> From: Rich Measures <measures@vc.net>
> Subject: Re: [AMPS] Re: Bent filaments
To: <amps@contesting.com>
> Date: Sun, 13 Jul 97 17:45:17 +0000
> >I wonder how the VHF current gets so high through VERY large RF
> >choke so as to melt the wire,...snip...
>
> Ian said this current is DC. I said it's DC. Who said that the current
> is VHF, Mr. Rauch?
> Rich---
Rich,
You now claim the VHF current is dc. But in another post you
claimed RF heats the grid of an 8877 and boils away the gold,
and you claim HUNDREDS of mA of dc can't hurt the gold.
Which is it??? Does it change to fit which kit you want to sell?
Page 16 of Care and Feeding of Power Grid Tubes says:
"At a typical operating temperature of approximately 1900k, a
thoriated tungsten filament will produce a specific peak emission
of about 70 to 100 mA per watt of filament heating power."
The same approximate figures appear in Reference Data for Radio
Engineers and other engineering texts. Reference Data for Engineers
defines the absolute saturated peak current as about 160 mA per watt
of filament power maximum.
Since the filament power is about 75 watts, the maximum specific peak
emission is 8.25 amperes, or 2.75 amperes RMS current for a normal
tube class B current waveshape. The absolute peak emission current is
about 10-12 amperes if the grid is held thousands of volts positive
with respect to the filament!
We find in text, including Eimac's text, the specific peak current of
about 8 amperes, which from an ac analysis limits the EFFECTIVE
current when amplifying or oscillating as 2.75 amperes dc. Even the
absolute peak current, if the grid could EVER be driven to 2000 volts
while the anode is held at several kV, would only reach 10 or 12
amperes.
Where does the dc current you think bends the filaments come from, if
the specific peak emission is 8 amperes and the absolute peak
emission capability is 10 or twelve amperes?
Once we find the source, we can calculate the force.
73, Tom W8JI
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