What "Class" amp is this?

[TheTubeAmper]

Hello!

Now this question may look very stupid, but forgive me, I'm still pretty green when it comes to tubes and audio.

I've designed and built myself a nice little low voltage, line level, tube preamp. I'm using 2x 12AU7A's, one tube per channel, using both of the triodes inside wired up so the audio from one channel goes into its respective tube, goes through the first triode of the tube, then through the second triode of the tube, and leaves the preamp. The other channel is the same side.

As I understand it, your bias is a determining factor in what class of amplifier you have, and as I understand it, I have NO bias? That is, I am not putting any sort of DC or AC into the grid of any of the triodes, all of the audio signals entering the grid are just straight how they are.

Like I said this may sound silly that I've made my own design, tested it, and built it, and all the while did not know what class amp it was, but that's how I learn, I pick things up as I go.

Any help is greatly appreciated.

Thanks.


PS: I'll leave you with a nice picture of said preamp. (Please note it is not completely finished, it will look a bit better when it is.)

 

[Eli Duttman]

Your circuitry is single ended, not push/pull. Only Class "A" can be linear, when the circuitry is single ended. Therefore, your preamp is Class "A".

Class of operation is defined by the duty cycle of the active device and applies to both tubes and SS.

A Class "A" amp exhibits a 100% duty cycle. A Class "B" amp exhibits an exactly 50% duty cycle. Obviously, Class "AB" lies between the previous 2, with a duty cycle less than 100% and greater than 50%. Linear push/pull circuitry can be built employing Class "A", Class "B", or Class "AB".

A Class "C" amp exhibits a duty cycle less than 50%. By definition, Class "C" is non-linear and worthless for audio purposes. However, Class "C" is extremely efficient and makes a "whale" of a power RF oscillator.

 

[TheTubeAmper]

Your circuitry is single ended, not push/pull. Only Class "A" can be linear, when the circuitry is single ended. Therefore, your preamp is Class "A".

Class of operation is defined by the duty cycle of the active device and applies to both tubes and SS.

A Class "A" amp exhibits a 100% duty cycle. A Class "B" amp exhibits an exactly 50% duty cycle. Obviously, Class "AB" lies between the previous 2, with a duty cycle less than 100% and greater than 50%. Linear push/pull circuitry can be built employing Class "A", Class "B", or Class "AB".

A Class "C" amp exhibits a duty cycle less than 50%. By definition, Class "C" is non-linear and worthless for audio purposes. However, Class "C" is extremely efficient and makes a "whale" of a power RF oscillator.

Thank you, that was a very good explanation of the different classes.

May I ask though, what do you mean by it being "linear"? And could you shed any light upon bias?

 

[lcsaszar]

I would add that the term "class" has importance mostly for a power output stage, where a reasonable compromise should be found between linearity and efficiency.

If there is a resistor between the cathode and ground, the DC current on this resistor sets the cathode positive vs. the grid. This is a common technique for small signal tubes.

 

[Yvesm]

Don't forget that "class" may (will) change according to the level of the signal.
Almost all power output stages are Classe A at low level and go to AB, B and even C (yes, some geetah amps intentionally do that) while level rise and become less and less linear !

Yves.

 

[jjman]

My understanding is that class is determined at an output that is at or near its design max. Perhaps at a specific THD? So running a "low" signal might be in the "100% duty cycle" range, but not class A if it's less than 100% cycle when near max output.

 

[Merlinb]

My understanding is that class is determined at an output that is at or near its design max. Perhaps at a specific THD? So running a "low" signal might be in the "100% duty cycle" range, but not class A if it's less than 100% cycle when near max output.

That is how you might label am amp for advertising purposes.

But amplifier 'class' is a technical definition that applies to any active device, at any signal level. It just depends on what conditions you're interested it any any given time. An amp can be class A for small signals, but class AB for larger signals (and may get pushed to pure class B for even larger ones) But it might be advertised as class AB, since the customer will care mostly about the max power condition.

 

[TheTubeAmper]

Thanks for all of the replies everyone.

Now I'd like to inquire about how a bias can change the class of the amp.

Thanks.