4 ms is like infinity for a power arc. This is shorter than one AC
cycle, so if you can trip off the mains power this quickly you are
doing good. But for the stored energy being released from the
capacitors, it is a long time. I am working from a background where
80-100 uF at many KV is used, so I may have the quantities exagerated
by a couple of orders, but in general, you really want to limit the
current first, then if you really want to open circuit, do it with a
relay or a semiconductor switch. A shunt switch as a crowbar might be
a better idea with your relay. Have a series R with it to protect it,
and then divert the energy that is otherwise destroying your tube or
tuning capacitor or plate RFC.
You should study the peak current ratings for the RF-4 and calculate
what your capacitors will supply into, say, a 50 volt drop of a DC
arc. If it exceeds the relay rating, expect poor lifetime (of tube,
rectifiers, and/or relay) and failure.
The purpose of a series impedance in the HV line is to drop voltage
when a huge spike of current is drawn, such as induced from a tube
internal arc (breakdown) OR a tank flashover due to running it
unloaded, or during a parasitic oscillation induced flashover. In all
cases, all of the tube manufacturers have recommended (for about 80
years now) to install small R (rated for big power and voltage
though) in series to limit peak current in such events.
It would be virtually impossible for a tube manufacturer to predict
how fast their device MIGHT oscillate uncontrollably, since they have
no control over the circuit design or layout. Parasitics are not a
tube characteristic but are a system problem, as has been pointed out
by many here in the past. Of course, as many have painstakingly
demonstrated in words here, some tubes are more prone to self
oscillation if rigorous techniques are not followed in building the
circuit around them. Yes, they can be mild to severe. They don't
always cause a flashover. You should rethink your design to protect
for whatever overcurrent event might occur, and consider some peak
current calculations.
Good luck, try some things out. In industry, we first sacrifice a
#29-32 AWG wire across the plate circuit to test protection. If it
blows open in a flash, then you are not protected for the 5-10 Joule
limits of the grid wires. This is a standard practice as demonstrated
by CPI/Eimac, RCA/Burle, Thomson/Thales, Econco, Litton, and others.
Another test is the foil test where a small solder wire is brought to
a sheet of thin aluminum foil. If the hole is larger than a few mm,
then you can expect damage during a tube internal arc.
>
>From: Bob Marston <k1ta@contesting.com>
>Subject: [AMPS] 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.
>
>Has Eimac ever done any studies delving into how fast an 8877/3CX1200/3-500
>transitions from stable to "full-on" parastic oscillation? 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?
>
>Bob K1TA
>
>-
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