and it's pretty pointless listening to a class A valve amp, because it's highly distorted and low quality anyway.
And it is that very distortion characteristic that blues and rock guitarists are after.
Although the distortion characteristics of a valve are much more desirable than that of a transistor.
sonuv said:
what are class A,Class B ,Class AB etc
Class A amplifiers are "single ended" (i.e. only need 1 output device) and that 1 output device amplifies the full sine wave. When the amp is in the quiescent state (i.e. when there's no input signal present...which we will call "zero signal condition"), the output device is dissipating its full rated dissipation. When under signal average dissipation drops to about 1/2 what it dissipates under zero signal conditions. Because it is single ended, there is no offset current in the output transformer to keep the idle current from magnetizing the core so the OT must have a sufficient air gap between the coils and the core to minimize idle magnetization. Also, in a typical Class A amp the power supply must be spec'ed to deliver LOTS more power than what the amp will actually put out (something over double the output power) as the amp dissipates its full plate dissipation rating at idle, which ends up being a bit higher than what ends up at the output under signal. Under signal, 1/2 of this power is handed off to the load while the valve dissipates the other 1/2.
Because Class A amps only transfer 1/2 the available power to the load while the valve dissipates the other 1/2, they are considered to be highly inefficient comparted to Class AB and Class B.
Current draw from the supply on a Class A amp is constant. When under signal, the voltage at the valve plate drops, which increases the voltage dropped across the OT while decreasing the voltage dropped across the valve at the same time. This trading off of the voltage drops between the load and the valve "hands off" the power to the load that the output device was dissipating at idle. In other words, you're just changing which device the supply voltage is being dropped across when under signal.
Class AB amps are push pull. Each output device passes current to the load for more than 1/2 but less than the full swing of the sine wave. This means that there will be an "overlap region" where both devices are on at the same time until at some point in the sine wave swing they transition into Class B mode, where one of the output devices turns off (goes into cutoff) while the other output device is still conducting. They alternate on/off at opposite times. On a transformer coupled amp, the center tap of the transformer primary is connected to the + rail while the start/finish wires go to the output devices. Because one side of the primary appears to be wound BACKWARDS from the other relative to the center tap, current through the transformer primary flows in the opposite direction, which reverses the magnetic polarity of the magnetic field, which induces a negative going swing at the secondary while the other induces a positive going swing at the secondary. With both output devices alternating on/off at each end of the transformer primary, this is how you end up with a complete sine wave at the output.
When at an idle state, both sides of the OT primary are pulling current through the output devices. Each output device is biased so that they are only dissipating roughly 70% of their dissipation rating while this is happening. Due to both sides of the OT primary pulling current, with one side wound backward from the other relative to the center tap this causes one side of the primary to be magnetized at an opposite magnetic polarity than the other side, which cancels the core magnetization at zero signal condition, which means no air gap needed on the transformer to reduce idle magnetization of the core.
Under signal at full output, damn near all of the available power is handed off to the load while the output devices dissipate very little at the peaks of the sine wave, which makes for a much more efficient amplifier. Output devices dissipate the full available power 1/2 way up the sine wave swing, which ends up being DOUBLE what the output device is rated to take, but due to being in cutoff for 1/2 the swing the average dissipation ends up equaling the output devices rated dissipation.
Current draw on a Class AB amp is constant when in Class A mode, then increases at the point in the swing where the output devices transition to Class B mode.
Class B is just like Class AB, but with no overlap region. When one output device is on, the other is off, and they switch on/off at opposite times. Each output device only passes current for 50% of the sine wave swing. Of course, on a valve design there still needs to be an overlap region to mask the non-linear region at the bottom of the response curve in order to eliminate crossover distortion.
