Fixed Bias Questions

[dunner84]

I have a few questions regarding proper biasing in a fixed bias amp. I recently competed my first fixed bias amp, and I needed to tweak the bias circuit a fair bit to get reasonable numbers, and this raised some questions. What is the relationship between the cathode current and negative grid voltage? Without "using your ears" what is the most important factor when biasing, max plate dissipation, cathode current, or grid voltage?

Example. with 6V6s I have -42v, 22mA, and 457 on the plates. That works out to be around 10watts or 70ish% for a jj6v6... With earlier bias circuit configurations, I needed much different grid voltages to get 22mA. Which item requires the most attention or care when it comes to the spec for a particular tube? Am I better off biasing to 60% if it means I can reduce the grid voltage?

[PRR]

Get the happy current.

To some degree you do not care what grid voltage is needed to do that.

But for 6V6/6L6/EL34 we "expect" Vg1 to be some fraction of Vg2 at a happy high-power point. Expect 0.06 for lower plate voltages and 0.1 for high plate voltages. 457V on a 350V tube is insane high. Assuming Vg2 is say 440V, we would expect 0.1*440V= 44V on G1. Which is about what you have.

This deep-tail of the tube's current range will vary a LOT between tubes of the same "type number".

[sluckey]

Cathode current is inversely proportional to negative grid voltage. IE, as negative voltage increases, cathode current decreases and vise versa. The voltage on the grid CONTROLS the cathode current. I consider max plate dissipation to be the most important.

With earlier bias circuit configurations, I needed much different grid voltages to get 22mA.

I find that odd, assuming plate voltage was the same and the only thing that changed was the bias circuit.

[jecars]

I am a fan of the following link to interactive tube data, kudos to Black Magic Amps. I like to compare/contrast plate vs grid curves (lower left graph) while varying inputs.

http://www.bmamps.com/Tech_tds.html

P.S.  Your post says 450V plate on a 6V6 from JJ.  I assume it's a 6V6S to run that high, am I right?

[dunner84]

Thanks for the info and quick responses. It all makes a lot more sense now. As for the 6v6s and the high plate voltage, it's an ab763 type amp that I designed for 6l6's; however, my test tubes are the jj6v6s. The voltage is a bit high because I use a SS rectifier for testing. That being said, I have read a lot about these tubes, and people seem to think that the JJ tubes can take the higher voltage, especially if they are biased conservatively. I also only seem to like the way they sound when used in amps designed for 6l6's. They don't quite sound/behave like a 6v6 to me

[Willabe]

The voltage is a bit high because I use a SS rectifier for testing.

Why?

Why not put in the rectifier tube your going to use so you have the voltages you will normally have.

[dunner84]

For this build I had a nice NOS 5ar4 on order. It's actually supposed to show up tonight, so I'm excited to get it all put together. There are some nos tung Sol 5881s coming too. They are for another project, but I will be trialing them in this amp.