[Amps] Best way to tune a Dentron ML2500 ?

[Joe Subich, W4TV]

> >
> > 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.  

> 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.  

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.  

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.  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.  

You can stick with tetrodes with handles in class AB1 
operation (no grid current) and avoid the issue. 

73, 

   ... Joe, W4TV