Common mode theory for newbie

[catalin gramada]

Hello. Built this 12bh7 driver. I.m not pretty sure I understood correctly how it work. Should I follow 542+88V as supply voltage to describe the load line ? I ask for a basic theory how this driver in common mode work please. The voltages are directly picked from my test circuit.Input signals are supposed to be perfect balanced.The voltage amplification is 15 from my measurements. Thanks.

[Helmholtz]

Why common mode and out-of-phase grid signals?

The supply voltage for loadline construction is always the voltage between plate and cathode at zero plate current, so 542V+88V=630V.

[catalin gramada]

There was two reasons from my point (noise reduction not included) : First : to keep output signal balanced (a semipot will be included in the plate circuit if necessary for adjustment), second : to benefit by full amplification (with individual unbypassed cathode resistors the voltage amplification is half by that 7,5)

[Helmholtz]

There was two reasons from my point (noise reduction not included) : First : to keep output signal balanced (a semipot will be included in the plate circuit if necessary for adjustment), second : to benefit by full amplification (with individual unbypassed cathode resistors the voltage amplification is half by that 7,5)

Confused. Is this circuit not used as a PI? Where do need common mode operation?
After all it's a differential amplifier with very low common mode gain.

[catalin gramada]

No it is not a phase inverter. The phase inverter I used is a dc coupled cathodyne as preceding stage. The signals at the inputs of this driver are inverted and supposed perfect balanced.
In common mode the gain is almost as much as the tube can provide : 12BH7 u=16. I get litle bit over 15 in my circuit.

[Helmholtz]

No it is not a phase inverter. The phase inverter I used is a dc coupled cathodyne as preceding stage. The signals at the inputs of this driver are inverted and supposed perfect balanced.

So you want to use it as an "afterburner" driver stage with grids fed from the out-of-phase PI signals?
That would mean differential - not common mode operation, the input signal between both grids being Vin= (V+) - (V-)=2V+.

[catalin gramada]

Yes, is a balanced voltage gain stage using a low u triode.

[Helmholtz]

It works as a differential amplifier (amplifying the difference between the grid signals) to each of the out-of-phase outputs. No difference to an LTPI.
Even feeding only one grid would produce output at both plates - but only half as much.

For (undesirable) common mode, both grid signals would need to be the same (not out-of-phase).