[AMPS] adding a second tube

Sun, 10 Aug 1997 18:53:48 +0000

> Date:          Sun, 10 Aug 97 19:41:21 +0000

> -  For example:  When the second tube is added to a 77dx, and the tubes 
> are driven to 2A of anode current, the stock tank's operating Q on 160m 
> drops to about 5.9.  IMO, this is not good engineering practice.

That may be your opinion, but it is most likely not a real 
concern. There is no rule or law that says the Q needs to be 
"twelve". The minimum Q for a conventional Pi is established by Qmin 
= 1+ (sqrt of Rhigh/ Rlow). If the pi-L has a intermediate 
transformation impedance of 200 ohms and a source impedance of 1000 
ohms, the required Q is only  1+ sqrt of 1000/200.... or a Q of ~3.23 

The "Q of 12 rule" stems from the days when PA's were commonly class 
C, and link coupled or conventional Pi networks were used.

Linear PA's, with their longer conduction angles, generate much less 
harmonic energy than the old class C PA's. Class AB PA's make
very poor harmonic generators, that's why doublers and triplers are 
all operated in low conduction angle modes (like class C). 

A SPICE analysis of the Alpha 77 pi-L tank circuit shows reducing Q 
from 12 to 5.3 increases the second harmonic level by just over 4 dB, 
and increases the third harmonic level only by about two dB.  The ETO 
I measured here had about -47 second harmonic suppression at 3800 
watts output. The FCC requirement for a 1500 watt PA is about -45 dB. 
Why worry if it is 45 or 49 dB??

Imagine the time and money necessary to get the tank Q back up to 
12....just to pick up 4 dB of additional suppression!

73 Tom

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