Ian,
I've beat this dead horse to death and if anyone doesn't think that IMD
can be increased by not enough filtering, the best thing to do is just
try it. Change the filtering on the power supplies for the screen and
control grids and note the differences as a before and after. I done
this long ago building smaller amps (RF and Audio) using non regulated
supplies. I was working around an original design by an engineer down in
Memphis at the time. He fed the screen off the center tap of a HV
doubler. The amp splattered like He** and always had that bad name
branded to it. After looking the circuit over, I added a diode off the
center tap and another 100 uF @ 450 Vdc filter cap, that cleaned up the
bad IMD and the hum that the circuit was known for. I then went and
upped the filtering on the bias supply from 22 uF to 80 uF and it came
down more. I was actually checking the output the way Richard Measures
said of using a receiver to tell the difference on a near by frequency.
I know and seen it was noticeably different. So it had to be the 120 Hz
ripple causing the IMD, or at least in my way of seeing it. The
bypassing was already well done enough by the original design using 5000
pF @ 1Kv ceramic discs at each tubes pin to ground. I did add some
additional bypassing to the control grid source behind the blocking
choke at the control grid pin. However, after adding this, I did not
tell any difference from testing. Also, I used the two tone test and
audible voice tests using a receiver set for the monitoring and there
was an increased IMD using the voice compared to the two tone even
though I cleaned up the filtering but the splatter was less after the
filter modification. Voice will raise it but it would have been worse
with 120 Hz hum too because it was! If 120 Hz ripple hum wasn't a
problem, we wouldn't have to use filtering at all in the power supply.
All be it that the regulator will drop it more with filtering, but if
the supply isn't filtered enough, some is bound to get through. This
being said, why is Rich attempting a resonant choke filter for the
screen supply but if not to kill out 120 Hz ripple? Last, why does Henry
go to the extent of using a resonant choke to filter the plate supply?
"I'd like to make a modest proposal here: how about not feeding a
"dirty" supply to *either* of the grids?".......... Ian G3SEK
Ian, that's exactly the point I'm trying to get across. Get the
filtering right and I'll bet you'll see a drop, I know I did. I'm not
saying that a regulator will not clean it up to, it will, but it needs a
clean supply, that's all I've been preaching here, nothing more. The
only other thing I mentioned was that voice modulation creates more IMD
than does the two tone test, and by what I've read, we are all in
agreement it does.
Will Matney
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