[AMPS] Tube load resistance calculation

[Tom Rauch]

Date sent:      	Wed, 26 May 1999 09:31:43 -0700
From:           	Vic Rosenthal <rakefet@rakefet.com>
Send reply to:  	rakefet@rakefet.com
Organization:   	Transparent Software
To:             	Amps reflector <amps@contesting.com>
Subject:        	[AMPS] Tube load resistance calculation

> 
> My ARRL handbook gives the following formula for calculating the load
> resistance of a vacuum tube power amplifier:
> 
>         Vp
>   Rl = ----
>         KIp
> 
> where K is 1.5 for class AB, 1.57 for class B, and 2 for class C.  I'm not
> sure how they get this constant, and I wonder if it has anything to do
> with the duty cycle of the signal being amplified:  in other words, I
> wonder if these constants would be different for a CW or voice signal?  My
> guess is probably not.  Is this correct?

It is an approximation based on the conduction angle of the tube. 
The longer the period of conduction in each individual RF cycle in 
comparison to off-time in each RF cycle, the closer to the DC 
resistance the source impedance of the tube becomes. For class 
A the factor is 1.

The formula is just a guesstimate, the exact answer is given by a 
Chaffe analysis where the available RF current at the desired 
frequency is compared to the maximum anode voltage swing.

(Notice that Warren Bruene's theory about plate resistance and 
output device gender determining the source impedance does not 
fit ANY of these formulas.)


73, Tom W8JI
w8ji@contesting.com

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