On Apr 8, 2005, at 2:39 PM, Joe Subich, K4IK wrote:
>
>> 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.
Over driving with even 200w could not heat the grid to the temp needed
to sputter gold. I have autopsied a number gold-sputtered 8877s that
were driven with 100w radios.
>
>> 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.
Eimac was the first source of the oscillation explanation.
>
>> 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.
No matter how many watts?
>
>>> 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.
The MLA's fan blows directly on the hottest part of the sockets.
> I've had an MLA2500 and
> have seen how hot the socket/chassis gets in comparison to other
> amplifiers with proper cooling.
You have extraordinary vision.
>
>>> 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.
I said it was UHF. Foote told me that the 8877 dev. team did not
measure the frequency of the 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.
The grid intercepts some of the cathode current, Joe. The anode gets
what remains
> That current (particularly when over driven) results
> in overheating of the grid. Electrostatic attraction will assist
> in stripping the coating from an overheated grid.
So why do a good number of gold meltballs stick to the cathode?
>
>> 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.
In 3-500Z amps, the parasite is typically below 150MHz. In 8877 amps,
the parasite is apparently in the UHF region - judging by the
evaporation pattern on the grid.
>
> 73,
>
> ... Joe, K4IK
>
>
>
>
Rich Measures, 805.386.3734, AG6K, www.somis.org
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