Why "flyback" diodes?

[rockon1]

Are these really necessary? It seems a lot of newer amps(non vintage) use them. Wouldnt an amp work without them? Bob

[Don Moose]

You mean on the OT primary? A safety feature in case of a no-load condition or poor/intermittent connection. Given the tendency to inter-board-lead/solder-joint breakage in modern-construction amps, this is a reasonable thing.

On relay coils? Noise suppression - keeps the current induced by the coils collapsing field in the coil and out of the switching contacts.

Hope this helps!

[rockon1]

You mean on the OT primary? A safety feature in case of a no-load condition or poor/intermittent connection. Given the tendency to inter-board-lead/solder-joint breakage in modern-construction amps, this is a reasonable thing.

On relay coils? Noise suppression - keeps the current induced by the coils collapsing field in the coil and out of the switching contacts.

Hope this helps!

Yes I meant on the primary. If one failed I figure it might just as easy cause a problem. I guess it would have to see a dangerous (to the OT) condition to do so anyways and better it than the OT? Thanks Bob

[tubeswell]

Yeah the amp'll work without them or with them. FWIW I took the ones out of my PV C30 (can't remember why) - haven't had any issues. They're there to protect the your valuables against insane spikes

[Enzo]

They are like the airbags in your car. The car will work just fine without them. Your amp will work fine without the flyback diodes. Amps also used to work fine with two-wire power cords. If you crank your amp without a speaker connected, or if a speaker cord gets yanked out, or a wire falls off in the cab, there is a very real possibility you will destroy your output transformer. These diodes help prevent that. They are invisible to the circuit under normal conditions. Cheap insurance I'd say.

[Robert M. Martinelli]

My .001 cents....
You can tell a bad design from a good one by the attention the designer puts on such things....a good designer should ALWAYS work with a "worst case" design philosophy IMHO and, though it's true that an amp can work without these features, they're always a good thing to be found, all the more so because they add protection at a very low cost, as Enzo wisely pointed out.
Regards
Bob

[rockon1]

Thanks guys for clarifying this topic fior me. Bob

[R.G.]

In the Workhorse amps, I ditched flyback/clamp diodes in favor of a pair of high voltage MOVs across the primary. Clamp diodes force the flyback energy to be dissipated in the winding as heat. MOVs eat the energy directly and I think put less stress on the windings.

I don't know if that's the reason, but I could never get a Workhorse to oscillate when operated with no load, and I did try.

If you do this, be sure the MOV voltage rating is at least twice the maximum B+ you'll ever have under high line and with lowest output tube bias. Preferably about 2.5-3x the max B+ to ensure that the MOVs don't get enaged under maximum signal conditions.