[Amps] Verify "Q"

[sm0aom at telia.com]

Q is usually chosen between 10 and 15, somewhat depending on how much losses can be tolerated,
and how much harmonic suppression is required.

Verifying loaded or operating Q can easiest be done by using an antenna analyser or VNA connected to the
output end of the pi-network and replacing the tube output impedance with a resistor of the proper value.

Then measure the bandwidth when adjusted for resonance between the  3 dB return loss or 5.8:1 SWR points and divide the centre frequency with
this value. Double the result for a first-order approximation for the loaded Q of the pi-network.

A pi-network has a slightly asymmetric amplitude response which makes the traditional Q = 2Cf/BW for a terminated network somewhat inaccurate.

73/
Karl-Arne
SM0AOM


----Ursprungligt meddelande----
Från : k2vco.vic at gmail.com
Datum : 2021-10-15 - 05:43 ()
Till : amps at contesting.com
Ämne : Re: [Amps] Verify "Q"

You have to leave the plates connected, because the output capacitance 
of the tubes is a significant part of the tank circuit.

The ARRL handbook talks about Q from 10 to 15. It is determined by the 
l/c ratio and the plate impedance of the tube. I don't think you can 
directly measure it because it is a property of the circuit when it is 
operating under load (it is not the same as the Q of the individual 
components).

73,
Victor, 4X6GP
Rehovot, Israel
CWops #5
Formerly K2VCO
https://www.qsl.net/k2vco/

On 14/10/2021 23:30, Joe wrote:
> Ok,
>
> Lots of reading, LOTS of help from others here. This should be the 
> last bit of questions I have the need of help with.
>
> When doing the like reverse plate load simulation test.  I have been 
> told, and have read, both things when doing the test, some have said 
> to lift the plate connections off the tubes.  While others say that 
> they need to still be connected. So what way is it?
>
> What value of "Q" should one desire to have?
>
> And once the amp is thought of to be set up with that desired value of 
> "Q"
>
> Is there a way to measure this value of "Q"?
>
> Joe WB9SBD
>
> _______________________________________________
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