#VOX AC30CC2 TUBE BIAS FULL#

The plot shown below illustrates the current and voltage waveforms associated with the above circuit, when the output level is set just below the clipping point (note the slight rounding of the tops and bottoms indicating the onset of clipping, prior to it flattening out on the top at hard clip). The current meter symbols show the quiescent DC plate current in each tube, and the voltage and current probe symbols show where the plate current and output voltage measurements were made. The output transformer reflects the specified 4K plate-to-plate primary impedance.

An input signal to the phase inverter of 0.43V peak (0.86V p-p) drives the output stage just to the point of clipping. This results in a quiescent plate dissipation in the output tubes of (320V-10.5V)*47.47mA) = 14.7W (note that the EL84 is only rated at 12W max plate dissipation).

#VOX AC30CC2 TUBE BIAS FULL#

How does the AC-30 output stage measure up at full unclipped output? The circuit below is the output stage of the common Vox AC-30 amplifier, which features a cathode-biased output stage: The plate voltage is set to match the factory schematic, which shows 320V. This is the approach taken in this article. From a design standpoint, the circuit can be simulated using a commercial program such as PSpice, and can be determined whether or not the design is theoretically possible to run in true class A, and if not, what it would take to get it there. The current probe is then clipped onto the plate lead of one tube on each side of the push-pull pair and the plate current waveform is inspected to determine if it is in cutoff for any appreciable amount of time. This can easily be done with a current probe and an oscilloscope, by cranking the amp up to just before clipping into a resistive dummy load.

So, all we have to do is look at the plate current in one tube on each side of the push-pull output stage to determine whether the amplifier is truly class A. In a class AB amplifier each tube amplifies more than half the signal, in order to reduce the distortion that occurs at the zero crossings of the waveform, which is called 'crossover distortion'. This is done to provide higher efficiency and greater output power. Effectively, one tube amplifies the 'upper half' and the other tube amplifies the 'lower half'. The output waveform is still a clean, unclipped sine wave, because the transformer sums the two 'halves' of the input signal into one composite signal. Cutoff refers to plate current cutting off on one side of a push-pull pair for a portion of the cycle, while the other side continues to function. Note that cutoff does not mean that the output of the amplifier is clipped, or distorting.

If the amplifier is designed such that the plate current flows for more than half, but appreciably less than a full cycle, it is then a class AB amplifier. If each side is in cutoff for exactly half the input cycle, it is a class B amplifier.

The Vox AC-30 is always brought up as an example of a class A amplifier, but is it really class A?ДЇirst of all, how do you tell if an amplifier is class A? A push-pull amplifier is defined as operating in class A if, at the full undistorted output (just prior to clipping), the plate current flows in each tube for a full 360 degrees of the input conduction cycle. General There is a lot of discussion on the topic of class A amplifiers.

Vox ac30cc2 tube bias