[AMPS] More on 91b Problem, Any Ideas?

[Dick Green]

> As a sanity check I * V * efficiency = output  (2550 V * 0.6 A * 0.65) =
995
> W
> which is pretty close to what I am seeing on the watt meter.
>
> Given this additional info... anyone have any ideas ?

Tim,

The HV readings sound OK, but the plate current is too low. That should be
closer to 1 amp. If the Icom rig can successfully drive the amp to 1500W
with 50W drive, then I can think of only two possibilities: 1) the power
from the rig is not getting to the tubes, or 2) drive from the rig is
pulsing, causing a low average power reading.

As I'm sure you've heard already, high input SWR could be the problem. But
if the solid state rig sees a high SWR, it will reduce the power output and
you will see that on the rig's power meter. Is the rig reporting 50W output
at the same setting of the RF PWR knob, regardless of whether the rig is
connected to the amp or a dummy load? When you test power output from the
rig to a dummy load, is it (and the SWR) the same whether the rig is
connected to the dummy load through the amp (in standby) or directly?

If you haven't already, I would put the Daiwa meter immediately after the
rig and before the coax link to the amp. Drive the amp to max power and
check that the Daiwa really reads 50W with zero reflected power. Repeat the
test with the Daiwa at the amp end of the coax link.

Did the 91b come with a coax link for hooking up the rig? My 87A did, and
I've found that I have to use that link or one of exactly the same length (6
feet) in order for the rig to see the best input SWR to the amp.

I raised the possibility of RF feedback because I've seen it happen on my
FT-990. It would be helpful to know if the plate current meter and wattmeter
in the 91b are average-reading or peak-reading. Alpha should be able to tell
you that. They're peak-reading in the 87A, but I don't know if that's true
of the 91b. If they are average-reading, then I'd suspect some sort of
pulsation going on due to RF feedback from the amp to the rig. Since you are
using the FT-1000D mechanical relay to key the amp, it seems unlikely that
RF feedback via the key line is the problem, although it's possible. The
relay itself could be pulsing, but you'd probably hear that. Did you try
keying the amp (i.e., shorting the relay line) manually while transmitting?
That simple test would probably eliminate the RF feedback theory.

73, Dick WC1M



--
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