Gents,
I am running one of these monsters of amplifiers and I must say that I have
had very few complaints from fellow amateurs regarding my SSB signal. I had
much more complaints when running a pair of 4-250's in GG before. I have
had many people check my signal and reports are certainly ok.
My intent when I built the amp was that if I received a lot of bad reports
or had other problems I would modify it and run screen and grid voltages as
per the book.
So far I have not had any indications telling me I am a bandit on the bands
because of my amp and I have had the amp in daily use for 2.5 years.
A few notes,
The tube I use is a big one, 3 kW plate dissipation tetrode (QBL5-3500)
with a lot to spare as far as power capabilities the way I run it. I derive
650 V of screen voltage over a 200 ohm resistor from grid to ground. Input
matching is done with toroidal pi nput circuits that gives flat and stable
load to the exciter (TR7). Tuning is smooth and stable and there is max
output at dip of anode current. The cathode is grounded. I have no problems
getting full screen voltage even on 28 MHz using a quadrupler bridge made
from schottky diodes.
The ALC light on the TR7 hardly ever comes on when operating SSB so drive
levels are held at a point where the exciter should be performing best (?).
So, are we talking about the same kind of problems here or what ? As
someone pointed out, the 4CX250B's can certainly sound bad if overdriven,
so if someone heard a DAF amp with those tubes that sounded bad they would
also be able to hear a conventional one that sounds bad if not driven
properly.
I know of a VERY well known operator in Europe who has a commercial 4CX250B
amplifier and he produces terrible splatter.
The point I'm trying to make is that setting the knobs on the TX right to
start off with is a good way to start producing good sounding signals.
Furthermore, and this is probably the point Jim SM2EKM made, if the amp is
able to take some high level spikes on the input without saturating the
result must be a better sounding signal. Many operators do indeed run their
amps at maximum. No wonder they sound bad if there are voice peaks that
saturate the tube(s). Then again, how would they know... mysterious reports
of splatter on the bands and tripped overload protection circuits 'wrongly'
set up at the factory... can't be the operator..
Finally a question, when talking IMD, enlighten a mere mortal as to where
this distorsion is to be heard. Is it within the passband, outside of the
passband or maybe both. I am not clear on this and the handbook I have
(1984) does not explain this in great detail. Can we hear a difference of
15 db IMD with our ears on the bands or is it academic ?
I do want to know what it is I am doing wrong when accepting a DAF
amplifier as a rather nice afterburner... I'd gladly modify it if I am
convinced that it is bad for my fellow hams.
73
Peter/SM2CEW
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