27-11-2016, 05:46 PM
Note that mA/V of a triode or pentode does vary with operating voltages. You need a minimum Anode voltage, correct heater (10% can make a big change) and be well away from Vg1 cut-off voltage.
Valves with variable spacing on the winding of grid are "remote cut off" or variable mu, the mA/V changes far more with Vg. E.g, EF80 and EF86 are sharp cut off (more linear) and EF89 is variable. The less linear variable spacing on winding grids is better for AGC.
The Vg2 (screen volts) dramatically changes mA/V. Hence Vg2 must be at correct setting. The DF92 or EF80 work better with amplified AGC on G2, the DF91 and EF89 work well with control grid AGC.
Valves with variable spacing on the winding of grid are "remote cut off" or variable mu, the mA/V changes far more with Vg. E.g, EF80 and EF86 are sharp cut off (more linear) and EF89 is variable. The less linear variable spacing on winding grids is better for AGC.
The Vg2 (screen volts) dramatically changes mA/V. Hence Vg2 must be at correct setting. The DF92 or EF80 work better with amplified AGC on G2, the DF91 and EF89 work well with control grid AGC.
