Transformer question

[Axtman]

Why is it bad to run a output transformer without a load (no speaker attached) yet it is okay to run a power transformer without a load (all the tubes pulled)?

[Helmholtz]

In principle no-load operation is no problem for a transformer.
But in the amp the OT is an integral part of the output stage and if you disconnect the secondary load, the power tubes may produce kVs across the large primary inductance, likely to damage the insulation of the OT.

[mhuss]

Just spitballing here, but I think OTs tend to have a much higher inductance, which means more reactance and higher 'kickback' voltages as a result. Hundreds of volts are typical not a problem for windings, but thousands of volts can arc and destroy the wire insulation.

[Helmholtz]

Well, there's real no reason for mains voltage to rise when you disconnect the load from a PT.

[The Dude]

In principle no-load operation is no problem for a transformer.
But in the amp the OT is an integral part of the output stage and if you disconnect the secondary load, the power tubes may produce kVs across the large primary inductance, likely to damage the insulation of the OT.

^^^^^^ That. The transformer is part of a circuit, and the question of loading has more to do with the circuit than the transformer itself.

[J M Fahey]

Mains is a voltage source (low impedance) so load does not change it.
120V no matter what, loaded or open.
Even on a HV secondary: you have 400VAC? ... you will always have them, or less (because of losses)

But tubes are current sources, they will keep injecting current into OT even if such energy is not absorbed at the other end (speaker secondary) so will develop HIGH voltages at primary side.

Even worse, since they are inductive, switching current ON-OFF (exactly what overdriven tubes do) will develop kilovolts, remember they *start* with around 400-450V DC which is already half a kilovolt