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This explains it pretty well:

http://www.opamp-electronics.com/tutorials/core_saturation_2_09_11.htm

At this point, it starts to get way over my head.

tung

To add to what DVM said:

The Hammond 125ESE is about 4X the size of the stock VJ OT. The Hammond is made better than the stock VJ OT using better materials. Since the VJ is a single ended amp, the...

Robert,

You are correct. An OT is basically an impedance matching device. Tubes are high voltage, high impedance, low current devices. Speakers are low voltage, low impedance, high current...