Vacuum Tube bias for designers

[richgwilliams]

You are trying to force the valve/tube to 'stay on' by using a constant current sink. This is not new. There are different ways to force the valve/tube to stay on. You probably have read about them all.

The current from Plate to Cathode (without AC signal) is always accurately 8.192mA the voltage across the 1K5 Cathode resistor is quite large usually maybe as much as 10 Volts, the HT (B+) is hopefully exactly 180 Volts so the Voltage at the Plate (without AC signal) is exactly 180 - (6000 x 8.192 e-3) which is 130.85 Volts exactly.

 

[soulmerchant]

You can do that even with a non-bypassed cathode resistor referenced to ground if you know what you are doing and understand the electrical characteristics of your triode well. You can also simply bias the grid - which is the standard way which plate curves in datasheets are generated (measured then averaged. This is often done in pro audio.

Are you using a constant current sink to bias the cathode? or just a negative rail? It seriously is not new - pretty much everything has been done before with valves/tubes. But they are still a pleasure to work with

 

[richgwilliams]

As a designer the whole point is not for my design to rely on the characteristic curves to stay constant during the lifetime of the valve, to stay constant between different valves of the same type or to be the same for different valve types.

That's the whole point of this design method, constant current and negative rail to allow the emitter resistor to hold the Cathode slightly positive of the DC Grounded Grid.

I never saw this done before until I designed it. Show me a circuit where it is done? If it had been used in equipment from many years ago it would have solved countless reliability problems and so many posts in diyaudio about the need to adjust Grid bias resistance etc.

If you build valve circuitry try it.