How does a Class A amplifier amplify the NEGATIVE part of the sine wave?

[boobtoob]

Hello:

Have built a few 'kit' guitar tube amps (5F1A, 5E3, etc.), and am learning about this stuff.

I understand how the plucked guitar string creates a sine wave signal with both positive and negative voltages.

And, I think I understand how the class AB amps take the sine wave, spit it into positive and negative, and sends each of the voltages to separate output tubes.

But, the class A still mystifies me. I understand that the output tube is always running full blast and it is biased at about 50%.  And, when a positive signal is applied to the grid, it subtracts electrons from the grid, thus allowing the cathode a clearer path to send its electrons to the plate.  This will cause amplification.

But, if the grid is sent a negative signal, it would BLOCK electrons trying to get from the cathode to the plate.  This would cause the sound to diminish, not amplify. 

What am I missing?

Thanks,

[PRR]

> How does a Class A amplifier amplify the NEGATIVE part of the sine wave?

When you turn it on, current rises to "half" of maximum.

With signal, it varies from half to zero to half to max and back.

[jjasilli]

boobtoob:  The power tube is not special.  The same thing happens in ea stage of a preamp tube, be it the usual triode or a pentode.  I think your error is believing that when the signal grid goes negative it necessarily puts the tube into cut-off.  This is wrong. 


When the tube is biased a current is flowing through the tube between cathode and plate.  The signal impresses an image (a negative image) of itself upon that flow.  Only if the signal is too strong for the bias operation point does the tube go into cut-off.  Before going into cut-off the signal will be clipped producing overdrive tone.