Grid Resistors

[quicksand]

I have a Fisher amp that uses the 7868 output tubes and has 330k grid resistors. I have read that the 330k resistors are too high for the 7591s and to replace them with 200k resistors and double the .047uF cap to .1uF to compensate.

Does the same apply for the 7868 output tubes? I am running old stock tubes - would the lower value resistor take ease on the 7868s in their old age? I have looked at the receiving tube manuals and the two tubes appear to be similar.

On a similar note I also read that new production 7868s require this change.

 

[battradio]

I would do it for the peace of mind . 7868's and 7591'are the same tube different basing .

Mark

 

[TubeGrashopr]

I have a Fisher amp that uses the 7868 output tubes and has 330k grid resistors. I have read that the 330k resistors are too high for the 7591s and to replace them with 200k resistors and double the .47uF cap to .1uF to compensate.

Does the same apply for the 7868 output tubes? I am running old stock tubes - would the lower value resistor take ease on the 7868s in their old age? I have looked at the receiving tube manuals and the two tubes appear to be similar.

On a similar note I also read that new production 7868s require this change.

I'm not an expert yet, but I've done some extensive reading and bookmarked many sites it seems to be standard practice for for 7868's and 7591's. Fisher 400 500 and 800 are all very similar this way.

 

[golana]

I'm not an expert yet, either:nerd:
but wouldn't you mean twice a 0,47 cap
to a 1,0mf.

 

[quicksand]

Good catch golana - .047uF is what I meant.

 

[golana]

I zigged---then you zagged

 

[quicksand]

I'm not an expert yet, but I've done some extensive reading and bookmarked many sites it seems to be standard practice for for 7868's and 7591's. Fisher 400 500 and 800 are all very similar this way.

Good point Grashopr - the 400 uses the 7868s. It just seems that every time I read about this mod it was refering to the 7591s. Just wanted to be sure.

Thanks for all the responses - Cheers!