On Sat, 18 Sep 2010 10:11:00 -0700 (PDT), "Randy S."
<rj3819@yahoo.com> wrote:
>Hi de N2CUA, Randy, Taberg, NY
>
>I'm new to the list..
>
>Just bought a nice home brew KW HF Amp
>With a pair of 4-400A?s for a couple hundred dollars..
>I don?t believe there is an RF Choke to ground on the Output .. should I add
>one? I see it in many schematics.
Yes, you should add one. It's purpose is to prevent HV from appearing
on the output if the plate blocking capacitor should short. It will
cause the fuse to blow if that happens.
>Also, untuned input .. and short of using an antenna tuner on the Input, not
>sure I can add it .. pretty crowded chassis. Ideas?
>Do I need it really? Input impedance is pretty low on grounded grid Service
>right? So not to worry even with a solid state rig with SWR protection? Thanks
>for any advice and tips ..
If you are driving it with an older tube-type transceiver with a tuned
putput, you don't need one. Newer solid state transceivers do need
one for two reasons: 1. Provide a more uniform load over the 350
degree RF cycle to prevent possible damage to the transceiver, and 2.
Provide the "flywheel" effect needed by the amp for best linearity and
IMD performance, especially on an amplitude-varying mode like SSB.
Linearity and IMD are not important on constant-carrier modes like CW
and RTTY.
>OH he also put a TR switch inside .. one unit .. nice looking one ..
>Looks to be expensive, has those smaller UHF type connectors on it.
>TNC connectors I think.
>
>Is there an issue with the TR sequence using something like that?
There could be a problem. The only way to be sure is to examine the
output waveform with a scope. Ideally, you want the antenna to be
connected to the amp before any RF drive is applied from the
transceiver. Connect the scope across the output of the pi-network
before it gets to the relay and look for a sharp spike of RF which
would last just a few milliseconds before the relay closes. Do this
check with a small amount of drive at first, then increase it up to
the amp's limit. Be sure your scope probe is capable of handling high
RF voltages If you don't see any spike, you are ok as is. If you do
see a spike, check your transceiver manual to see if there is a menu
setting to introduce a delay in the RF output when PTT is activated.
If your transceiver doesn't have such an option, then you need to add
a sequencing circuit. A full-power spike like that can be damaging to
the tube and the components in the output tank.
And welcome to the group. Have fun with your amp.
Bill, W6WRT
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