Colin,
I don't see how you figure this as running the screen and bias at the
manufacturers rated voltage should make the IMD products show at their
published specs for that tube. The way your saying, this aint so, says the
tube manufacturers dont know what they're talking about. The manufacturers
ran the tests to see if the IMD products were where they should be. That
is if one builds the circuit with the values they give, and is operated
within the range they give. If the tube in question had terrible IMD
products where the tube was ran at its specified values, they wouldn't
sell it as nobody would buy it. One might try to do other things to
improve the IMD products from their published values however. This is what
I'm saying, if they're ran at the manufacturers published specifications,
the tube will produce IMD products within the range they publish which is
deemed acceptable.
Will
On Fri, 21 Jan 2005 09:54:08 -0800 (GMT-08:00), Colin Lamb
<k7fm@teleport.com> wrote:
"As long as the bias and screen voltages are clean, the IMD will be low
as it should be."
This statement is not correct. Look at the Eimac imd curves for a
4CX250 or a 4CX1000A. The charts are made with absolutely stable bias
and screen voltages. I do not have access right now to the charts but
they are about -22 db 3rd order distortion, which I consider
unacceptable except for cb use. If the screen voltage is not regulated,
the distortion will increase. A number of hams are running 4CX250 or
4CX1000A tubes on 6 meters, grid driven and with no feedback and you can
hear the splatter when the band is open.
Now, compare the 4CX1500, which is designed for sideband service. In
exactly the same configuration, it will yield something like -45 db 3rd
order distortion. It is extremely clean. I remember a ham running a
the 4CX1500 tube in his amplifier and the difference was night and day.
The first time I tuned through his signal, I noted there was no sideband
splatter.
Something else can also be learned by looking at the curves - that
distortion does not necessarily increase as the tube is pushed harder.
There seems to be an assumption that if you take a high power tube and
run it at low power that the imd will improve. That may not be so.
Look at the 4CX250, for example. If you drop the power down to 50
watts, the distortion products actually increase (this from memory).
All this means that it pays to read all the information you can get from
the manufacturer and other critical tests, and then do not assume that
results you get will be similar unless you are running under the same
conditions. Although a spectrum analyzer can be useful, a reasonably
good receiver and knowledge can tell you a lot. Rich probably has the
procedure, using a receiver, mentioned on his website.
It was probably a good thing that I never did examine the distortion
products of my 4 x 6AG7 amplifier I built many years ago. I ran a
couple of hundred watts but never burned out any of the finals, and
never could get the metal case of the 6AG7 to glow.
Colin K7FM
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