> Two tone tests have been used as a standard to evaluate amplifiers since
> donkeys years. Now this test is not telling the truth say's the gurus..
> ok, I accept the doubts on given grounds. But maybe we should start
> testing other amps than DAF's without using the two tone test. A poorly
> regulated screen power supply in a grid driven amp should maintain a
> pretty stable voltage with a two tone constant modulation not giving away
> that the amp would sound bad when modulated by speech. True or not .. ?
It's well established a two-tone does not show up dynamic
regulation problems in the supply. A two-tone test does work OK
when the supplies are of known good dynamic regulation quality,
especially if the two tone test is made at several power levels.
So yes, the two tone test fails to root out dynamic regulation
problems. A poor power supply for the screen or anode can look
good on a two-tone test, and be unacceptable on SSB.
> Lars, SM3BDZ has published a number of plots on his website showing the
> performance of his DAF amp under different conditions. To rule out "the
> error" of a two tone test on a DAF he has plots of speech instead. Plots
> were made using first class commercial test gear. Check them out and you
> will see that the thing works.
....and the Web page address is????
> But I do most certainly agree with Marv and Colin that the DAF circuit is
> not going to work with every tube on the shelf, the 4CX250's for instance
> are a NO GO for sure. Just like some tubes don't work in GG.
Put me in that class too. I have no doubts some PA's can sound OK. As I've
said, I've seen class C PA's that were acceptable.
> I hear so many poor IMD signals on the band where I know the guys are
> using state of the art GG or grid driven amps that I am convinced that the
> -35db two tone IMD they "should" produce is just BS. Then again, there are
> many amps of the same kind that sound marvellous. What's the difference ?
> Let's find out !!!!!!! Let's start measuring speech modulated rf instead
> of steady two tone rf on ALL amps!
The biggest problem, in my experience, is many people do not
know how to tune a PA.
73, Tom W8JI
w8ji@contesting.com
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