>
>I have a question for the group that I have seen discussed before. It
>regards glitch protection.
>
>It has already been pointed out that fusing the HV supply line provides
>little if any protection due to the way fuses blow when faced with a
>castatrophic event. What I am considering is inserting a normally closed
>vacuum relay in the plate voltage line and triggering it with an electronic
>over-current sensor monitoring plate current. I see that the Jennings RF-4
>series relays can open as quickly as 4 msec.
// Can the contacts break 200A at 4kV?
>Has Eimac ever done any studies delving into how fast an 8877/3CX1200/3-500
>transitions from stable to "full-on" parasitic oscillation?
// Eimac has been somewhat silent on the subject of parasites. I have
never seen a Eimac Engineering Bulletin on such. QST ex-staffer David
A. Newkirk theorized that a parasitic could reach full throttle in 10 to
100 micro-seconds. Eimac was slow in recommending a glitch resistor be
put in series with the anode supply.
>One would have
>to take into accounts that Parasitic Oscillations can range in degree from
>mild to severe. Would such circuitry add any degree of protection?
>
// My guess is probably not. However, a resistor will do the job
providing it can pass the short to ground test -- i.e., subsequently
exhibit no physical damage or change in resistance. The beauty of a
glitch resistor is simplicity. For instance, typical grid-current
"protection" circuits for 8877s use a 1A to 2A transistor to interrupt
the flow of grid-current. However, during a parasite, grid-current goes
out of sight. An 8877 cathode is capable of >50A-peak emission. This
amount of current will C-E short a 2A transistor in short order. Thus,
there is essentially no grid protection during a major parasitic
oscillation. A photo of a parasitic-damaged 8877 is at Fig. 24 on my Web
site. The amplifier that this tube came from uses grid "protection".
cheers, Bob
- R. L. Measures, 805.386.3734,AG6K, www.vcnet.com/measures.
end
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