> To: amps@contesting.com
> From: "Ian White, G3SEK" <G3SEK@ifwtech.demon.co.uk>
To: <amps@contesting.com>
> Date: Sun, 13 Jul 97 08:28:42 +0000
> >This is still somewhat puzzling to me Ian. The peak grid current during
> >the 110MHz oscillation is sufficient to melt the copper wire in the grid
> >RFC.
I wonder how the VHF current gets so high through VERY large RF
choke so as to melt the wire, when full boogie HF drive doesn't melt
the wire?
This claim seems to be a case of pure "speculation".
> > . According to the fusing current ratings of copper wire, the
> >current (and the resulting electromagnetic force) must be substantial.
> >The tungsten wire is fairly hot. Apparently, the electromagnetic force
> >is strong enough to bend the hot wire.
Apparently? That sounds like pure "speculation". The question is...
"Could the current be high enough or not?"
The saturated emission current of a 3-500 is in the neighborhood of
ten or 12 amperes. No matter how much grid or anode voltage is
applied, the filament just CAN'T give up any more electrons.
Picture how difficult it is to reach the 12 amperes of total cathode
current. Obtaining this current requires almost 2000 VOLTS of
positive grid voltage with 4 KV on the anode!
It's difficult to understand how 12 amperes of filament-cathode
emission could be obtained through the VHF source impedance of
the PA power supply. The impedance of the VHF path from the power
supply to the anode, and through the cathode circuit back to the
power supply is very large. ALL those very big VHF current limiting
RF chokes and even the wiring impedance is in the way!!
Look at the filament. Does it make ANY sense a time-varying current
reaching only 12 amperes on peaks could bend a filament?
Remember the PEAK current is limited to 12 amperes, while the
AVERAGE current is about 1/3 that value in the case of the half
sinusoidal waveform typical in a tube. The average current is the
only current of long enough duration to overcome the mechanical
inertia of the filament and actually cause movement.
> Now can you take it down to the next level of detail, please:
> what currents flow in which wires, in which directions, and how do they
> produce the force that bends the wires?
We all know the current is absolutely limited to 12 amperes peak, or
under four amperes average effective dc current during an
oscillation. This would be with a zero impedance anode and cathode
supply impedance for VHF.
It is physically impossible for three or four amperes of short term
average filament current to bend the structure, even if we could
obtain the FULL saturated emission current of the 3-500Z.
> Bear in mind that a valid explanation has to cover all three of the
> following points:
>
> 1. According to the photographs, it's the heater wire that moves, not
> the grid.
>
> 2. Currents in wires can produce their own magnetic field, but you have
> to consider ALL the relevant wires.
>
> 3. RF current cannot bend the wire - above a few kHz it's too fast for
> the wire to follow. To produce a net physical movement there has to be a
> net DC component (or a low-frequency transient).
>
> Sorry, I don't know what the right answer is - only the conditions that
> it must meet.
Good point Ian. It would be interesting to see if these conditions
are possible to meet, short of having an arc. We know the limitation
on emission, so the only question is if three or four amperes of
worse case average RF current could bend a 15 ampere filament.
73, Tom W8JI
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