Plate current Question

[Rob11966]

Here's something that confuses me.

I am talking about the 845 tube but I imagine that this question applies to other tubes as well.

With respect to the operating plate current, the ratings differ depending on the plate voltage. Looking at the 845 Amperex data for example the typical operating values are as follows (plate voltage plate current) -

750v 95mA
1000v 65mA
1250v 52mA

I understand why you have to adjust the bias voltage to limit/adjust the plate current but I don't understand why the recommended typical operating current goes down with increased plate voltage. If the plate is capable of handling 95mA at 750v, why can't it handle 95mA at 1250v? More specifically what the problem with running a tube at say 1000v and 95mA.

I am sure I have missed some basic point. Any advice appreciated.

Cheers,

Rob

 

[rtsang]

Plate dissipation

Plate dissipation = plate voltage drop x plate current

The power dissipation is the fixed value.
For 300B, it is 40W.
For 2A3, 15W.
Of course too much voltage and too much current will also kill a tube, but usually we hit the power limit first.

 

[sawreyrw]

Power dissipation limit.

 

[Rob11966]

Obvious - now. Thanks guys.

Rob