Determining necessary bias voltage from tube data sheets

[soma_hero]

I have no idea how to interpret tube data sheet graphs, or draw lines to determine current and bias points and all that jazz.

I'm building a submini express using 5899 output tubes. I want to know where I should bias them at initial startup, so I can determine the circuitry for my bias tap. I'm using back to back 12V transformers and I've got a HV+ of 140V with no load. My existing bias tap circuit gives me a max negative voltage of about -12V. I don't think this is low enough so I'll put together a voltage multiplier to get me in the right range.

But I need to know that range first. How do I figure out my bias voltage from the datasheet info?

[Structo]

Well, I'm not a tube expert but I'll take a stab.
Looks like max plate voltage is 100v.
And plate current is 7.2 ma.

Not sure if the transconductance voltage is the bias voltage but since it is listed as negative - 14v that is probably the bias voltage.

The max plate dissipation is 1.1w, multiply that by .7 (70%) and that would give you the 70% dissipation goal of .77w

Then you divide that by the plate voltage, say it is 100v and you get 7.7ma which is fairly close to the number above for the plate current.

Because you didn't take the screen current into consideration that number is probably alright.

If this amp is cathode biased then you can usually safely bias it at 100%.

That would be 11ma for 100v plate voltage.

Here is the formula.

(W * .7) / V = I where w is max dissipation in watts, V is plate voltage and I is current.
That is the 70% bias rule.
But with cathode bias, you would simply divide the 1.1 max power by the measured plate voltage.

[jjman]

I'm just starting to understand the load lines and such. I see it but I don't know what the spec showing "-14v" is referring to exactly. The load lines and resulting suggested cathode math point to an expected -1.1 volts bias. It's a low current low voltage tube so low bias voltage is to be expected.

If you consider 100v on the plate and 7.2 ma of plate current you end up near the -1.0 volt grid line on the pentode graphs. I drew the red lines to show that vicinity.

[soma_hero]

Thanks so much guys, that sounds like my bias supply should be fine the way it is then..

Thanks for the pic JJman