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

[Jim Barber]

Absolutely. I learned that trick from Carl, seems like a few hundred 
years ago.

This design uses 3uH of "lead" inductance for just that reason. It 
raises the tune capacitance at 10M to a practical value, but only raises 
the load capacitance on 160M by a few picrofarads.

Ian's spreadsheet accommodates it, which is why I eventually want to 
borrow the math from it and formalize it into a desktop app. (with Ian's 
permission, of course)

73,
Jim N7CXI

On 10/21/2013 11:12 AM, Bill Turner wrote:
> 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
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