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- Thread starter ChasFred
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ivan H
Ambassador of Tubes & Grooves
Class A can be described very simply as "current flows through the tube for the full 360* of the sine wave". It can never be driven into cutoff or it is no longer operating in class A. The same criteria applies for class A push/pull.
It (class A push/pull) is a very inefficient way to use output tubes. Without naming names, you often see amplifiers advertised as "class A push/pull" designs when in reality they are advertised as having too much output power for the tube types used to actually be running in class A. Even the Vox AC30 is not a "class A push/pull" design (though often referred to as, thanks Mr Aspen Pittman). It is simply a hot biased class A/B design.
I believe that Randal's simulclass designs do run the class A tubes in true class A. Here phase inverter drive to these tubes must be limited so as not to drive them into cutoff. Ok, rant over. Cheers
It (class A push/pull) is a very inefficient way to use output tubes. Without naming names, you often see amplifiers advertised as "class A push/pull" designs when in reality they are advertised as having too much output power for the tube types used to actually be running in class A. Even the Vox AC30 is not a "class A push/pull" design (though often referred to as, thanks Mr Aspen Pittman). It is simply a hot biased class A/B design.
I believe that Randal's simulclass designs do run the class A tubes in true class A. Here phase inverter drive to these tubes must be limited so as not to drive them into cutoff. Ok, rant over. Cheers
ivan H
Ambassador of Tubes & Grooves
That audio amplifier would sound phenomenal. "True" class A biases the output devices in the center of the linear part of the plates characteristic curves & as such, to stay in class A the device cannot be pushed into saturation (as it will simultaneously be pushed into cutoff). It is a very pure form of amplification, especially class A triode. Audiophile's love their class A triode amplifiers (like that Mr Plexi has shown).
A word on "triode connected" pentodes. A "triode" has 3 elements (heater isn't counted here)
A "pentodes" has 5 elements.
Note the "suppressor" grid (g3) that the pentodes features. Its job is to repell electrons that bounce off the plate (secondary emission) "back" to the plate. Most guitar amp designs tie this suppressor grid to the cathode, though some tie it to a negative voltage, like the bias supply (runs the tube more efficiently).
When a pentodes is (so called) "triode connected", the "screen" grid (g2) is tied to the plate, or anode. This still leaves the suppressor grid as an active element in the tube. Tying the screen to the plate does greatly reduce the tubes efficiency & change the way in which it reacts & sounds, but it does not make it into a triode (doesn't "react" like a triode). In short, it is impossible to make a 5 element tube into a 3 element tube. Cheers
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