> How would one go about checking the resonant frequency
> of the grid
> ckt of a tube such as the 572B or the 811A or any similar
> tube for that
> matter ???
It's not too complicated Carl.
You have to know what frequency is the parallel resonance.
Series resonances, which mean the grid is most effective,
will show up on a GDO. You are also measuring outside the
actual grid structure so you can't just assume the grid
behaves anything like the external terminal impedance would
suggest.
But there is an easy way to do it accurately with the right
test gear.
If you apply a leveled source to the cathode (and don't have
any resonant systems connected to the cathode) and measure
the output voltage passed through to the anode, you actually
can determine the frequency where the grid is parallel
resonant (high impedance). There is a sharp peak in
feedthrough because the grid is no longer grounded.
Somewhere about that frequency is where the tube goes into
parasitic oscillation.
If there is enough interest, I can sweep several tube types
and show the difference.
By the way, there are several ways to keep the tube from
oscillating. One is to have the anode circuit resonant far
from the grid frequency. Another is to make the anode load
impedance, looking towards the tank, largely a resistance
with the value of resistance large compared to the reactance
of that path at that particular frequency.
The problem you can get into with the series-L from the tube
to the tank (as per ARRL Handbook) is the series L can and
often does have a very high impedance over a wide frequency
range, and if it happens to hit the same frequency as the
grid you have a dandy TPTG oscillator. This "luck" or
"misfortune" is why some people are successful with a
series L and some are not, even when using the same tube. Of
course with a network analyzer and the right fixtures you
can sort this all out and have a stable amp the first time
out.
By the way, if the Handbook implies this lowers tank Q that
actually might or might not be the case. In many cases the
additional coil will simply increase tank Q, although it
will allow resonance in a seemingly impossible situation.
Myself, I think that method is overrated. With a good tank
it doesn't matter of the loaded Q is 15 or 50. I've built
some very high power VHF amps with tank Q's of 50 or higher
with very good efficiency.
73 Tom
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|