>At 08:52 PM 12/12/97 -0800, you wrote:
>>>At 08:36 AM 12/12/97 -0800, you wrote:
>>>
>>>>There are no smallish wires in the the grid of an 8877. There are 100+
>>>>rectangular bars. At Eimac's maximum ratings of 4000v and 250MHz, the
>>>>grid of an 8877 carries over 40 RMS amperes. // There are no small
>>>>wires in the 8877 cathode element.
>
>Small is a very general term...compared to what?
- You could take a look and decide for yourself. If you don't have an
8877 grid handy, say so and I will mail you one. The choices are
heat-dam shorted grid, and gold-sputtered grid. The wires look pretty
substantial to me.
>and With > 4 Joules @ 50 volts
>flowing, small Eimac tube wires are small enough to be zapped, according to
>Eimac.
According to one source at 'Eimac'. However, according to a different
source at 'Eimac', thin layers of gold evaporate off of the grid during
an 'oscillation condition'.
- I have seen a number of grids from kaput 8877s, and I have never seen
one with an arc-mark on the grid wires. Roughly 2/3 of the kaput grids
have gold-boiling problems.
> "Thoriated tungsten filaments are capable of withstanding higher energy
>arcs than are the oxide cathode types. However, some of the smaller tube
>types such as the 4CX1500A, 3-500Z,
The 3-500Z is operated in grounded-grid, so how could an anode to grid
arc reach beyond the grounded grid to the tungsten filament/cathode?
>... ... they are
>trying to get us to invest $1.50 for a resistor to protect our investment
I doubt that a $1.50 resistor will limit things to 4J.
>
>Eimac states that a properly designed glitch resistor will limit the peak
>current during a fault to 40 amps. This is why your grid dissipation
>analogy doesn't wash.
Grids and cathodes are apparently not being damaged by an arc.
>IMO, the current available from an unprotected typical
>capacitor input filtered power supply is as much as 200 amps! Certainly
>well over 100 amps even in a very modest amateur table top unit.
>What is the energy delivered by a 4 watt-second charge in 5us?
Assuming that the voltage drop is constant, which is unlikely, assuming
that the filter capacitor can be discharged in 5uS, which is even less
likely - - on a sunny day, maybe 0.8MW.
... ...
>The electrons are emanating from the negative terminal, plus space charge
>neutralization. In all my amp designs, the Zener is right in this path.
>Do not get locked up on the grid; the cathode is just as much of a target.
If the grid is grounded, there can be no arc from the cathode to the
anode. The path of the alleged arc's electrons are: from the negative
terminal of the filter capacitor, through the anode current shunt R,
through the grid current shunt R, through the chassis to grounded-grid,
to the anode, through the HV-RFC, through the glitch resistor, and
finally to the positive terminal of the filter capacitor. .
>>Maybe not. Mr. Rauch and I have run a 3-500Z grid so hot it glowed, yet
>>no damage was done.
>
>I say again; run 10 Joules or so through there and report back.
I used roughly 30J. I have no idea what Mr. Rauch used.
>
>>How would you calculate the size of an 8877 glitch resistor using a 35uF
>>filter, operating at 4000v?
>
>The 4000 volts and 35 mfd are the easy part. The follow-on current from
>the power supply is the variable, so every case will be different. That
>is why Eimac devised the copper wire test so that the average builder
>can arrive at the proper value resistor without calculations or test
>equipment. They state that 4 Joules of energy (to give us an idea of
>what 4 Joules looks like) is stored in a 2 mfd capacitor in a 2000 volt
>power supply. So even it would need a glitch resistor to absorb the
>energy from the follow-on current.
However, there should no follow-on current if an arc was due to
rauchian-gas because such gas supposedly vanishes after the alleged arc,
which is why we can't find it a couple of minutes later with our
high-pot. testers. .
>What I am wondering now, is if the glitch resistor will also limit the
>current in a VHF parasitic arc to 4 Joules, and damage nothing?....Hmmmmmm
In my experiences, the arc from a VHF parasitic tends to make a clearly
audible noise, and is often visible -- such as intermittent arcing at the
tune C or across open bandswitch contacts. It seems to me that an arc in
a near vacuum would not produce much noise in the room. I have yet to
find a gassy tube which clearly made a sound when zapped on the bench.
It's mostly a light show.
>"Twilight Zone" theme; fade to black........
>
amen, Phil. A tangled web indeed.
cheers
Rich...
R. L. Measures, 805-386-3734, AG6K
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