Power MOSFET for tube biasing

Thread Starter


Joined Oct 23, 2011
Hello everybody,
I am new to the website, but long time DIYer.
I am playing with a power tube cathode biasing with a MOSFET.
I have two circuits in mind which I'll post.
I need your opinion and suggestions on both.
What will be the difference or benefits of using MOSFET(second attachment) over the Bipolar transistor(first attachment). Also am I correct about power dissipated from cathode transistor? I expect to be around 32W for the bipolar transistor version and 18W for the MOSFET version. I build already the MOSFET version and it dissipates a huge amount of heath. I tried to measure the temperature on the MOSFET itself and it shows 145~150 centigrade.
Thanks alot,



Joined Dec 26, 2010
They are quite different circuits, not just alternatively biased. The cathode output version with the BJT has no DC current in the transformer, and presumably has the perceived advantages in linearity and damping usually ascribed to cathode followers, likely at considerable some cost in efficiency. Presumably efficiency is not the aim though, given the use of triodes.

The second version puzzles me more. The output is from the anode, so a conventional output transformer with good DC tolerance is required. What then is the advantage of the complex biasing circuit? The triode grid voltage is not fixed, so this is not really a cascode.

In each case, the transistor takes the full cathode current, so you can find the dissipation by multiplying by the cathode voltages, less whatever is dropped in the small resistor in the emitter/source lead. Large heatsinks would be in order.

Thread Starter


Joined Oct 23, 2011
Thank you for your reply. Actually I am not concerned about the output transformer. I know that in first schematic there is no DC on the primary hence you can lower the size of the output transformer and have other benefits etc. Actually I am more concerned about stability of the biasing and thermal runaways of the tube or biasing transistor. One difference between two schematics is that the one with MOSFET is autobiasing the tube. The other with the NPN transistor is fixed by the LEDs drop voltage. I don't know if 20~30 watts is a lot of dissipating power? It's definitely creates a lot of heath. I mounted the MOSFET on the chassis of the amp using it as a heath sink.

Jon Wilder

Joined Oct 25, 2011
I honestly don't see the point of MOSFET biasing a Class A single ended amp. Why not just use a power resistor/bypass cap? What is it that you're hoping to gain via BJT/MOSFET cathode biasing?

Thread Starter


Joined Oct 23, 2011
Dynamic resistance of the MOSFET is zero or close to zero. So for the signal it is like cathode is sitting on the ground. Also you avoid bypassing el. cap.
No local feedback.


Joined Mar 24, 2008
Those are not MOSFETs, they are BJTs. Both are transistors, but entirely different animals.

MOSFET = Metal Oxide Semiconductor Field Effect Transistor (Voltage operated component)

BJT = Bipolar Junction Transistor (current operated component)