Ok…I gotta chime in on this again.
While I don’t completely disagree with the information from Mercury, I find it isn’t adequately explaining what is going on when a transformer rusts. I will say that I feel they are accurate when they state that a rusted transformer can sound great. I have always felt that losses in transformers yield good tone…as long as they are controlled.
First, the rust (iron oxide) isn’t the problem; the transitional layer (amalgam) that lies between the oxide and the unaffected metal is the problem. Eddy current losses in a transformer are a function of the lamination thickness and the metallurgy of the steel. The amalgam layer is much thicker than the oxide layer and has undergone significant change to its metallurgy and grain structure. This can equal some hefty eddy current losses. It can also cause some significant hysteresis losses as well; this is the part that I feel adds to the tone of the amp in a positive way. Mercury makes no mention of this important attribute at all. Many references lump eddy current and hysteresis losses together; they are completely separate phenomenon.
Eddy current losses are the electrical equivalent of resistance, except it is in reference to magnetism. Eddy current impedes the flow of magnetism, effectively slowing it down and dissipating some of it as heat. All transformers that use paramagnetic steel laminations have some degree of eddy current loss. Eddy current losses are directly proportional to the thickness of the individual laminations.
Hysteresis losses are a bit more complicated; they are the resistance to changes in magnetic flux and flux density. Hysteresis losses change the shape of the waveform. This also results in heat but rather than being constant like eddy current losses, the degree of losses can change with load. This is the phenomenon commonly referred to as “saturation”, though saturation never occurs in any guitar amplifier. Hysteresis losses are directly proportional to the physical size of the lamination stack. This is why “big iron” often results in a “clinical” or “HiFi” tone while “smaller iron” sounds more organic and musical.
Finally, the oxide layer in transformer laminations is not iron oxide. I’m not saying that nobody in the history of transformer manufacturing hasn’t done it, I’m just saying that once the process starts, it is nearly impossible to stop it. Copper oxide and Sulfur oxide are far more controllable, though some anodic action can occur which can result in a transitional layer of its own. This is controllable by design.
A rusted transformer is not necessarily a bad thing…it is, however, a different thing…which can result in better tonal characteristics. But, it is the beginning of a death cycle…which may take years or decades to manifest.
