[Amps] Best way to tune a Dentron ML2500 ?

[R.Measures]

On Aug 2, 2005, at 7:07 AM, Joe Subich, W4TV wrote:

>
>
>>>
>>> You've got to be kidding - 333ma is nearly the rated PLATE current!
>>
>> -  I am not kidding, Joe, and neither is either Ohm's Law or
>> the Eimac spec sheet.  The rated plate / anode current for a
>> pair of 8875s for SSB is 1000mA.   Is 333mA "nearly the rated
>> PLATE current" ?
>
> OK ... it's half the rated plate current (Svetlana spec sheet
> for their 3CX400/8874 is 350 mA plate current).  Still, there
> is absolutely no way the 8873/8874/8875 will survive 150 mA of
> grid current per tube or an 8877 survive 300 mA of grid current.
>
**  With the typical cathode-grid driving potentials and the rated grid 
dissipations, they will - provided that Ohm's is valid.  Both Tom Rauch 
and myself have turned a 3-500Z grid red-orange -- with no ill effect.

>> Eimac autopsies of the two 8875s I sent them from a MLA-2500
>> showed they died from internal leakage caused by gold sputtering
>> off from the grid.
>
> You keep saying that ... based on one letter from a low level
> employee who was not qualified or authorized to be making any
> assumptions on the cause of the gold stripping.

** W. B. Foote was Varian-Eimac's  Chief Specifications Engineer, Power 
Grid Division.  Mr. Foote assumed not, he told me that gold-sputtering 
from the grid was observed during autopsy of a failed tube by the 8877 
design team and that the consensus was that it was the result of an 
oscillation condition -- presumably in the UHF region -- due to the 
thinness of the gold layer that evaporated.  At first, I too was 
skeptical, but when I autopsied a tube, peered through my low power 
microscope and saw the cathode littered with tiny gold melt-balls, I 
bought into their conclusion.
>
> As you know from your own tube autopsies, the grids in the
> small external anode tubes are quite fragile ... much lighter
> than the grids in glass bottles of the same power rating ...
> and can withstand much less grid current.

** I have never seen a grid's molybdenum (melts at 2617ºC) base metal 
damaged, its gold (melts at 1063ºC) plating is what's fragile, Joe.
>
> Unless the grids are made of materials designed to withstand
> very high temperatures, e.g. pirolytic graphite, they are
> very easily damaged by excessive dissipation.

**  Getting to 2617ºC is easy?

> This is not
> the time or place to get into the weak bonds between the gold
> plating and the base metal of the grid structure, current
> concentration in the plating because of skin effects, and
> other phenomena that contribute to plating loss in tubes
> with excess grid current.  They have all be raised before
> but they don't fit your unique theory of parasites so you
> ignore them.

**  Unique?  It wasn't my theory, it was the 8877 design team's theory.
>
> You can stick with tetrodes with handles in class AB1
> operation (no grid current) and avoid the issue.
>
**  You may rest assured that I won't because I have seen gold-sputted 
triode grids through a microscope.

Cheers, Joe

> 73,
>
>    ... Joe, W4TV
>
>
>
>

Richard L. Measures, AG6K, 805.386.3734.  www.somis.org