[Amps] Runaway grid

Tom W8JI w8ji at w8ji.com
Fri Jul 21 07:03:10 EDT 2006


>>>If the tube is not gassy, and the grid is floating, the
>>>electron
>>>stream from the cathode probably raises the grid-cathode
>>>(fil-CT)
>>>potential to a few volts negative
>
> So if it doesn't matter, why do tube manufacturers specify 
> a maximum grid - cathode resistance for a lot of tubes? 
> Even low power audio tubes used in Class A?
> My experience is that exceeding the the maximum 
> grid-cathode resistance can (but not always does) lead to 
> a plate current runaway, even in tubes with no measurable 
> gas (as measured by grid current)

That's the same experience anyone who has built audio amps 
and played with grid resistors runs into.

I had a pair of 6146's in a bass guitar amp and in an effort 
to get better bass used very high grid resistors, and had 
runaway in the 6146's when they got hot. Grids went 
positive. Either every new 6146 RCA I bought that year was 
bad, or I had too much grid leak for hot tube temps.

73 Tom







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