Adjusting bias in a tube microphone.

[vmanj]

Hello everyone.

In the U87 solid-state microphone, the bias (source resistor) is adjusted individually for each fet transistor.

Does it make sense to do the same in tube microphones, for example 251, C800g. adjust the bias (cathode resistor) for each tube individually?

 

[Khron]

Tubes may or may not have actually less variation in parameters, than JFETs..?

Don't quote me on this, but i'll gladly welcome the input of wiser people than i

2N3819's quoted pinch-off voltage is 0.5V to 7.5V. That's pretty huge, especially considering its 25V drain-to-source max rating...

 

[Gus]

tube parameters are set by the physical construction.
grid to cathode distance, grid winding, grid to plate distance etc. so they have more controlled specs

 

[kingkorg]

On the other hand cathode and plate resistors seem to be just copied and pasted in many schematics, but they don't necessarily give symmetrical clipping, which might be something you want or not. I like to adjust cathode, and sometimes plate resistor as well so i get ½ of B+ voltage at plate.

 

[vmanj]

Maybe I'm wrong, but I think the fet in the u87 is set to close to symmetrical clipping.

It’s also possible in tube microphones; manufacturers do this by selecting tubes for a specific circuit.

But I’m not a factory and I don’t have the opportunity to select a tube, so maybe I need to use a cathode resistor to adjust the clipping close to symmetrical for each tube?

 

[kingkorg]

But I’m not a factory and I don’t have the opportunity to select a tube, so maybe I need to use a cathode resistor to adjust the clipping close to symmetrical for each tube?

Well that‘s at least what i do. Some people like asymmetrical clipping tho.

 

[Matador]

Note that half-B+ idle point may not necessarily guarantee symmetrical signal swing, due to where the load line passes through the various grid curves as the tube enters cutoff.

In fact, in the C12 design, having 1V at the cathode with a 0.7mA bias point (roughly a 1.5K cathode resistor) and a 100K plate resistor, even tubes from different manufacturers and made at different time largely sat all at the same idle point (roughly 50V at the plate). And all were capable of swinging pretty cleanly through 70-80V of output, meaning with 30dB of gain, even a few volt signal on the grid passes through pretty cleanly.