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

[Jim Thomson]

Date: Mon, 21 Oct 2013 13:13:33 -0700
From: Vic K2VCO <k2vco.vic at gmail.com>
To: amps at contesting.com
Subject: Re: [Amps] 200-ohm tank and "un-un" in HF amp design?

##  No way in hell is 3 uh gonna work for the  lead inductance.  That
circuit is commonly called a  L-PI.    Typ coil values are .3 to .7 uh. 
And no, it doesn’t make the amp unstable.   A buddy recently completed
a 80-10m YC-156 GG amp.   Now that tube is 35 pf  from anode to grid..
and rises to 50 pf when the grid flange is bolted to the chassis.    The min C on the
vac tune cap is 10 –12 pf.   So now we have  60-62 pf of tune C....with tune cap
unmeshed.  

##  We used the GM3SEK spread sheet to design the L-PI.   Runs like a swiss watch
on all bands...including 10m..with high eff on all bands... including 10m.   Flat strap coils
were used for both the L coil..and the main 10m coil.    IE:  2 x diff coils.   On a buddys
8877 6M amp, only one coil was used..and tapped in the middle with the hot end of the
tune cap. Globs of power on 6M. 

##  The coil b4 the main pi net trick works slick... and lowers the Q  on the highest band
in use.  I used the same trick... with a .66 uh coil b4 the main pi net... to lower the Q
down to 8  on my 3CX-3000A7  160-15m amp.   The 15m position is re-used on 17M,
where the Q rises to 12. 

Jim  VE7RF 





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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-- 
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco/