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> Date: Thu, 15 Oct 1998 10:40:33 -0400
> To: "Norell Bennett W." <bnorell@lge.co.kr>
> From: Pete Smith <n4zr@contesting.com>
> Subject: Re: [AMPS] SB-220 TUNE Cap Arcing
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>
> At 08:43 AM 10/15/98 EST, you wrote:
> >Hi Jon,
> >Just a followup and maybe the epilogue....
> >I checked the resistors in the (resistor-nichrome wire) plate
> >suppressors and they each read about 100 ohms as indicated.
> >I cleaned the TUNE-C, which had a couple of specs (I mean real
> >small specs) of solder sticking to a couple of fins. Also had
> >slightly sanded a couple of TUNE C fins that had black or rough-
> >ness where there had been some of the arc spitting.
> >The TUNE-C was quite rough to turn, so I retensioned the nuts
> >for smooth but not loose condition.
> >Here is the present condition:
> >Into a dummy load, using a Bird wattmeter and on amp meters:
> >No apparent arcs on any band, at any power. Temporary key-down:
> >
> >band Grid(ma) Plate(A) HV(SSB pos.) POut
> >10m 300 .88 2300 900w
> >15m 250 .75 2400 1000w
> >20m 240 .70 2500 1100w
> >40m 225 .68 2450 1100w
> >80m 235 .725 2450 1159w
> >
> >Of course the meter readings on the amp are not precise, but
> >is the Plate Amps or Grid amps too high on 10m?
>
> You're overdriving slightly on 10. Max grid current should be 250 MA.
> What are you using to drive it?
Pete, I am using a Icom 756 with about 90w to the input of the SB-220.
I have no measurable input SWR, after doing Rich's input mods.
If I am overdriving it, you mean I should reduce input drive power
below 90w?
>
> The dropoff in output on 10 and 15 is a function of the suppressors and
> maybe, to an extent, the age of the tubes. At rated grid current with
an
> unmodified, high-swr
> input circuit, I get about 750 Ma. and about 850 W. out, but KM1H's
> suppressors are less lossy than Rich's.
>
> I may have read on the
> >reflector that max Plate current is 400ma on a single 3-500.
> >Do these average readings look average for this amp, given that
> >the tubes may or may not be showing their age?
> >Other than a little foreign material on the TUNE C, I cannot
> >explain the original arcing.
> >After doing the above tests, I did the same connected to my
> >outside antenna (SWR was below 1.7:1 on all bands). No arcing
> >here either.
> >Any comments before we close the subject?
>
> Those specks just could be enough to start the arc.
>
> My advice is that you put index card stock behind the tuning dials and
mark
> the settings for each band and antenna, so that you can get close
before
> you apply power. That's nicer to your tubes and will reduce the risk
of
> arcing because you will never have too-light loading, which causes both
> excessive grid current and high tank voltages with the risk of arcing.
I have the dials marked, so tune up is quick and near perfect.
What is the amp control position in case of "too light loading" as you
refer to? Pretty novice to amp design here...
>
>
> 73, Pete Smith N4ZR
> In wild, wonderful, only middlin' rare WEST Virginia
>
>
>
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