In a message dated 4/4/06 8:01:09 AM Central Daylight Time,
amps-request@contesting.com writes:
<< But then we have the Eimac data sheet for the 4CX250R, which includes a
set of operating conditions to squeeze 400W PEP out of a single tube.
Not surprisingly, the 3rd-order IMD then increases to only -23dB
"referred to signal level" (which I take to mean "below PEP"; if so, it
equates to only 17dB below either tone). In other words, Eimac were
recommending operating conditions that produce totally unacceptable
levels of IMD.
This data sheet has had several bad consequences:
1. It led many users to be too greedy for output from 4CX250Rs.
2. It gave some users to claim that -23dB 3rd-order is OK "because Eimac
recommend it".
3. It gave the entire 4CX250 range a bad reputation for "-20dB IMD".
>>
Do not blame the data sheet for a poor design. Data sheets are as much of a
sales tool as they are an engineering tool. As such, the tube tabular
parameters for a given class of service are given to put the tube in the best
competitive light possible. So unless the data sheet gives you specific
performance
specs for the given set of tabular parameters, don't be surprised if your
measured performance is not what you expected. the only way you could blame the
tube
manufacturer or the data sheet is if the sheet said some thing like: Use the
following operating parameters to secure 30 db of IMD. You used those
parameters and got 20 db. OK, you have a case.
Eimac and other tube manufacturers build their tubes for a variety of
applications with a variety of probable performance requirements. Deriving a
set of
tabular operating parameters that are all encompassing would be impossible.
Having had the experience of manually deriving the given tabular parameters
using
the Eimac Tube Performance Computer for about 300 sets of parameters,
generally speaking the distortion can be all over the place. Generally, one set
of
parameters will show the tube running right up to its dissipation ratings. The
other set will be for the tube running near its maximum rated plate current. I
think you get the picture.
Recently I derived the parameters for class B modulator service for about 50
different tubes, then compared that data to parameter derivations done for
broadcast transmitters (with the help of their typical meter readings table)
using the same tubes. The conclusion was that the manufacturer's suggested
parameters, with one or two exceptions, gave parameters resulting in more
distortion
than the set of parameters derived by the transmitter manufacturer. I've also
done class B linear amplifier analyses and found similar issues.
So if you are used to using tabular data in your design, it is vitally
important that you completely understand the assumptions that were made when
the
given parameters were derived. Lacking that information, you are faced with
either getting out the strait edge and the tube performance computer and doing
some
calculations, or doing some lab work in determining a useable set of
parameters. Don't blame the data sheets for poor performance unless the
performance
figure is specifically stated on the data sheet, and the given parameters fail
to produce that performance. With the number of calculations I've done, errors
on data sheets are not unheard of either. Otherwise, you need to plot and
analyze your own Operating Line on the CC curves
As a general rule, the tabular parameters are reliable only as a starting
point and/or for comparison purposes for a more exacting design.
Jeff Glass, BSEE CSRE
Chief engineer WNIU WNIJ
Northern Illinois University
WB9ETG since 1970
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