Tim,
Good point.
I measured slightly more output on 10 and 15M - previously around 900W out
on 10 and now about 950W. It seems to tune more smoothly but this is pretty
subjective. Input SWR was slightly improved from about 1.4 to 1.2 (I checked
just before doing the direct grounding of the grids and immediately after).
The amp was not exhibiting signs of instability previously.
Whether this is worthwhile to you is a personal judgment. What I was
addressing with this modification was theoretical instability contributed by
the capacitors/resistor combination. This was described as making the
amplifier less stable above and below a certain range of frequencies. I
think that if the amp is prone to oscillation at VHF perhaps it is a design
problem associated with the grounding of the grids through R/C or L/C
networks and we are band aiding the problem with suppressors, specific R/C
values/combinations for grid grounding, and an occasional incantation. I
think using the grid as a proper shield goes a long way toward this kind of
amp stability.
I built a 3cx1000a7 6M amp that has the grid directly grounded, using the
SB-220 design and even some SB-220 components. Obviously, we are talking
apples and oranges here but that amp is stable as can be.
-ap
-----Original Message-----
From: owner-amps@contesting.com [mailto:owner-amps@contesting.com]On
Behalf Of Tim Duffy
Sent: Tuesday, September 26, 2000 6:20 PM
To: A P
Cc: amps@contesting.com
Subject: Re: [AMPS] SB-220 Smoke Test
So what benefit did directly grounding the grids bring?
More output on any or all bands for the same drive power?
The same output on any or all bands for LESS drive power?
A better VSWR between the radio and the amplifier input? The stock 220 input
VSWR on 10 and 15 is pretty poor.
Does the amp tune smother on 10 and 15?
In short, you said it works, but what does the change do to make it worth
while?
73,
Tim K3LR
A P wrote:
> First. . .good news Jose. Just keep up the high voltage respect . . .
>
> Second. . . SB-220 with directly grounded grid (three rg58 braids on each
> tube). Smoke test was a complete success. The amp tuned up on all bands
> (though with more load capacitance than expected into the dummy load) and
> put out rated power. For 70W input on 28.885 it put out 900W (120VAC
mains).
> Tuning was smooth and all functions were as expected. I think I'm going to
> leave the amp this way. . .
>
> Thanks to all for the advice . . . .
>
> 73,
>
> de k3xf
>
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