The 425 amp has a very keen anti-hotswitching “ watchdog” circuit. You’ll
notice if you try to use a foot pedal for example to key the amp and you have
the radio feeding a signal in before you switch the amp, the amp won’t key.
Like wise if the radio is putting out a signal and you unkey the amp it won’t
return to rx until you cut the drive power. This circuit totally protects from
hot keying. Totally impossible. This circuit monitors the input so239 for RF.
A resistor is in series with any RF at the input jack, rectifies it and it
disables the keying circuit if RF is present there before the switching of the
output relay. The sample resistor is only a small quarter watt resistor.
Normally when driving the amp you’ll never need more than 55 watts or so. At
55 watts the resistor can handle the current. If you run a full 100 watts that
resistor always passes current. Sustained power burns up that resistor over
time. Many I see have a burn mark on them, but they still work. One guy send
me one for repair and he sent the power of an SB200 through the amp on bypass
and roasted the resistor and burned a hole in the PC board. That’s when I
discovered this. When I asked him if he ran high power thru the amp when it
was off he said “ how did you know that”.
Sent from AOL Mobile Mail
Get the new AOL app: mail.mobile.aol.com
On Tuesday, December 15, 2020, doug ferguson <thenewt@msn.com> wrote:
Thanks for the suggestions. I’ll look at the coax and path to the tubes. As
for the degradation of the tube(s), I get full output
with exactly the same drive, about 50w on 40m, as I’ve always gotten, so if it
is an emission weakness it doesn’t effect output
at all.
As for putting 100w through it when off, Guilty! Not very often as I don’t
often need that much power for high duty cycle modes
But have done it, I’m sure. What in the amp could that damage? I’ve not been
able to find a stat in the manual for how much power
It can handle in “pass through” mode. I only would have done it when turned
off and only found a referenct to how power is routed
from the exciter when the Amp is off.
I’ve had the Amp over 20 years and as far as I know it’s got the original
Eimacs in it, certainly not Chinese changeouts but that’s good
to know if I ever do change tubes. I’ve been extremely careful with grid
current, etc., over the years and it’s rewarded me with great
performance and the K3 tuner does the trick but not sure how this SWR issue
might affect it long term. It doesn’t seem to be getting
any worse although as I said, it’s very slowly changing and I first noticed it
a couple years ago.
Doug – N7NM
Sent from Mail<https://go.microsoft.com/fwlink/?LinkId=550986> for Windows 10
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|