[Amps] RE: 4CX250B Screen Supplies

[Ian White, G3SEK]

Will Matney wrote:
> The very worst problem of IMD is the 120 Hz ripple that's left from a 
>power supply. Everything else is a minor cause except for the tube 
>design itself and the circuit placement/shielding, and bypassing. When 
>120 Hz ripple hits that PA tube, it's just a very large mixer 
>amplifier. Keep in mind that the control grid supply can be infected 
>with 120 Hz too, especially if it's grid driven!

I don't think that is true. Looking at the IMD traces for the three 
different amplifiers reviewed in Sep 1997 QST (the expanded traces on 
the ARRL CD) there are no major effects from 120Hz modulation.

There is some "raggedness" on the peaks that could be due to 120Hz 
modulation, but it is some way down on from each of the two main tones, 
and more generally down in the -60 to -70dB region where it's very 
difficult to distinguish from analyser noise or the analyser's own hum 
sidebands.

These were triode power amplifiers, so if there was going to be any 
influence from 120Hz modulation, it would be significantly worse than 
for tetrodes. (The screen grid makes tetrodes quite insensitive to minor 
variations in anode voltage, but it also makes them very sensitive to 
minor variations in screen voltage.) I also looked for another possible 
120Hz effect: the expanded test results show traces at 750W, 1000W and 
1500W output, and you would expect any 120Hz effects to get worse at 
higher power levels, because the amps are pulling more anode current and 
the % ripple would increase. But that isn't there either.

If the screen supply (or the anode supply for a triode) has a lot of 
120Hz ripple, then of course you'll see 120Hz modulation on all signals 
going through the amp... but it's still not strong enough to 
intermodulate significantly with itself.

Sorry, Will, I don't believe the facts stack up to make 120Hz ripple a 
significant contributor to IMD at all.



>So why super clean the screen when we're feeding a dirty supply to the 
>control grid (tetrodes and pentodes) and mixing it there too!

I'd like to make a modest proposal here: how about not feeding a "dirty" 
supply to *either* of the grids?


-- 
73 from Ian G3SEK         'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek