First I have to say I based my view of the circuit on an old article in
an USA magazine, and other amplifiers I have seen.
I see, after looking at Ian's page, the original G2DAF circuit is
much worse than I initially thought.
It has no negative feedback. It simply runs a tetrode as a grid
driven class AB2 triode amplifier, with a tiny amount of time-varying
screen voltage. The 300 ohm swamping resistor is almost nothing
in this circuit compared to the peak loading of the exciter when the
grid swings positive.
The circuit certainly looks like someone didn't have the foggiest
idea what they were doing at all when they "designed" it.
> I simply stated that some grounded grid amps, which happen to use sweep
> tubes, have poor imd performance, yet that does not mean that all sweep
> tube amps are unacceptable. Rich measured one bastardized "DAF" amp for
> imd and feels qualified to condemn all of them.
I think we can all agree with the DAF circuit, it is pure dumb luck if
it works as well as even the poorest sweep tube
amplifiers...anecdotal claims aside.
> "DAF" circuit. I understand tests have been done and posted to a website,
> yet no one seems interested in looking at empirical results. The attitude
> of a few is "it cannot work, therefore any results which tend to indicate
> that it does are flawed".
You can find all sorts of things that aren't true on Web sites. Web
sites simply present the owner's personal opinion, presented
without any form of peer review.
For a good example of this, look at the Web sites touting the CFA
antenna as a breakthrough, and the Double Bazooka antenna as
having gain over a dipole.
All it takes is one or two to half-way work, and then suddenly it is
proclaimed a "good idea".
> At least a few "modified" DAF circuits are in daily use in Sweden. Tests
> have been made and posted, and other hams who seem competent, have
> listened and not heard objectionable splatter.
How many dB is "objectionable"? I can't find that quantity in my dB
tables. Are these Hams actually competent, or do they just "seem
competent"? How are they modified?
> To dismiss an idea for the wrong reason, or because of a lack of adequate
> testing or understanding, is perhaps as objectionable as listening to a
> lot of splatter. In fact, it is much worse. I have a neighbor who runs a
> 4-1000 grounded grid, driven by a SB-200. He overdrives everything and
> has horrendous buckshot. But, even though I operate in the same dx window
> as he, it does not bother me that much.
One source of pollution isn't so bad. We just don't need hundreds
of them.
It's very plain the design of the DAF, if you can call it a design, is
very poor. No one can say they all won't work, I've even seen class
C amplifiers that are "acceptable".
But the bottom line is the DAF circuit is certainly a very poor
design, with little thought given to what actually happens in the
system.
73, Tom W8JI
w8ji@contesting.com
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