Re: [Amps] 200-ohm tank and "un-un" in HF amp design?

["Carl" <km1h@jeremy.mv.com>]

----- Original Message ----- 
From: "Bill Turner" <dezrat1242@yahoo.com>
To: "Amps" <amps@contesting.com>
Sent: Monday, October 21, 2013 2:12 PM
Subject: Re: [Amps] 200-ohm tank and "un-un" in HF amp design?


> ORIGINAL MESSAGE:          (may be snipped)
>
> On Mon, 21 Oct 2013 13:36:18 +0000, Manfred wrote:
>
> > On the highest bands, the problem is another one: All that tube
> > capacitance and stray capacitance forces us to use a Q higher than we
> > would like! For example, on 10 meters with a 4000 ohm plate, a Q of 10
> > would need a tuning capacitance of just 14pF. Good luck finding a tube,
> > and a construction method, that keeps the tube and stray capacitance
> > lower than this!
> REPLY:
>
> The solution to this problem has been known for decades. A simple coil
> between tube anode and the rest of the circuit provides the equivalent of 
> an
> L-network which steps down the tube impedance to a value that is easily
> handled. Details can be found in any recent ARRL handbook in the amplifier
> section. The best part of this "trick" is the coil is most effective where
> it is needed, i.e. the higher frequency bands, and effectively 
> "disappears"
> on the lower frequencies. Problem solved for about a penny's worth of
> copper.
>
> 73, Bill W6WRT

This "trick" goes back to the 30's when the shortwave broadcast bands kept 
going higher to chase the sunspots and tube tecnology wasnt keeping pace. 
The high output C required a bit of simple engineering. Some hams might have 
done it first but the earliest I remember it was a Hints and Kinks in the 
50's or so but a lot of those are repeated on a regular basis.
For ham amps Ive been championing that method on here and elsewhere for 
decades and if you make that L coil resonant on 10M it results in a 4X 
increase in the Tune C requirement. Tap the coil in the middle with the Tune 
cap and it is 2X.  A random inductance coil gives random results...
It is very apparent that the ARRL ants in the labs read these forums as they 
have corrected several long existing foulups in the Handbooks as well as 
adding "new" ideas as if they got a bright idea all on their own one morning 
on the can..... Outside "consultants" dont bother giving any credit either. 

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