Tom,
Sure, you can run over the rated current temporarily, but to advise one to do
so sets you up for liability. I can not tell someone to operate a tube outside
the published curves. What if I did, then they turned the tube in, and the
manufacturer refused to replace it after they told them I recommended doing
that? The person then could come back on me for the costs incurred. That is a
very possible scenario! I will not recommend to anyone on this list to ever run
a tube outside the listed curves or maximum ratings. That's one of the first
things I learned when working in engineering. Never say something will do more
than the published ratings. If I had where I worked as an engineer, I'd have
been fired or severely disciplined for doing it.
Your missing the whole point I'm trying to make, I guess, as I'm not alluding
to anything about the published Ameritron rating, or what has happened in the
past. This is just the way any working engineer should advise anyone with a
question concerning maximum ratings and published curves.
Best,
Will
*********** REPLY SEPARATOR ***********
On 6/25/06 at 11:10 AM Tom W8JI wrote:
>> I'm not refering to Eimac but the numerous other suppliers
>> and manufacturers.
>
>Richardson's looked at the AL80B manual also. They had no
>problem with it. They own Amperex.
>
>Anyone with a basic understanding of tubes should know a
>thoriated tungsten's life isn't compromised in the least by
>brief excursions over the CCS current rating. IMO all the
>fuss about this is a personal issue. It has nothing to do
>with what the manual says or any other technical problem.
>
>> I should have worded the above differently and was my
>> mis-wording. It should have said deviate from the curve,
>> or be outside the curve. Are you saying Eimacs curves are
>> incorrect and one can operate the tubes outside the
>> recommended curves? This being any tube, not just the
>> 3-500Z. I'm not asking about any products by Ameritron or
>> the 3-500Z question from Rich. This is about all the
>> published information and curves that Eimac has
>> published.>>
>
>Eimac and any other manufacturer can't possibly anticipate
>every operating condition. Their data almost always came
>from a single group of tests in a test fixutre and assumed
>more of a commercial application. Because they don't publish
>a particular set of operating parameters doesn't mean those
>conditions won't work or are unacceptable. Anything I have
>ever done that was outside a clear boundary has always been
>run past engineers at the primary source.
>
>For example I designed a medical pulse amplifier using two
>MRF150's. The peak pulse power was 1200 watts or more.
>Motorola had no problem with it and out of hundreds of
>generators there was never a failure of FET's.
>
>>>It is impossible to look at one constant current line and
>>>predict transfer characteristics or odd-order IM
>>>distortion
>>>levels. If the constant current characteristic curves at
>>>one
>>>current value set the distortion level why would negative
>>>feedback reduce distortion? How could the output be linear
>>>when the tube is cut off for more than 140 degrees of the
>>>RF
>>>cycle?
>>>
>>>It is a transfer function problem. The level of IM
>>>distortion products is a function of the slope and shape
>>>of
>>>the gain change as drive level is varied.
>>
>> I understand this, but isn't the published curves there to
>> show the maximum operating areas?
>
>Constant current curves primarily allow the designer to
>establish a load line that presents the proper load to the
>tube, and of course can be used to esitmate bias and power
>output. The end result should always be tested. That is the
>only final and accurate way to determine IM. The 550mA in
>the later tune step has been the focus for many years by one
>person, while the manual's suggested reduction of drive to
>400mA steady current in the next step has been ignored. The
>reason 550mA is chosen is that happens to work out to
>establish a load line where peak clipping won't occur, it
>increases negative feedback by reducing anode load
>impedance, and it optimizes IM performance with drive
>characteristics typical exciters.
>
>As for distortion products, even at 1000 watts PEP out a
>single 3-500Z in GG is significantly cleaner than nearly any
>modern solid state rig. First, Eimac and most commercial
>manufacturers use dB below one tone. Radio manufacturers use
>dB below PEP, making them look 6dB better than they really
>are. Many of the modern radios we use are barely 10db over
>what a class C PA would do, and that's with a steady tone
>test where the lousy ALC systems and bias fluctuations on
>transistor base bias systems don't come into play.
>
>I can't even use a regular modern amateur transceiver to
>measure IM in many tube amps. They are too dirty, and that's
>when the test is a two tone and ALC attack isn't an issue. I
>normally have to either dig out a modified KWM-2 and run it
>though a class A amplifer, or run two RF generators into a
>combiner and amplify up to drive power levels through very
>clean driver amps.
>
>If someone wants to get on a campaign to clean up the
>airways they better do some IM testing on radios and on some
>of the solid state and tetrode amplifers being sold.
>Actually they should test everything before they publically
>complain about it, and not go off attacking based on a wild
>guess.
>
>>>If your read Care and Feeding of Power Grid Tubes
>>>(published
>>>by Eimac) all of this information is there. They even say
>>>they intentionally design to create an even-order
>>>curvature
>>>slope to avoid creating odd-order IM!
>>
>> I've read it front to back.
>
>Respectfully then, you should re-read this section:
>
>>>I suggest you look at page 96 section 4.3.2 of Care and
>>>Feeding. It's all there in black and white and worth
>>>reading. It will help sort unfounded opinions from the
>>>true
>>>facts. The ideal transfer characteristics for power
>>>amplifier conduction angles less than 360 degrees and more
>>>than 180 degrees (class AB) aren't even a straight line!
>
>Please point out anything there that disagrees with what I
>have said above. It's the transfer characteristics that
>matter, and a straight line is not always optimum in an HF
>PA.
>
>73 Tom
>
>
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