[Amps] reading a tube datasheet

[R L Measures]

On Jun 27, 2006, at 1:23 PM, Tom W8JI wrote:

>> On Tue, 2006-06-27 at 12:22, Tom W8JI wrote:
>>>  You are reading more into the data sheet than it is
>>> saying.
>>
>> It is somewhat disconcerting to have someone say the
>> datasheet
>> isn't a reliable source of information.
>
> No one said that that I know of. Although I haven't been
> reading all the posts, at least I certainly didn't say that!

You implied that the bends in Eimac's  constant current curves for  
currents above 1.2a were virtually meaningless and also that the  
400mA max rating for linear service was meaningless.
>
> What I said was someone wasn't reading the datasheet
> correctly and applying the information correctly.

I agree with that.
>
>> I always assumed when it said something like
>>
>> RF Linear Amplifier
>> Grounded Grid Class B
>> Maximum Ratings
>> ________________
>> DC Plate Current 0.400 AMP
>>
>> that's really what it meant.  Now it seems  maybe not!
>> (Why would they put that in there?  Did they just pull
>> numbers
>> out of the air?)
>>
>> Maybe you could step back a little from the current debate
>> and say a word about what the datasheet actually does say?
>
> The data sheet is generated early in the design of a new
> tube by looking at samples under certain limited test
> conditions and by the design goals of the engineers.
> Operation outside the exact letter of  "typical operation"
> is widely accepted and very common. Eimac and other
> companies often OK operation outside the guidelines. The
> data sheets themselves even state "If it is desired to
> operate this tube under conditions different from those
> given here, contact the factory......".
>
> This entire worn out thread centers around two things, tube
> life and operational performance.
> As I said before and I'll say one last time:
>
> 1.) Everyone agrees tube life is NOT an issue.

Correct

> 2.) Multiple measurements show the tube very linear with the
> load line established at 550mA,

Says who?

> and it has far less IM than
> any typical exciter with that load line.

Transceivers have load lines?

> 3.) IM is caused by deviation in transfer characteristics,
> NOT by following one constant current curve.

When a constant current line bends, it is because the transfer  
characteristic - or gain factor -  is changing along the line.
Where the line is straight it indicates that the transfer  
characteristic is constant in that region .  For example, the 1.2a  
line is virtually flat from an instantaneous anode potential swing  
from a few hundred V to 4000V.  Thus, linearity is maximal and impure  
matter is minimal..  OTOH, the 3a constant current line bends upward  
at lower anode potentials, which means that the transfer  
characteristic changes along that part of the line, so operating with  
an anode current of 3a peak will result in the production of impure  
matter.

> 4.) The manual doesn't even tell people to operate at 550mA,
> so that is a non-issue. They are told to establish the load
> line at 550mA and back drive off to 400mA for operation

Good point, but what I hear on the air speaks for itself. Perhaps the  
mistake people make is driving the amplifier with 100w instead of 70w?

In any event, your tune-up procedure is not recommended by any tube  
manufacturer, and tuning up for max out with 70w drive per 3-500Z  
results in pristine cleanliness - provided the exciter has the same -  
which should come as no surprise to us since the 3-500Z is rated at  
better than minus 40db of total IMD. .
>
> I really can't contribute anything else to this thread. I
> think everything that needs to be said has been said several
> times.

Agreed, and if you hang in this thread any longer, some wascally  
person might ask you about the 550mA is OK letter from Eimac you  
promised us c. 6-yrs ago.

cheers, Tom

> People can always get a copy of "Care and Feeding of
> Power Grid Tubes" and read much of what was said here in
> that book (and read the book several times if they need to
> see it over and over again).
>
> 73 Tom
>
>
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>

R L MEASURES, AG6K. 805-386-3734
r at somis.org