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/
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