2 wrote:
>
>>
>>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?
>
Quite so - the purpose of the glitch resistor is to limit the current to
a maximum that will not damage the tube. The resistor (along with the
series resistance and inductance of the smoothing cap) limits the peak
current, even in the event of a crowbar short from B+ to chassis.
For tubes up to 1500W dissipation, Eimac suggests limiting the current
to about 40A, which is why glitch resistors have a value of a few tens
of ohms for the typical 2-3-4kV B+. Without the resistor, peak currents
are much higher.
>
>>Has Eimac ever done any studies delving into how fast an 8877/3CX1200/3-500
>>transitions from stable to "full-on" parasitic oscillation?
>
CONFUSION ALERT - who says *all* these events are due to parasitic
oscillations? What about gas arcs (there are a billion neutral gas atoms
inside a typical so-called "vacuum" tube) or direct shorts from B+ to
chassis?
Let's just say that big current surges sometimes *do* happen, and the
amp needs protection against them.
>// 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.
Maybe so, but if the current surge is caused by a physical crowbar
short, the rise time could be much faster. In either case, even a 'high
speed' relay is waaay too slow.
However, there *is* a place for a B+ over-current relay in glitch
protection - that is to disconnect the mains supply to the B+
transformer. Otherwise, whatever caused the glitch may just keep on
going (been there, seen that... it seemed to take me forever to hit the
switch). For this backup protection, a reaction time of a few
milliseconds is fine - but ONLY if there is a glitch resistor to catch
and limit the initial surge.
>Eimac was slow in recommending a glitch resistor be
>put in series with the anode supply.
>
Slower than when? Eimac's Bulletin #17 on 'Fault Protection' (which I
quoted above) was published in 1987.
>
>>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.
And also speed - the resistor has no 'reaction time' of its own, and
actually helps slow down the rise-time of the current.
>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.
Please can you give an example or reference to such a deficient circuit?
I've never seen one like you describe (but maybe I've led a sheltered
life). Where exactly is this 2A transistor?
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.com/g3sek
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