Hum through speaker with NO TUBES! Help!

[big_teee]

Inocent Bystander reading this interesting Thread.
Can someone explain the purpose of the Flyback Diodes for us Not so Veteran Builders?
Thanks,
Terry

[Krwkka]

From WikiPedia:

A flyback diode (sometimes called a snubber diode, freewheeling diode, suppressor diode, or catch diode) is a diode used to eliminate flyback, the sudden voltage spike seen across an inductive load when its supply voltage is suddenly reduced or removed.

As I understand it, a speaker motor actually produces a voltage when it returns to its rest position. This voltage goes back into the transformer through the secondary and is multiplied in the primary, ultimately creating a sort of "backfire" in the amp's circuit.

I'm sure somebody here will improve on this explanation.

[Enzo]

When you have to replace one of those diodes, you really should replace its partner. If the amp is now working, you can safely take some readings. Measure voltage drop across each of those diodes. COmpare the readings of the two on the good side to the readings of the ones on the side that failed. Any difference between the one yo didn;t replace there, and the corresponding one on the side that didn;t fail? I am not talking about junction drops on diode test, I am talking about actual voltage found across the things in a live circuit.

Remember, your diode test only sends a little over a volt to the diodes under test. You measure the junction voltage at that level. That doesn;t tell you much about how the diode handles 500v. The one you didn;t replace could still leak like a seive at anything over a few volts, and your meter couldn't detect it. That is why I suggest measuring the voltage found across each while the circuit is operating. If I had to guess, I'd be thinking the one diode shorted and the remaining one leaks. Replacing the shorted one was enough to prevent the leak from finding ground, but now we really only have the one diode blocking it instead of two.

Of course that is just a hypothesis.

[Krwkka]

Good points. Unfortunately, I can't do the testing now. The amp went home with its owner this afternoon. However I did replace both diodes on that side.

[Enzo]

Well good, you didn;t take any shortcuts, and whether I was right or wrong on the second diode is now moot. SO good.