Originally Posted by jefrs
An 8ohm load on a 4ohm tap is a +100% load, which is fine if you don't mind risk of blowing the output valves and/or the OT cooking - the the OT will get hot enough to fry an egg with the correct load. Remember that 8ohm load is actually somewhere between 50ohm and 8ohm.
You know I hate questioning you on some of your posts but either I miss the logic or there is not enough info for me to process the information so I hope you do not mind my asking a few questions.
So I take it an 8 ohm load will cause a transformer to get hotter than if it is feeding a 4 ohm load that it was designed for. How does that happen? Let's take a case of a short circuit across the transformer output. Not much voltage but a lot of current. And since the only real resistance (although we are dealing with AC the correct term is impedance but since those less technically confident have an easier time with resistance we will use it) is due to the output winding, the voltage drop occurs across the winding and since the current is equal in the whole circuit the power is dissipated in the transformer (Power = Volts x Amps).
Now the other extreme is if the transformer is lightly loaded. we will get a higher voltage but a lower current in the secondary. Given that the current is equal in the secondary loop and that there is more of a voltage drop in the load rather than in the secondary winding I would think we would assume that most of the power will be dissipated outside the transformer. In my neck of the woods this would cause the transformer to run cooler.
Now if you chart the amount of power dissipated in a transformer, with a dead short as all the heat developing inside the transformer and with the transformer lightly loaded with most of the power turned to heat outside the transformer, you might take it with a leap of faith that driving a four ohm load will heat the transformer up more than driving an eight ohm load.
Or you might not. I am scratching my head on this one.
What happens when the load goes wrong? - I had an AC15 blow the speaker (G12-15) which shorted and caught fire, which caused a fly-back on the OT, I got an electric shock which knocked me on my back, meantime the main fuse blew and all the lights went out, the girls started screaming as the boys started a fight.
Actually I think the speaker shorting would load down the transformer. Maybe the speaker opened after the short and then the transformer ha no where to loose the energy resulting in the fly-back?