ORIGINAL MESSAGE:
On Sun, 3 Apr 2011 11:32:54 +0200, "BEHIELS JEAN-PIERRE"
<ON4AEF@base.be> wrote:
>
>I have to now what the minimum capacitance is,of the input circuit of a
>homebrewed amplifier PI-filter system.
>Can I measure this value with the vacuum capacitor already mounted and
>connected on his place into the input of a PI-circuit.
REPLY:
Yes, you should measure the minimum capacitance with the tune capacitor
connected but with the tank inductor disconnected and also the plate RF
choke disconnected. The tube must also be in socket and connected but no
power applied, not even filament power.
If you then find the minimum capacitance is too high for resonance on 28
MHz, there are two options:
1. Use the minimum capacitance and reduce the value of the tank
inductor. This will give you a higher tank Q than you want, but if you
can accept the slightly higher losses and sharper tuning peak, it will
work.
2. Or you can use the method described in any ARRL Handbook in recent
years. Install a small inductor (typically .5 uH or less) between the
tube anode and the plate blocking capacitor. This acts as an L-network
and transforms the tube's load impedance down to a value suitable for
normal Q with the relatively large minimum capacitance. See the Handbook
for further details.
I have used both methods in different amplifiers and either one can be
made to work well.
73, Bill W6WRT
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