On Jul 22, 2006, at 10:06 AM, Joe Subich, W4TV wrote:
>
> Measures writes:
>
>> If the anode potential fell to a very low level during parasitic
>> regeneration, electrons from the 3, 8874s' cathodes would mostly
>> flow into their grids, causing a >18A burst of grid-current which
>> would quite likely cause the 5A-rated Q1 to C-E short. .
>
> Anode potential falls to a very low level without corresponding high
> anode current? Amazing but not possible.
High anode-I corresponds with low anode-V.
> The plate current pulse
> would cause the control board to take the HV off line and quench
> any activity.
>
> In any case, VDR1 shown between the cathode and chassis would likely
> short if the cathode flew high enough to support your claimed 18 A
> grid current pulse.
Eimac specs the repetitive peak anode current at 6A per tube, and the
one shot capability would undoubtedly be a bit more than that.
>
>>> Once again, fast electronic protection takes the amplifier
>>> off-line. I repeat: Q1 is a bias transistor, it is NOT part
>>> of the protection circuit, and it is NOT required to interrupt
>>> large currents.
>>
>> Even during a VHF parasitic?
>
> Yes .. yes ... and yes again. The power for the parasitic has to
> come from somewhere.
??
> The parasitic must also generate an anode
> current pulse to "feed" itself - properly designed safety circuits
> will remove both HV and drive (and generally open the cathode as
> well) when high grid and anode currents are sensed at the same time.
For an 8877 amplifier, are these safety circuits designed to handle
the peak emissive ability of the tube??
>
>
> 73,
>
> ... Joe, W4TV
>
>
>
R L MEASURES, AG6K. 805-386-3734
r@somis.org
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