813 Plate Resistance

[W1RKW]

Anyone know what the plate resistance of a pair of 813's would be looking into a Pi network that is running about 2.0kV and I have no idea on plate current but figure maybe 300 to 400mA for the pair?  Can I assume  Vp / Ip to get an approximate value?  I'm trying to reverse engineer my tank circuit because I do not have a good match at this point. It's resonating as best as I can tell somewhere between 80m and 40m.  And how does one treat a pair of tubes vs. a single tube.  Does parallel resistance for two tubes come into play or does one treat the total current as one vs two?

[k7yoo]

Class C plate resistance is 3333 at 300 ma. This is a good level and where I run the pair in my T-350XM at.  Remember ---2X IP for class C RF
Modulating impedance at full load is as you stated, or 6.6K

I suggest going to the James Tonne homepage and downloading the PIEL program
http://www.tonnesoftware.com/

Skip

[k4kyv]

Plate voltage vs plate current is the modulating impedance, but that is not the same thing as plate resistance.

The plate load impedance is based on rf current vs rf voltage at the plate of the final.  For class-C operation there is a formula based on DC plate voltage and current for a given Q, and the ARRL handbook, at least the ones form the 50's and 60's, gave a set of equations for calculating the values of the components in a pi network.

It doesn't matter whether it is a single tube or a pair; it's the total plate voltage and plate current that matters.

The set of formulae is rather complicated, so I would look in the plate tank capacitance chart for the plate tank tuning capacitance required for a Q of 12 at the desired frequency, and using cut-and-try, build a tank circuit that would hit resonance with that value of plate tuning capacitance while the loading capacitor is adjusted to maximum capacitance (minimum loading).  As you increase the coupling by decreasing loading capacitance, you will have to increase the main plate tuning capacitance to maintain resonance because the two capacitors (plate and loading) are effectively in series.  If the maximum setting of the loading cap still gives too much plate current but you can get a dip, then you need to increase the size of the loading capacitor, or switch in additional fixed capacitance in parallel.

[stevef]

Tonne's PI-EL program uses "Peak Plate Swing" as a required entry, which I've been told is not the same as the DC plate voltage, but there is ambiguity in the answers I've received as to what it actually refers to.  The EIMAC handbook page 46 alludes that it is the peak value minus minima value of RF plate voltage.  But EIMAC uses constant-current curves to calculate it (and so does Orr's handbook).  I have never found such curves for the 813.  And I have yet to locate the equations to calculate it.

[W3SLK]

My HN-500 I run the 813's into a Pi network with an Ip of 350mA @ 2.0KV. 500 watts out on the nose.

[AB2EZ]

The rf output impedance (a somewhat confusing name for what is being talked about) is defined as the impedance of the pi network at the frequency of operation (e.g., 3.885 MHz) which drops the full plate voltage when the plate current component at the operating frequency passes through it.

Refer to the attached JPEG

In a Class C amplifier, the plate current component at the operating frequency has an amplitude that is twice the average plate current. This comes out of the math. Note that the peak Class C plate current is much larger in amplitude than the average (DC) plate current.

Thus the desired value of the pi network's impedance at the operating frequency is equal to: the plate supply voltage / [ 2 x average plate current]

With this value of pi network impedance at the operating frequency, the voltage on the plate of the tube will vary from 2x the supply voltage down to zero on each cycle of the rf. This results in the maximum output power for that combination of plate supply voltage and average plate current.

Example:

Plate supply voltage = 2000 volts
Average (DC) plate current = 400 mA

Modulation impedance (presented by r.f. stage to the modulator): 2000 volts / 0.4 amps = 5000 ohms [assuming a triode, or a tetrode that has the appropriate amount of screen modulation... so that, under modulation, the average (averaged over an rf cycle) plate current is proportional to the modulated B+ voltage]

Class of operation: Class C

Average (DC) plate current (as stated above): 400 mA
Fundamental frequency component of plate current (in mA): 800 sin(2pi x f x t), where f is the operating frequency. [Again: note that, for Class C operation, the amplitude of the fundamental frequency component of the plate current is twice the average (DC) plate current.]

Required impedance of pi network to cause the plate voltage to from 2x plate supply voltage to zero volts (i.e., swing between 4000 volts to 0 volts) = 2000 volts (amplitude of plate voltage swing at the operating frequency) / .8 amps (amplitude of plate current swing at the operating frequency = 2500 ohms.