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

[Carl]

It means squat unless you believe Measures voodoo. Kill the parasitic 
tendency right at the anode with a correct value L.

Proper layout and construction practices even work with a pair of 4X1's on 
10M.

It also helps to have a SA turned on to watch for possible problems but even 
a GDO in diode mode is a good tool; I used one for decades before investing 
in my first SA, an old HP 141T system.

Carl
KM1H


----- Original Message ----- 
From: "Vic K2VCO" <k2vco.vic at gmail.com>
To: <amps at contesting.com>
Sent: Monday, October 21, 2013 4:13 PM
Subject: Re: [Amps] 200-ohm tank and "un-un" in HF amp design?


> 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
>>> _______________________________________________
>>> Amps mailing list
>>> Amps at contesting.com
>>> http://lists.contesting.com/mailman/listinfo/amps
>>>
>>
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>
> -- 
> Vic, K2VCO
> Fresno CA
> http://www.qsl.net/k2vco/
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