KM1H wrote:
>>I would use the power output control that is on most all the
>>transceivers
>>these days to not drive the AB1 amp into grid current.
>>Terry - W6TG
>
>Not very easy to accomplish with some of the newer rigs.
>Of particular concern is the prevalence of 4CX250 type tubes on the
>VHF/UHF bands; they are a boon and a bane at the same time.
>The lack of ALC circuitry in almost every published article for these
>tubes creates a problem for many VHF/UHF ops who have to operate on the
>band along with overdriven amps.
>
With the correct drive levels and power supplies, 4CX250, 4CX350 and
probably 4CX400 amplifiers can give excellent-quality signals. A lot of
work was done on this in Europe 10-15 years ago. At that time, which was
the peak of VHF/UHF tropo DX activity, it was quite common to have three
or four 4CX users in the same small town. We forced one another to clean
up our act, so we all learned very quickly! (Most of us are still
friends too.)
>I am not even sure if the typical VHF multimode xcvr has any provision
>for ALC input; perhaps Ian would have that info.
Most of the "table models" have, but you can't rely on it working well.
Power overshoots are very common, so it's not a good idea to rely
totally on ALC for power control. I'd rather set the drive level
correctly, and use the ALC as a safety net. Ideally the ALC should
hardly ever operate, and ALC should NEVER be used as a speech processor.
Taking up Tom's point about over-enthusiastic guest operators, there's
one "chief engineer" in a contest group over here who uses internal
trimpots for all the mic and RF power controls - all the front-panel
controls are dummies!
The other important point is to build the G1 supply with a low
impedance, so that a small amount of grid current is not an IMD
disaster. Also use grid swamping resistors so that there is no sudden
change in imput impedance when grid current starts to flow.
>The inclusion of ALC circuits in various VHF oriented publications would
>be a start in the right direction since AB1 tetrodes are more common at
>those frequencies.
Gimme time, I'm working on it!
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
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