Carl (>) wrote in reply to me (>>):
>>It looks like GW4FRX can make some more measurements of IMD to answer
>>your question more specifically, Carl. What he proposes to do is:
>>
>>1. Set up 2*4CX250R with Va 1500V, Vg2 350V and adjust Vg2 to give
>>ZSAC
>>133mA, all as specified in the EIMAC datasheet.
>
>Why on earth do you want to vary screen voltage Ian? That sounds like
>one of Rich's ideas but is not real world practical.
Whoops, sorry, t6ping error on my part - I meant to type "adjust Vg1".
Vg2 will be set to EIMAC's datasheet value and left there.
A great deal of the rest of your posting was affected by that mistake of
mine - in fact we seem in pretty good agreement.
>Also at no point did you specify what the test setup consists of.
In a word, adequate. It will be two-tone through an exciter that
produces low enough IMD to avoid affecting the results from the
amplifier under test (this amplifier needs only a few watts of drive in
AB1 for full output, so low source IMD isn't hard to achieve). GW4FRX's
tests over the last 20 years have always had to use whatever equipment
he could borrow from organisations such as the Air Force, the BBC and
whoever. That's the way it has to be. The analyser this time around will
be a modern HP instrument, and we should have digitally recorded spectra
for the record.
>>2. Adjust drive, tuning and loading to reproduce as closely as
>>possible
>>the two-tone operating conditions and output power in the EIMAC
>>datasheet. The output power will be adjusted to the datasheet value of
>>262W PEP per tube, and IMD levels will be reproduced as closely as
>>possible (see note below).
>
>Is this with NIB tubes? My experience is that most tubes will load to
>their own best point.
"NIB" = ?
>If you are going to run pure AB1 why not load for
>maximum output at whatever voltage you are running? Heavier loading is
>always preferred anyway.
>Just set the bias for idle and tune for max. You will probably find
that
>the best output is reached with Is well under the specs. I wager that
>if you INCREASE loading until output just starts to drop that
>will be the point of best IMD. My feeling is that a tetrode should
always
>be tuned by watching the Bird meter and screen current only. All else
>falls into place.
That's exactly what we say - always peak the tuning for max, but use the
screen current to adjust the loading to a point that gives optimum IMD
(ie the best possible balance between IMD and power output). This is
definitely NOT the loading for maximum output - it's much heavier.
>I may be getting in a bit deep here but I have a feeling that Eimac specs
>were not derived at best case for ANY particular voltage. I have always
>found ANY tetrode in good condition to perform better at well under "
>typical" screen current specs.
It depends what we mean by "better" and "best". Are we talking about
best power output, best IMD or best efficiency? Usually it's a balance
between the first two - but favoring which side?
> GW4FRX has found that stabilized screen
>>supplies
>>tend to give better IMD than EIMAC found. (This is possibly because
>>EIMAC were using simple potential-divider screen supplies with an
>>impedance of several thousand ohms, which represented the state of the
>>art back in the 1950s when the measurements were made.)
>
>Gee whiz, VR tubes were available too back then. Even I could afford WW2
>surplus in the 50's on my paper route money.
Well, they didn't tell us so we may never know. That's one of the
reasons for being prepared to do our own measurements, to meet modern
requirements using modern power supplies.
>>5. The tubes will be significantly under-loaded at the higher anode
>>voltage, so it's quite possible that the IMD will get worse in tests 3
>>or 4. But if the loading is adjusted correctly, staying well on the
>>heavy side, there should be a significant increase in power and the
>>IMD
>>could also improve.
>
>Under loading will always produce inferior results Ian.
>
Agreed.
>>What do you think, Carl - will those proposed tests answer your
>>questions adequately?
>
>
>With reservations as mentioned above Ian.
Which I think were 90% due to my silly typing mistake.
>Since hams...including
>myself....tune for maximum output that is how you should run the tests.
That isn't the way to run the tubes, but I'll ask FRX to do a three-way
comparison at 2000V between:
1. Loaded as required to simulate the EIMAC spec when the anode voltage
was 1500V
2. Loaded for maximum output
3. Loaded for good IMD, according to FRX's own recommendations in 'The
VHF/UHF DX Book' and using screen current as the main indication.
That should cover everything.
>
>To recap:
>Use new or as new tubes
These tubes have about 4500 filament hours, but much less transmitting
time. They can be regarded as "as new".
>Vary Ep in some predetermined steps.
It'll be 1500V and 2000V.
>Set screen voltage as per spec sheet
>Set idle current to specs by varying Eg
Yes.
>Adjust loading for maximum output and then continue to increase loading
>until power just starts to drop.
Yes - see above.
>
>Adjust drive to maintain zero grid current when in key down fully loaded
>config.
>
Drive and g1 bias will be constant after the initial setup. Afterwards
we may do some other tests with different levels of ZSAC. Depending on
time, we may even venture into the AB2 zone (close your eyes, Rich :-)
>Take measurements with those parameters and only then vary screen voltage
>to test IMD. This also requires readjusting bias for idle current.
>
Yes - screen voltage will be 350V throughout.
>All in all this is not a simple 10 minute exercise.
>
>I wish I could be there! Sorry to be so long winded also.
>
Reading back over this post, I notice my wording has shifted from
"GW4FRX will" to "we will" - I'll probably have to go up and lend a
hand. Pick you up from Manchester airport, Carl?
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
www.ifwtech.demon.co.uk/g3sek
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