> From: Richard Measures
>
> Denny Had did not use an Eimac grid-grounding collet, and that
> increased the chance of a parasite.
No fooling - and exactly what I've been saying. However, that is
not the only reasonable explanation for grid overheating, particularly
in a tube known for running excessive grid current when over driven.
> Eimac told me that the 8875s I
> sent them from a MLA-2500 were probably damaged by an oscillation
> condition.
Foote told you the tubes had been "badly overheated internally."
He ascribed that overheating to an "oscillation condition" but
you had already given him that "parasite" diagnosis.
> Are you suggesting that overdrive could melt and evaporate gold?
I am saying that any mechanism that causes overheating of the grid
can cause damage, including stripping (or evaporation) of the gold
plating.
> > 4) the tube is not well cooled (same internals as the 8874 with
> > a less efficient anode cooler)
>
> 8874s suffer from gold sputtering.
Of course, they are also very easily over driven. If I recall the
8873/8874/8875 is rated for 20 to 25 watts peak (cathode) drive and
a maximum grid dissipation of 5 watts!
> > 5) the socket end of the tube gets almost no cooling air in
> > the MLA2500
>
> Taurine feces.
DO you have real measurements? If so make a rational argument -
don't stoop top playground scatology. I've had an MLA2500 and
have seen how hot the socket/chassis gets in comparison to other
amplifiers with proper cooling.
> > 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 idea wasn't mine, it was Eimac's 8877 development team's.
The development team did not posit VHF parasites as you have been
claiming. Their explanation was a high UHF or SHF oscillation.
The same problem befell another Eimac product with which I was
involved ... it caused localized heating of the output window and
resulted in sudden, premature, catastrophic failure.
> I thought electrons travelled toward the anode, not toward
> the cathode.
The grid is still in the electron beam and intercepts some of the
anode current. That current (particularly when over driven) results
in overheating of the grid. Electrostatic attraction will assist
in stripping the coating from an overheated grid.
> The evaporation pattern on the grid suggested that it was UHF
> current - not VHF.
Sure, high UHF/SHF oscillation *NOT* the VHF parasites you claim
to cure with resistance wire.
73,
... Joe, K4IK
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