Here is the Sim comparing fixed bias to cathode biased operation on well aged 6V6s with the normal cathode emission dropped by 10 %.
The in circuit fixed bias tube emission dropped by ~9.5% while the cathode biased tube plate current dropped by ~4.1%.
Cathode biasing minimizes the need for bias adjustment. And also protects the tube in case of a problem.
Some might wonder where all the Ls, Cs & OPTs went. Biasing is done at DC, all the reactive components don't do much at DC.
And at DC the power pentodes look like triodes for biasing, even when the plate voltage is higher than the screen.
The important number is the screen voltage for biasing calcs. In a good pentode plate current doesn't change much.
Thats where screen curves are more useful than grid curves on the plate family graph. But watch the plate watts.
Not many tubes have that published but there are workarounds. Refer to the GE spec sheet p5 for data on screen vs the plate family
Sure beats shot gunning the design & hoping something good will come out! With one exception I've never used battery bias.
And my PP 815 red plated as the battery eventually sagged. Long ago, when I still had hair,
There is a highly detailed data sheet here by AWV (Amalgamated Wireless Valve) of Australia covering the 6J7-G.
The extra data pulled out by AWV on some tubes is very useful.
The in circuit fixed bias tube emission dropped by ~9.5% while the cathode biased tube plate current dropped by ~4.1%.
Cathode biasing minimizes the need for bias adjustment. And also protects the tube in case of a problem.
Some might wonder where all the Ls, Cs & OPTs went. Biasing is done at DC, all the reactive components don't do much at DC.
And at DC the power pentodes look like triodes for biasing, even when the plate voltage is higher than the screen.
The important number is the screen voltage for biasing calcs. In a good pentode plate current doesn't change much.
Thats where screen curves are more useful than grid curves on the plate family graph. But watch the plate watts.
Not many tubes have that published but there are workarounds. Refer to the GE spec sheet p5 for data on screen vs the plate family
Sure beats shot gunning the design & hoping something good will come out! With one exception I've never used battery bias.
And my PP 815 red plated as the battery eventually sagged. Long ago, when I still had hair,
There is a highly detailed data sheet here by AWV (Amalgamated Wireless Valve) of Australia covering the 6J7-G.
The extra data pulled out by AWV on some tubes is very useful.