>> From: Rich Measures <measures@vc.net>Rich,
>
>We've been over this a hundred times.
maybe several - - - unless I'm coming down with Alzheimer's, Ted.
>You wrongly insist you can simply multiply the RMS drive voltage
>against the indicated meter current.
>
>> - 100mA equates to about 2.7w.
>
>That's nonsense.
In a previous reply Mr. Rauch wrote:
>"When I was testing a PA with 3CX800's, I had no overload system
>installed. I had a few "woops" that lasted only long enough to see
>the grid meter get near the pin for a few brief instants. I ruined a
>new pair of 3CX800's in one day, after about five hits on the grids
>with a few hundred ma for a very brief period. When I cut open the
>tubes, they had gold "melt balls".
I find it nonsensical that Mr. Rauch messed up when he tuned up this
amplifier. My guess is that the amplifier was not being grossly mistuned
when the gold suddenly boiled off of the grids and formed melt-balls. In
the Ten-Tec 425 that I worked on, even with mistuning there was
remarkably little grid current in the 3CX800A7s -- possibly due to their
dispenser-cathode design. OTOH, with 3-500Z amplifiers, I see plenty of
grid current during mistuning.
Mr. Rauch seems not to understand how much energy is involved in heating
radiating objects to high temperatures. As I recall, Stephen's Law says
that the amount of energy required is equal to T raised to the 4th power.
Thus, to raise a grid to the temperature need to boil gold (probably
between 1500 and 2966 deg. C in a vacuum), a large amount of energy would
be needed.
>We have no idea what the grid dissipation is unless we know the
>driving voltage AND waveshape of the grid current at the particular
>operating point. Since it is under a condition of mistuning, it is
>not in Eimac's data.
>
Arriving at a semi-exact figure is undoubtedly a calculus problem, Mr.
Rauch. However, the current waveshape is pretty much a clipped third of
a semi-sinewave. The voltage waveshape is a sinewave. IMO, a
garden-variety grid current meter probably comes pretty close to
indicating the average grid current.
>Eimac does say (as I-- and others--- have pointed out to Rich several
>times) grid dissipation can be estimated by taking the grid cathode
>PEAK voltage and multiplying it times the indicated grid current.
Eimac does not say this. Does it make sense that one would use peak
voltage to determine average power? No matter how you stretch it,
according to Stephan's Law, there is not nearly enough energy. There
might be a more plausible scenario.
What does Eimac say about parasitic oscillations?
{In the letter regarding parasitics that Mr. Foote refers to in the
following letter, I described the unusual condition of the 100 ohm VHF
parasitic suppressor resistors -- i.e., they looked pristine on the
outside, but the resistance had increased to c. 400 ohms}
=======================
VARIAN ElMAC, 301 Industrial Way
San Carlos California 94070 1 U.S.A. / Tel. (415) 592-1221 TWX 910
376-4893
February 18, 1986
Mr. Richard L. Measures
6455 La Cumbre Road
Somis, CA 93066
Subject: 2 pcs returned 8875 S/N G8AD-241 and F8VD-428J, for evaluation.
Reference: Your letter dated 21 January 1986; EIMAC RPA #SC-2303.
Dear Richard:
Your letter about parasitics is quite interesting, and it appears your
two tubes have had the same trouble. The emission was poor on test, and
consequently other test results looked bad. The tube engineer then cut
them both open for an internal examination.
Both have been badly overheated internally, the apparent result of an
oscillation condition. The grid in these tubes is gold plated and if
overheated the gold vaporizes off, of course, and some of it inevitably
lands on the oxide cathode, and that poisons emission.
Though your tubes are quite oid (mid-1978 vintage) we will replace them
in the interest of good user relations. We will scrap the two you sent in
here, as we have to be sure they are out of circulation when we replace.
Good luck in your .continued operation.
Sincerely, WILLIS B. FOOTE, Chief Specifications Engineer, Power Grid
Division.
=========================
... ... ...
Rich---
R. L. Measures, 805-386-3734, AG6K
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