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@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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