The 8874 and 3CX800 parasitic frequencies are well into the higher VHF
range. As such the Alpha designed suppressor is nowhere close to 10M and the
suppressors should be fine on 6M....normally.
That leaves open other problems such as a poorly constructed conversion with
no regard to proper design of the input, output, and plate choke. Any or all
could be at fault.
Id start with a GDO to examine the plate choke for series resonance and go
from there.
Ive converted many 76CA and PA to 6M with 8874's and they are very stable.
Ive also done several LK-800A's (three 3CX-800A7's) and their original
suppressors work just fine also.
Another thought...was the amp originally converted with the 8874's and the
3CX800's just stuck in with no changes? For all I know it could have been
one of mine that the 8874's finally gave out. Ive also done a few that had
seen better days but customers didnt want the expense of retubing and
figured 800W or so was enough.
Carl
KM1H
----- Original Message -----
From: <TexasRF@aol.com>
To: <K8LEEWMM@aol.com>; <amps@contesting.com>
Sent: Wednesday, September 09, 2009 3:40 PM
Subject: Re: [Amps] 76ca
> As a wild guess (have not been there/done that), the inductance of the
> suppressor is too high. There will be a voltage drop across that
> inductance
> related to the current through it. If this voltage is high enough, the 100
> ohm
> resistor will be above rated dissipation.
>
> The power dissipated in the resistor is equal to the voltage squared
> divided by the resistance.
>
> If the suppressor was designed for 10m, it is not surprising that the
> inductive reactance is greater on 6m resulting in more dissipation in the
> resistor.
>
> You can reduce the inductance (fewer turns) or increase the power
> dissipation of the resistor.
>
> The 3CX800 is a quite high frequency tube with short internal leads. You
> may find that the suppressor is not even needed. Again, beware: I have
> been
> there/done that. Use caution.
>
> 73,
> Gerald K5GW
>
>
>
>
>
>
> In a message dated 9/9/2009 2:30:14 P.M. Central Daylight Time,
> K8LEEWMM@aol.com writes:
>
> gentlemen
> I have acquired a nice 76ca that has been converted to a pair of 3cx800,s
> .
> The challenge now is that it also has been converted to 50mhz.
> On my first testing done, it worked very good. After running some test I
> slowing loaded
> the amp up for near max output. Within about 2 minutes the original plate
> lead suppressor went
> up in smoke. The 100ohm resisters opened.
> I have talked too and tried some of ag6k,s fix,s but no real steps
> forward.
> I have tried parasitic suppressors from a Drake L4B , the single "U"
> type
> with no change.
> It does have a tuned grid input unit mounted on the back, to me it seems
> to
> tune much too sharp?
> The plate choke looks non original, issue there?
>
> thanks for ur thoughts
>
> wayne k8lee
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