[Amps] Minimum Value of RF Plate Chokes

[Alex Eban]

...guys, remember that adding capacitance on the input side does modify the
loaded Q, besides reducing the tuning range of the network!
This technique of "neutralizing" part of the output reactance of the tube is
in any case frequency sensitive, being resonant, so that departing too far
from the design frequency is going to wreak havoc to it. This is exactly the
way to smoke a plate choke!
Alex	4Z5KS

-----Original Message-----
From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com] On
Behalf Of Paul Kelley N1BUG
Sent: Thursday, February 26, 2009 7:35 PM
To: amps at contesting.com
Subject: Re: [Amps] Minimum Value of RF Plate Chokes

Hi Eddy,

Edward Swynar wrote:
> By effectively changing the circuit values in a tank circuit to
accommodate
> a less-than-perfect choke, what is the fall-out going to be to the
operating
> parameters of the tube...? After all, we are now CHANGING computed values
> for specific "Q" in the tank components, to make-up for the shortcomings
of
> the choke --- correct?

To my way of thinking, we're not changing the computed (or optimum) 
values of components in the tank circuit, nor its Q, *if things are 
set up and tuned correctly*. I look at it this way: Picture a tank 
circuit built with the computed values. Now picture a plate choke 
with less than enough reactance at the operating frequency to be 
"invisible". Assuming the bypass capacitor at the cold end of the 
choke has reasonably low reactance at the operating frequency, 
essentially the choke looks like an inductance shunted to ground. 
Now if we add a capacitor in parallel with the choke of sufficient 
value to form  a parallel resonant circuit with said choke, the 
combination of L (choke) and C (added capacitor) presents a very 
high impedance to the rest of the circuit and becomes "invisible".

Now look at the circuit. The tank input C is in parallel with the C 
we added to resonate with the choke. So why not replace them with 
one capacitor equal in value to the combined value of these two 
capacitors? If we do this by simply increasing the value of the tank 
capacitor, then when we tune the tank to resonance we are in effect 
resonating the tank properly, but at the same time providing the 
right C to resonate with the choke as a parallel resonant circuit, 
making it essentially "invisible".

Or am I looking at this wrong?

> Also, what would happen when the final tank circuit is off-resonance
> slightly --- will RF end-up back into the power supply?

Maybe. One would hope the bypass C at the power supply end of the 
choke has very low reactance and shunts most of the RF to ground. If 
there is a glitch resistor in series with the HV that should at 
least aid somewhat in forcing RF to ground rather than sneaking into 
the power supply.

> Again, I may well be missing something critical here, but by having to go
> through all these "gymnastics" in the values of the other parts of the
> pi-tank system, it is very obvious that the presence of a "sub-standard"
> choke is hardly "invisible" in the whole scheme of things, and it seems to
> fly in the face of a properly-designed final pi-tank network, on a given
> band...

I tend to be a purist and therefore would prefer to have a choke of 
high enough inductance to look invisible on its own. As a practical 
matter in a multiband amp (mine covers 160-10m) while also 
preventing potentially disastrous series resonances in the choke on 
the higher bands, I can live with the choke needing a little 
"assistance" in the form of extra pi tank input C on the lower 
bands. My plate choke certainly doesn't have enough reactance at 1.8 
or 3.5 MHz to be very invisible without help!, but with the cold end 
of the choke bypassed well and a somewhat higher than usual glitch 
resistor (I use 100 ohms) I've never had any apparent problems with 
  RF in the power supply (or otherwise).

However... I'm pretty much a stickler for keeping the amp optimally 
tuned as I QSY. Near the amp I have a little chart with tune and 
load settings for the various bands. For 160m the chart contains 
optimum settings at 10 kHz intervals, and for 80m 25 kHz intervals. 
With vacuum variables and turns counters, it's pretty easy to just 
tweak the knobs per the chart as I move around. It has become second 
nature.

73,
Paul N1BUG
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