17of the September, 1990 *QST*. As I understand it, the Alpha 87 and
88 have grid protection, but they do not have cathode protection. The
simplest way to protect a cathode is to place a low-L MOF resistor in
series with it. In multi-tube configurations, individual cathode
resistors tend to equalize the cathode currents as well as to make the
cathodes less vulnerable to "cockpit trouble". However, cathode
resistors increase driving Z, so the optimal tuned input values for L and
C2 will be different.
- R. L. Measures, 805.386.3734,AG6K, www.vcnet.com/measures.
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