This cap is a DC blocking Capacitor. It's main duty is to deny DC
voltage/current flowing into the next stage, usually the tuning tank. As
such it's value is not critical. However, it is connected to the input of a
tuning stage and as such it's reactance will influence the working
parameters of it. Thus, when simulating/computing
the next stage this capacitor value is taken in consideration. Usually in
the simulation it is connected in series with the RF generator thus it's
reactance varies with the bands. The bigger it is, the lower the frequency
response becomes (i.e. allows more RF to pass at low frequency) It actually
behaves as a Hi Pass Filter. The tuning components are influenced somewhat
by this capacitor but not much.
Some times this cap's value is chosen such it will nullify the internal tube
rectance by presenting to it a complementary one. It is often done in Solid
State amp. design
If you build an amp and simulate the output tank, then for the beginning,
chose an arbitrary cap value and then as you go thru simulation you will
optimize it for the best tradeoff between price/value/performance.
Need more? tell me :)
Alex, N2NNU
alex@sandlabs.com
http:/www.sandlabs.com
----- Original Message -----
From: <jstrohm@texas.net>
To: <amps@contesting.com>
Sent: Wednesday, November 28, 2001 12:38 PM
Subject: [AMPS] Another Stupid Question
>
> OK, only stupid because I never asked it. Yet.
>
> What determines the capacitance value for a plate-blocking capacitor? By
this I mean the capacitor (usually a doorknob cap) between the plate choke
and anode connection, and the start of the tank circuit / pi network.
>
> Obviously, this cap has to resist DC and RF voltages present, be capable
of passing the RF current present, and resist heat and other environmental
conditions present.
>
> What determines the capacitance?
>
> In reviewing a couple dozen amp schematics this morning, I saw a range of
500 to 2000 pF for HF and VHF amps. The only excursion from this range was
an .047 uF (4700 pF) in a 6146 final, which ran at significantly lower
voltage and power levels in HF.
>
> I don't need a detailed mathematical explanation -- just wonder if this
was an empirical "we always did it this way" design decision, or if there's
an electrical basis, such as time constant or reactance or a contribution to
the tank circuit elements, or a physical basis such as reactance vs.
physical size of the device in the circuit (e.g., parasitic inductance on
the structure of the cap vs. allowable value of capacitance).
>
> Jim N6OTQ
>
> --------------------------------------------------------------------
> mail2web - Check your email from the web at
> http://mail2web.com/ .
>
>
> --
> FAQ on WWW: http://www.contesting.com/FAQ/amps
> Submissions: amps@contesting.com
> Administrative requests: amps-REQUEST@contesting.com
> Problems: owner-amps@contesting.com
>
--
FAQ on WWW: http://www.contesting.com/FAQ/amps
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
|