[Amps] Starting up a NEW Henry amp

[Joe Subich, W4TV]

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
>>