> Rich Measures writes:
>
> After Eimac autopsied both tubes and discovered that both contained
> gold melt-balls, Mr. Foote telephoned me and related the strange
> finding of the 8877 development team. Near the end of our
> conversation, Mr. Foote said that the timing of this episode was
> perfect since all of the men who were on the 8877 development
> team were being forced out the door and that in a couple of months
> the story of their finding would disappear with them.
Some of the 8877 team were still with Eimac in late 1988 - early 1989.
> If the MLA-2500 was a power oscillator, it therefore could have no
> parasitic oscillation and you would have a point, Joe.
The MLA2500 (I had one for several years) is particularly bad for grid
overheating not necessarily due to parasitic oscillation.
1) The grids are not well grounded - the grid shunt is not
sufficiently bypassed
2) Many users clip out the input swamp (or it fails)
3) the tubes are very easily overdriven
4) the tube is not well cooled (same internals as the 8874 with
a less efficient anode cooler)
5) the socket end of the tube gets almost no cooling air in
the MLA2500
The MLA2500 just simply cannot be operated safely at 1500 watts
output.
> The 8877's grid weighs c. 2oz. The amount of energy needed
> to heat it to the boiling point of gold would be prodigious.
You cannot extrapolate an 8875 grid (weight?) to an 8877 grid
even though both are known to be easily overdriven.
However, it is not necessary to heat a solid to the boiling
point to convert a portion of it to gas. Sublimation (phase
change from solid to gas) is also a possibility, particularly
in a near vacuum, with temperatures close to, but less than the
boiling point and under the influence of strong electrostatic
forces. (Anyone who lives in cold climates has seen sublimation
in the direct evaporation of ice on a bright sunny day.)
Sublimation occurs only in the surface layer which also explains
why you do not see Molybdenum contamination of the "gold melt
balls."
Your claims that the presence of gold melt balls as proof of
parasitic oscillation is quite simply a case of hearing hoof
beats and looking for zebras. The evidence is quite clear that
the grid surface was overheated. However, in the presence of a
known sensitivity to over current, bombardment from the "beam,"
and (significant) electrostatic forces, parasitic oscillations
in the VHF range as you propose are far from the most probable
explanation.
73,
... Joe, K4IK
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