speaking of bias, do you believe any of this:
"I read about this trick several years ago, and even did several experiments. The theory goes like this:
Every transformer (or electromagnetic device) has two non linear regions. One is near zero current, and the other is the saturation region. Look up "BH curves". We all know about saturation in a transformer, and the resulting ugly sounding distortion. There is another, much smaller non linear region where some of the initial energy applied to the tansformer is used up magnetizing the core without generating any current in the secondary. This leads to a "dead zone" around the zero net current region.
This effect does not come into play in a SE amp since the DC current flowing through the transformer primary biases the transformer right into the center of its linear region. The transformer in a P-P amp ideally has no net magnetizing force at idle, so the non linear region occupies the center of the important "first watt" zone. When a P-P amp is used with efficient speakers the effect can be a loss of detail and a "dull sound".
It is possible to move the "dead zone" away from the "first watt" region by applying a significant offset in bias current of the output tubes. The tubes must be biased hot enough so that the tube with the lowest current does not generate distortion. This technique works best with class A P-P amp designs. A significant DC offset requires a gap in the transformer core, which then reduces the primary inductance, requiring a much larger transformer.
My experiments used an 80 watt guitar amp transformer ( I have a lot of them) similar to the one I used in the 300Beast (30 watt P-P). I restacked the core to include a gap, which was optimized to allow the transformer to work in a SE amp.
I tested the modified transformer with the idea of building an asymetrical P-P amp for HiFi purposes, and believe that the idea has some merit for lower powered P-P amps (I was making 10 watts). I came to the conclusion that it would just be easier to build an SE amp.
These experiments quickly lead to the idea that the asymetrical P-P amp had some serious use as a guitar amp. It is possible to use two different output tubes, at two different bias currents, to build an amp that sounds clean up to a point, then transitions to nasty abruptly without needing to be extremely loud. Much more experimenting is needed here.
also of interest:
"In E/I and C core OPT's you can add DC bias for reduced distortion. Chicago Transformer, Peerless and others did this by deliberately unbalancing the DC resistance in the primary halves. It also messes with the tubes and is not good when the system is called upon for maximum performance.
Better to put a gap in the core and flatten the permeability hump that way instead. That is what DC off set and gaps do and it has a direct relationship to how much distortion you have under 400 Hz. Commercial core does not provide anything but a ferrous focusing window for the B Field event, above 400 Hz, and the increased permeability in the core, as frequency drops, just interferes with the antenna event and creates distortion.
Only problem with either scheme is that you will eat inductance, which is what gives you the reactive load match that provides low frequencies in the first place. Only free lunch is no saturation induced reminance in the core so no zero crossing distortion in PP and no permitivity defined "settling" time on the back half of a signal in a SE OPT either. No way out of inductance equals distortion either.
