Hi Peter,
> bias supply. AB2, you do. If you can't guarantee (by Rich's increased
> bias voltage, ALC, black magic or whatever) NO grid current, I submit
> that you need a stiff bias supply.
Only a fool would argue with that! Yet many people suggest using NO
grid regulation.
> worked on the design of some professional PA's, and we found better
> consistency by using either a regulated bias and regulated screen, or
> unregulated bias and unregulated screen - which is what you'd expect,
> since the supply variations tend to cancel themselves. But don't
> regulate just one of them.
Absolutely, and you also have to be sure the time constants of each
supply are the identical if they are unregulated.
That is why I use a three tone test on SSB PA's. A two tone test will
NOT show regulator tracking problems you would encounter during
normal voice transmissions. It is almost guaranteed IMD will be worse
than indicated by a two tone test.
> This suggest that a really stiff bias supply
> probably means a regulated screen supply, and for some tubes, the ideal
> is a shunt regulated supply, although as has ben said, in many cases,
> zeners are adequate. (They really improve 6146 PA's, helping prevent
> plate current runaway)
Right again. Most zeners have much too high ESR to be good screen
regulators. Good for class C, but not for AB.
Some recent designs have suggested screen supplies that flop around
from 400 volts or more down to 300 volts, because the "designers" go
by hear-say reports that regulation is unimportant.
That's one step above the G2DAF IMD enhancing system.
> As far as the 'audio sounds OK, so I can't be splattering' discussion
> goes, some tests have shown that you can get up to intermod products
> 17dB down before the audio starts to sound poor. Since infinite clipping
> gives 3rd order products 13dB down, the in channel audio quality is (as
> so many of you have said) no indication whatsoever of the adjacent
> channel performance.
Some class C amps sound pretty good. Audio quality reports mean
nothing, except the exciter and mike have nice response.
> Now, what sort of signal would an AL1200, or an Alpha or a QRO produce
> on this test? Anyone able to find out?
What you really need is an independent lab to make three tone tests.
Your noise test looks good, but it does not include syllablic
variations in level.
73, Tom W8JI
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