Of course! In any grounded grid amplifier the exciter sees
both the input and output loading. Output tuning is highly
influenced by the power level (maximum impedance at minimum
plate current, ie, no/low drive). Thus the exciter will see
a significantly different total load depending on the power
level.
About the only time input and output tuning will be truly
independent is when using a (swamped) grid driven tube in
class A or AB1 (no grid current).
73,
... Joe, W4TV
On 12/12/2023 4:37 PM, Alek Petkovic via Amps wrote:
For what it's worth, I have had terrible problems getting the input
matching right on several amplifiers using the VNA, yet when using
normal drive levels, as reccommended in the user/service manuals, tuning
is a breeze. Therefore, I think that there is some truth in the
assertion that tuning at low levels does not work properly.
73, Alek VK6APK
On 12/12/2023 7:27 pm, Rob Atkinson wrote:
If you try
to use the internal auto tuner it will not tune properly as it tunes
up at low
power and the input impedance of the amp changes as the power increases.
Power level should not affect the impedance of a load. If it does,
then something is wrong. A part may be heating up and changing value,
in which case the matching network should be fixed. You should be
able to put a vswr analyzer on the amp input and see the R + X for
some frequency, and those values should hold for whatever drive power
you use on the same frequency.
73
Rob
K5UJ
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