[Amps] Verify "Q"

[Thomas Walsh]

What I do is leave everything including the tube and even the plate choke in circuit but add a non inductive (carbon) resistor from the plate to ground, the resistor represents the plate impedance when in operation, then with an antenna analyzer looking
back into the output connector while holding the t/ r relay closed you should see
resonance and be able to dial it in with the tune and load caps to where you want it. Example: I recently built an amp using a yc-156 (3cx5000A7) in grounded grid configuration, after everything was in place I placed an 1100 ohm resistor from the plate to ground then I hooked up a lab power supply to the antenna relay to hold it in the tx position, then use an Mfj-259 at the output connector to find the resonance of the entire network including the tube C. If you use a graphing analyzer you can move the tune and load caps to dial it where you want it visually. This will put it very close for 1st try. This method also indirectly shows the Q by the width of the low swr curve, for instance if the lowest part of the swr curve is 20kc wide then after changes are made in the tank but now the low is say 5kc wide, that represents the higher Q setting. It is not meant to be dead nuts on but rather only a good starting point for first power up with rf. I prefer the wider swr low which makes it easier to tune during actual rf. The key is to use only a pure Carbon Resistor from plate to ground, no wire wounds! 

A.R.S. W2CO


On Oct 14, 2021, at 9:43 PM, Victor Rosenthal 4X6GP <k2vco.vic at gmail.com> wrote:

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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