Measured Voltage results of Alpha 9500 key
down testing.
AC Mains Voltage at Amplifier switch box.
142 VAC Amp on at Standby.
239.6 VAC Amp at 1.3 KW into Dummy load
?
Filament Voltage
5.3 VAC at initial power on.
5.45 VAC after 180 Second warm up remains
at 5.45 during subsequent Standby (not Transmitting).
5.16 VAC on Key Down 1.3 KW.
?
Plate Voltage
3.5 KV during any and all standby
conditions.
3.2 KV during key down 1.3KW.
?
Looks like normal Voltages to me.? Strongly
points to a weak tube.
?
Thank you, Alan and Alex for urging the
Voltage tests.
Alan, the filament voltage check was really
simple through the bottom plate.
Having two Fluke VOMs made this testing
easy and less dangerous.¡ªno having to move probe tips after
set-up.
?
73,
Dave, w6de
?
?
Hey Dave,
If your amp has been operating "normally" for a length of time
(i.e., with apparently normal/full output) and only recently
started showing signs of decreased power, then it's probably a
bad tube.
However, it would be prudent to check your filament voltage,
to ensure that it is within spec. If anything will kill an
8877 tube, it's LOW filament voltage.? An under-temp/cold
cathode will cause striping of the coating off the cathode,
dramatically shortening its life. The
filament voltage must be checked at the tube base, at both
static (no load) and loaded (max RF) conditions.
Tom Rauch, W8JI, has documented
this... it is worth the read. HERE:??
and HERE:
On the 9500, you can access the tube base by setting the amp
over on its right side (viewing from the front), and removing
the cover on the amp bottom, below the tube. Obviously, for
safety: Do this with the amp unplugged, and be certain the B+
is fully discharged (although no B+ voltage on the bottom).
Make the connections to your meter, then plug-in the amp and
power up. No harm in running the amp on its side for the short
period of testing.
Connect a DVM to the filament pins, and then fire-up the amp.
Allow the tube to come up to temperature (3 minutes), then
make note the filament voltage. Then apply some drive and get
RF output (preferably near legal-limit) and again make note of
the filament voltage.? If the filament voltage is NOT within
specs, then there's an issue.
It's better to have slightly high voltage unloaded, and at or
near specification (or slightly high) under load, than it is
to have low voltage, especially under load.
While replacing your existing tube will likely cure the low RF
output problem, continuing to run with low filament voltage
will kill the new tube also, although it may take a while.
Just as a point of interest, my 9500 runs at closer to
3450-3500 volts on the plate, and my input line-voltage is
always very close to 250v (247-249).
Good luck. Let us know what you find...
73,
~Alan
W5ARM
