On Sun, 24 Aug 1997 22:29:39 -0400 (EDT) KA2YKC@aol.com writes:
>I have a Commander HF2500 amplifier that seems to be experiencing high
>grid
>current readings. I am wondering if I have some flaky 3cx800's, the
>meter(s)
>are out of calibration, or I am just worrying for nothing.
You have a real concern.
>On 20m I apply approx. 35w drive CW. The plate current is 1050ma or
>so. THe
>plate voltage is around 2000-2100v. The load control is around the
>22-25
>mark, and the tune is at the 52-55 mark. I get 1500w dead-key on the
>Bird 43.
>I have used 2 slugs and 2 meters just to check the RF readings. All
>ofthe
>above seem to be close to the quoted values in the manual, with the
>excepion
>of the Grid Current. The factory test(s) sheet quotes the grid current
>at
>approx. 35ma. However, my grid ma meter is showing approx. 80 ma when
>I
>first key down and it slowly drops to approx. 60 ma or so after 15 -20
>seconds.
Those factory sheets ARE ONLY into a pure 50 Ohm resistive load. They are
only meant as guidelines to get you started.
You actually have to manually tune the amp into your own antenna system
for proper performance.
On 160-80M that amp will drift in Ig readings due to some component
heating but your readings are still way too high for long term tube life.
I suggest that you increase the loading ( LOAD control to more capacity)
while also peaking the grid current. A pair of 3CX800's should run about
10-20ma total when loaded properly and even with 100W of drive. The best
way to tune that amp is with the full 100W from your exciter and go for
maximum output which will be in the 2500W range.
THEN, use your exciter to drop power down to wherever you wish WITHOUT
touching the amp controls.
>
>Line voltage is a stable 238-240vAC and doesn't drop on keydown.
>
>After tuneup, I switch to SSB. With my TS950 and the processor (which
>results
>in a strong continuos 35wPEP drive), the grid current reading still
>peaks at
>60ma or so.
>
>However, with my Icom 738 also running 35w CW the reading (grid) is
>the same,
>but on SSB the grid current is around the 35ma point. On the Bird I
>still
>show 1500w when the meter is in PEP, but obviously in RMS mode the
>output is
>nowhere near that on both input and output . Also, the Plate current
>is
>somewhat lower.
>
>My feeling is that the Kenwood is really driving the amp on SSB with a
>solid
>35w PEP whereas the 738 is only occasionally peaking the 35w drive
>level,
>thus the lower plate current also.
Different rigs have different PEP SSB outputs and it is mostly due to the
ALC circuit time constants. Check the various IRCI bulletins to see if
there are any mods (factory or otherwise) for your rigs. Low SSB PEP was
a well documented problem with the TS-940 which appears to be corrected
in your TS-950. I have no ICOM experience; but it sounds related from
your brief description.
>
>Therefore, my questions are,
>
>a: Is the manual/factory measurement sheet quoting grid and plate
>readings in
>CW or SSB-PEP ?
CW Key Down into a 50 Ohm load and driven at 100W output.
>b: Are my experiences similar to those of other HF2500 owners ??
Not really, it is a typical question from first time amp owners in
particular. I think all will agree that it is best to ask questions
before making very expensive mistakes.
>
>c: Am I worrying too much as the tubes are rated at 80ma MAx per pair
>???
Your concern is completely justified. Drifting readings are usually a
sign of something wrong with a tube or circuit components.
>d: Could there be something wrong with the amp I am overlooking (all
>looks
>well. I
> even measured some of the components in the grid metering
>circuit.) ????
I'm still just going by your initial observations and feel that it is
just a tuning and familiarity problem.
>e: Could I have some odd 3cx800's causing the high grid current ?????
Odd 800's blow fuses; they are not a very forgiving tube.
Sorry for the slow answers but I've been off fishing for 2 days and then
off to a VHF/UHF conference for 2 more days. Didnt even look at other
replies yet.
73....Carl KM1H
>
>Otherwise, the amp works great and I am happy with it, although
>somewhat
>concerned.
>
>
>Your input and suggestions are greatly appreciated.
>
>73 Eric
>KA2YKC@aol.com
>
>
>
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