> I still have one major concern.
> I am to use this amp for HF SSB speech service.
> One of the respondents warned against serious distortion
> ( I guess IM3 ) of
> the G2DAF concept.
> I therefore did reread and reread the original G2DAF
> article but he
> enumerates under his concept's advantages a.o. " very low
> intermodulation
> distortion product level ".
> This confuses me.
> Though I am fully aware of the unusual dynamic
> characteristics of the G2DAF
> amp I still wonder if and why IM would be disqualifying or
> not.
Peter,
People sometimes promise anything when selling an idea. The
fact is there is NOTHING about the G2DAF system that helps
IM distortion. It does not help linearity, it generally
makes it worse. The same tube in traditional grounded grid
without the G2DAF circuit has almost the same gain and often
has much better linearity.
Think of where all the power comes from. The power for the
screen comes from the exciter, and the loading on the
exciter varies with the amount of screen current. Not only
that, the screen voltage is not at the manufacturers
operating range and it is very unstable.
I used to have a G station write letters, make calls, and
stop by the booth when at trade shows. He used to always
"push" the G2DAFcircuit for commercial amplifiers. When I
would tell him I measured IMD in the -20 to -30dB range from
tubes that normally did -40dB in a similar circuit, he
would always say "that isn't so bad". When I would ask him
what he measured, he would say "about -25 dB, but that is
perfectly reasonable".
It was a great deal like Bill Orr, who pushed and pushed the
favorite "super cathode driven" circuit he copied from
Collins. The fact is, outside of the a grounded grid tetrode
that NEVER runs grid current, that super cathode drive
circuit is a horrible idea. There was major pressure on
manufacturers from Orr to use that circuit despite the fact
it could be proven it did nothing good for the amplifier for
stability or distortion, and generally did a whole lot that
was bad.
Ideas often become a game of promotion, and some pretty
strange things make it into the mainstream because one or
two individuals take what really is nonsense on as a
"cause", almost like a religious experience. When only one
person is at the core of saying something is a good idea we
have to be extra careful to confirm it. Good ideas become
nearly universally agreed to, while bad ideas generally wind
up with one or two people dismissing the results of
measurements or tests by several other people.
73 Tom
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|