Hi Werner,
Unfortunately, the tuning must be made at full power.
After a friend of mine was killed adjusting the input coils
on a 4-1000A project, I quit using the pi input method.
I now use a "T" match on the input. This idea was "stolen" from
Creative Electronics. I simply install an MFJ antenna tuner ($59.95)
near the tubes. For two 8877's, I use a 2:1 unun transformer between
the tuner and the cathodes like Alpha did on their 77SX. This allows
a perfect 1:1 input VSWR on ANY frequency between 1`.8 and 30 mhz
and works every time! If you want to duplicate the MFJ circuit, all
you
need are a couple of small variable caps (200-300 pf) and a multi
tapped
coil/switch. By the time you add up the cost of all the components for
a
tuned input, the $59.95 for the MFJ is probably about the same, and
you
have the safety of not having to tweak an amp running full power with
the
cover removed.
(((73)))
Phil, K5PC
>
>I designed a switchble input pi-net for my 'two-holer' (a pair of
8877's). The
>calculated Z is aprox. 23 ohms. The circuit is located close to both
input
>blocking caps and tubes. I tested this circuit with both tubes
connected to
>the sockets and a 23 ohms resistor cross to both blocking caps. I
tuned this
>pi-net for all bands. As source generator I used the MFJ Antenna
Analyser 259.
>All settings was nearly SWR 1:1 and I was satified. Later, under real
power
>conditions all values are away up to 2:1 and higher. I know my first
test was
>made under static circumstance and in the real world it works under
dynamic
>conditions. But I do have no chance to tune the network under
operation
>conditions (no possibilities).
>Is there to go another way to fix it right under 'could conditions'?
Otherwise
>I have to make a new metal design to operate the input pi-net parts
under
>power conditions.
>Maybe it would be better to test this circuit with more r.f. power
than the
>MFJ 259 produces???
>73, Werner DJ2HZ
>
>
>
>--
>FAQ on WWW: http://www.contesting.com/ampfaq.html
>Submissions: amps@contesting.com
>Administrative requests: amps-REQUEST@contesting.com
>Problems: owner-amps@contesting.com
>Search: http://www.contesting.com/km9p/search.htm
>
>
--
FAQ on WWW: http://www.contesting.com/ampfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|