# fixed bias change the grid voltage on clipping

#### Celsius

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

#### Chris Hornbeck

A vacuum valve's grid acts as an anode if it is driven positive of the cathode. The driving voltage charges up the capacitor coupling signal to the grid. It's an important yet often ignored issue in amplifier design.

All good fortune,
Chris

#### Celsius

A vacuum valve's grid acts as an anode if it is driven positive of the cathode. The driving voltage charges up the capacitor coupling signal to the grid. It's an important yet often ignored issue in amplifier design.

All good fortune,
Chris

sorry, too much summarized to understand

something are wrong?

#### disco

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.

#### Diabolical Artificer

See - Analysis section 9. He mentions a few methods to slow it down including making grid leak R bigger and coupling cap smaller, both have consequences also mentioned are catch diodes and a zobel network but I'd be interested in members approaches to design in order to ameliorate this.

Andy.

#### analog_sa

Use a transformer and be done with this.

#### Chris Hornbeck

An inductive source also has (dI/dt) consequences.

All good fortune,
Chris