Class A Class AB Single Ended Push Pull

[tot_Ou_tard]

I believe that this link has been posted here before, but it bears another look:

http://www.mesaboogie.com/US/Smith/ClassA-WebVersion.htm

 

[tunghaichuan]

I believe that this link has been posted here before, but it bears another look:

http://www.mesaboogie.com/US/Smith/ClassA-WebVersion.htm

Very interesting. In a nutshell:

For single ended operation, a guitar amp can only be operated in Class A. It is possible to operate it in Class B or Class C, but it wouldn't sound good.

For push pull operation, a guitar amp can be operated in class A or Class AB. Class A is inefficient and produces lots of heat. Class AB is more effiecient and produces more power.

Biasing is independent of Class of operation. Either a Class A or a Class AB amp can be either fixed bias or cathode bias.

I was taught that Class A operation is only about 33%-40% efficient. For two EL84 tubes, this means that a Class A amp only would produce about 8-10 watts, not 15 like the article stated. (The EL84 has a maximum anode dissipation of 12 watts).

Here is another good explanation of Class of operation:

http://www.aikenamps.com/ClassA.htm

tung

 

[tot_Ou_tard]

Thanks tung! I think that I'm really starting to understand these terms.

Here's a question that didn't seemed to be answered.

Can a single ended Class A amplifier have global negative feedback?

The global feedback was described in the Aiken article as running from the output (the output of the output transformer?) back to the input of the phase inverter, but a single ended Class A amp has no phase inverter (at least that is my understanding).

I suppose the feedback could just go into the input of the power tubes directly.

 

[tunghaichuan]

Thanks tung! I think that I'm really starting to understand these terms.

Here's a question that didn't seemed to be answered.

Can a single ended Class A amplifier have global negative feedback?

The global feedback was described in the Aiken article as running from the output (the output of the output transformer?) back to the input of the phase inverter, but a single ended Class A amp has no phase inverter (at least that is my understanding).

I suppose the feedback could just go into the input of the power tubes directly.

Yes, if you take a look at the 60s Fender Champ schematics, you will see that there is global negative feedback. It is injected into the cathode connection on the second gain stage, i.e. the 12AX7 triode driving the 6V6 tube.

tung

 

[tot_Ou_tard]

Yes, if you take a look at the 60s Fender Champ schematics, you will see that there is global negative feedback. It is injected into the cathode connection on the second gain stage, i.e. the 12AX7 triode driving the 6V6 tube.

tung

Thanks tung!

So both triodes of the 12AX7 are used and the negative feedback is injected after the first triode & before the second?

 

[tunghaichuan]

Thanks tung!

So both triodes of the 12AX7 are used and the negative feedback is injected after the first triode & before the second?

Just the second triode. The two 12AX7 triodes are cascaded one into the other to increase the gain.

If you take a look at this schematic:

http://www.diycustomamps.com/images/princeton/champ_5f1_schem.gif

(that is a 50s champ, and it also has negative feedback)

You'll see a 22k resistor connected from the secondary of the output transformer connected to the cathode of the second 12AX7 triode. Notice that there isn't a cathode bypass capacitor there. If there was a cap there it would swamp the the negative feedback and effectively remove it from a circuit. This is a cool mod to do to champs: put in a cathode bypass cap but make it switchable on the ground side. This dramatically increases the gain and distortion.

tung