To: <amps@contesting.com>
> Date: Fri, 22 May 1998 10:01:57 +0100
> From: Peter Chadwick <Peter_Chadwick@mitel.com>
> Subject: [AMPS] parasitics
> To: 'amps' <amps@contesting.com>
> I question the parasitic bending, because I don't see where the current
> comes from. In an oscillation mode, as opposed to an arc, current is limited
> by emission, and that's around 65mA per watt of heating current for a
> thoriated tungsten filament. This gives an upper limit for the 3-500Z
> filament assembly of around 4 to 5 amps.
Actually the saturated emission current can be as high as 100 mA per
watt at normal operating temperatures of 1900k (the section on
Electron Emission, page 9-9, of Electronic Designers Handbook by
Giacoletto).
In pulse service, with 5.5 volts on the filament, Eimac supplied
data indicates the saturated current is just over 10 amperes with
3000 volts of positive grid voltage and 5000 volts on the anode.
I think we can be very sure a typical amateur PA won't reach
ten amperes.
> Re Tom's point on neons. My father taught me that in about 1958. I've found
> it true, and I can't explain it. Another indicator is a loop lamp, where at
> VHF you get a blue glow in the bulb - I don't know why.
Interesting, I always though excessive VOLTAGE could also change the
color, because the ions got "over-excited". Isn't the problem one of
energy levels, and not frequency?
>As far as
> neutralising GG stages is concerned, it seems a good idea to me. And you can
> do it cold.
Yes, it works quite well...and is used in many GG amplifiers (despite
Rich's claims it can't be done). By the way, there is so little
difference from tube to tube in tubes like the 811A and 3CX1200A7
that neutralizing capacitance almost never needs any tweaking from
tube to tube, unless vendors are changed.
> On the 6146 variability, I do not agree with Tom that they are on the knee
> of the curve. These are DC conditions, and the plate voltage is 750, the
> screen at 210, and the bias around negative 45 volts. I'll admit tube
> variability is not as bad as transistors (and ICs). Hard experience has
> taught me that with RF analogue IC's, second sources usually aren't.
I tossed my 6146 data sheets years ago. At 210 screen volts, what is
the anode current delta when grid bias is changed 10%. Conversely,
what is the anode current delta when the bias is fixed at 45 volts
and the 210 volt screen voltage is varied 10%?
73, Tom W8JI
w8ji.tom@MCIONE.com
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