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

[Vic K2VCO]

Note that this lengthens the VHF 'parasitic tank circuit' between the plate, the frame of 
the tune capacitor, and the cathode. So it can cause instability.

On 10/21/2013 12:21 PM, Jim Barber wrote:
> 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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>
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-- 
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco/