[AMPS] G2DAF Circuit

[Tom Rauch]

> Although the load would change with drive, I am not convinced that is as
> big a problem as it might first appear.  When drive is less, the output is
> less. Buckshot usually occurs at peaks, as amplifiers flatten out.  What

?? 

"Buckshot" (IMD products) can appear any level transition during 
the envelope change, but it is most harmful if it occurs during the 
periods where envelope power has the highest time-averaged power.

I'd rather have someone operating next to me with occasional peak 
clipping than non-linearity at about 50% of the envelope peaks or 
less, because the splatter is more constant! Speech processors 
actually reduce peak-clipping in the amplifier stages. The reason 
they have a reputation for increasing splatter is they increase the 
time the envelope stays at higher power levels. 

If you run a processor on a system with clean amplifiers, the 
system actually gets less objectionable because occasional peaks 
and overshoots are removed.

If the PA or amplifier stages have non-linearity at medium to high 
power levels, a processor will make the system sound worse on 
adjacent channels.

The G2DAF system would be the last system I'd run with 
processing of any type.         

> is more troubling is whether the exciter is properly loaded on both half
> cycles at peak power.  That is the reason for using a tuned input in the
> grounded grid circuit. 

If the G2DAF system is working, it probably is working because 
the screen circuit ISN'T doing anything at all. 

Every bit of power applied to the screen that supposedly increases 
output comes from the exciter. The cathode would have to be 
"robbed" of a significant amount of drive power to supply power to 
the screen.

A simple "triode" connection of the tetrode, with attention to voltage 
division between screen and control grids, would have less 
distortion and just as much gain.

The DAF system is similar to the "forward-acting speech 
processor" in the ARRL Handbook that samples input signal, 
rectifies it, and uses it to limit gain. That circuit doesn't work at all 
like the article claims, has poor envelope characteristics, yet 
people swear by it.

A two-tone test will not show up many types of splatter-causing 
problems. It certainly doesn't show most power supply regulation  
problems! The screen grid in the DAF is poorly regulated! 

While a two-tone test is one of the least effective ways to tell if a 
PA is clean, many people use something even less reliable. They 
depend on on-the-air testimonials from a few people with S7 noise 
levels.

One thing I commonly have heard is an operator getting a 
complaint often asks another random person (who often has a S-5 
or stronger background noise) how his "signal sounds". Of course 
it sounds fine to someone who doesn't know what to look for and 
who might even have a high background noise, especially when the 
other person wants to be a "nice guy".


73, Tom W8JI
w8ji@contesting.com

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