• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

fixed bias change the grid voltage on clipping

Celsius

Member
2011-05-23 11:21 pm
Spain
Last week testing the power of my 211 amp begin to measure different point to see what happen when 211 begin to work on clipping.


Plate 1040V no change with or without clipping
Cathode 0,65V on testing resitor 10R change from DC + AC = 0,65v rms
Grid : -55V standby
Grid: -100V( more or lees ) on clipping

I never measured this point on clipping before, but I would like to understand why. The grid resistor load is 220k why on clipping have dc current in negative voltage to change the voltage from -55v in standby to -100v at grid
 

disco

Member
2006-04-17 6:27 pm
Holland
In baby steps ;) : What happens when your final tube is driven to clipping?

Normally the grid is driven from the input signal with both positive and negative phase. There is only very small current involved, just enough to overcome the input capacitance. At clipping, the positive phase charges the coupling capacitor because more current starts to flow. That is because the grid holds no longer a negative charge and gets bombarded by electrons. So, the positive grid loads the coupling capacitor which has to discharge through the grid leak resistor. This gives blocking distortion.