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[AMPS] G2DAF Circuit

To: <amps@contesting.com>
Subject: [AMPS] G2DAF Circuit
From: W8JI@contesting.com (Tom Rauch)
Date: Thu, 26 Oct 2000 16:08:36 -0400
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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