Flyback Diodes

[Wilder Amplification]

I know some amps have them from each side of the OT to ground, but wouldn't placing them backwards across each 1/2 of the OT primary work also (from each outer winding to the center tap)? If so, would this way work better than the "plate to ground" flyback diodes? Any input on this would be much appreciated.

[Steve Conner]

No. One diode would short one half-cycle of the audio, the other diode would short the other half-cycle, the result would be no sound and plenty of smoke.

[Wilder Amplification]

Ooh why didn't I think of that!? That's right because one plate swings higher than B+ for 1/2 the cycle. Duh! lol

Sorry...just now having my coffee this morning.

[defaced]

There was a really good these on OT protection a few months ago that had some good suggestions to protect against the various failure modes of an OT. Would be worth the search.

[R.G.]

Steve is correct, as usual. Can't put the diodes on the B+ side. Those primaries swing both directions relative to B+.

Those flyback diodes work by clamping the down-going side of the transformer winding to ground. The power tubes can only pull down, from B+ to ground. When you try to interrupt this current, the inductive kickback flips this voltage upwards, above B+, to try to force the tube to accept the same current it was carrying. The voltage flys upwards until the current can be satisfied. This also makes the voltage on the non-current-carrying half head downwards from B+, because the two primary halves work in see-saw fashion.

Power tubes in Class AB normally experience something a bit less than twice the B+ voltage. When one tube pulls down the voltage across its half-primary, the other half primary swings up by an equal amount, but above B+. The inactive tube can see nearly twice B+; it's lower by the "saturation" voltage across the active device, and that can be as low as 50V for some power tubes.

On a current-stop transient the transformer half-primary that was being pulled down flips in voltage above B+; so does the inactive half-primary, so the inactive side heads for ground. Now the clamp diodes come it. As the inductive kickback forces one side above two times B+, it also forces the other side below ground. When that happens, the flyback diodes conduct and clamp the down-going half to ground, so there's no more than B+ across that side. By see-saw action, that also limits the up-going side to two times B+, not a lot more than happens in normal action.

What this action cannot protect against is the inductive kickback of the transformer's leakage inductance. Leakage inductance is by definition the "inductance" that is unique to one half-primary, not the other. The leakage inductance component of the flyback voltage cannot be clamped by the other half primary. On the other hand, the energy carried in the leakage is enormously smaller, so even an arc forming will not cause much burning.

The inability to clamp leakage inductance flyback spikes is why I like a chain of high voltage MOVs across the primary. If their voltage is more than twice B+, they never conduct in normal operation, but when they are tripped over, they eat any discharge energy including leakage inductance energy. Diode clamps are nice, but the leakage inductance flyback may degrade them over time and eventually short them. This is why those diode clamps are usually strings of three 1N4007s - you have two chances to not die. One 4007 may die at something just over 1KV, two gives pretty certain normal operation, and three gives you an active spare.

[Wilder Amplification]

Steve is correct, as usual. Can't put the diodes on the B+ side. Those primaries swing both directions relative to B+.

Those flyback diodes work by clamping the down-going side of the transformer winding to ground. The power tubes can only pull down, from B+ to ground. When you try to interrupt this current, the inductive kickback flips this voltage upwards, above B+, to try to force the tube to accept the same current it was carrying. The voltage flys upwards until the current can be satisfied. This also makes the voltage on the non-current-carrying half head downwards from B+, because the two primary halves work in see-saw fashion.

Power tubes in Class AB normally experience something a bit less than twice the B+ voltage. When one tube pulls down the voltage across its half-primary, the other half primary swings up by an equal amount, but above B+. The inactive tube can see nearly twice B+; it's lower by the "saturation" voltage across the active device, and that can be as low as 50V for some power tubes.

On a current-stop transient the transformer half-primary that was being pulled down flips in voltage above B+; so does the inactive half-primary, so the inactive side heads for ground. Now the clamp diodes come it. As the inductive kickback forces one side above two times B+, it also forces the other side below ground. When that happens, the flyback diodes conduct and clamp the down-going half to ground, so there's no more than B+ across that side. By see-saw action, that also limits the up-going side to two times B+, not a lot more than happens in normal action.

What this action cannot protect against is the inductive kickback of the transformer's leakage inductance. Leakage inductance is by definition the "inductance" that is unique to one half-primary, not the other. The leakage inductance component of the flyback voltage cannot be clamped by the other half primary. On the other hand, the energy carried in the leakage is enormously smaller, so even an arc forming will not cause much burning.

The inability to clamp leakage inductance flyback spikes is why I like a chain of high voltage MOVs across the primary. If their voltage is more than twice B+, they never conduct in normal operation, but when they are tripped over, they eat any discharge energy including leakage inductance energy. Diode clamps are nice, but the leakage inductance flyback may degrade them over time and eventually short them. This is why those diode clamps are usually strings of three 1N4007s - you have two chances to not die. One 4007 may die at something just over 1KV, two gives pretty certain normal operation, and three gives you an active spare.

So essentially what you're saying is that there are about 9 phases of diode operation going on here correct?

[R.G.]

So essentially what you're saying is that there are about 9 phases of diode operation going on here correct?

Actually, I count 15 phases. There's new moon, - oh, wait! that's not right! 8-)

[Billy R aka DynaFreak]

RG: excellent description of something for which I had only had a partial understanding! How do you select the correct parts for the MOV protection you describe?

[Steve Conner]

Yes, there are some fairly complicated things going on. That's the thing about tube amps in general, most of the components do about 5 different things. When you go tinkering with them, like Garrett Hardin said, you can never do just one thing.

The MOVs were covered in that other thread that was mentioned above. I think I remember RG saying that they have capacitance that affects the top end a little.

Nowadays, TVS diodes are available as an alternative to MOVs. You can get unidirectional ones that are like beefed-up zeners, these could be strung from the plates to ground like the old flyback diodes, except they'd also protect against leakage inductance kickbacks.

You also get bidirectional ones that could be substituted directly for MOVs the way RG suggests. I've used the 1.5KE400CA before, a 400 volt part. 400V is the highest voltage they make, and you need a rating about twice your B+, so you'd have to string them in series.

[R.G.]

RG: excellent description of something for which I had only had a partial understanding! How do you select the correct parts for the MOV protection you describe?

Steve is correct. You pick devices which will NOT conduct until over 2 times B+. Under twice B+, they conduct on signal peaks often and die, perhaps taking something else with them. So give yourself a bit more than 2 times B+ at high line voltage, considering what happens if someone puts in solid state rectifiers and raises B+ some more. In general, you don't have to get super-close to B+, as the tubes aren't going to be damaged. It's the KV-range spikes you're trying to catch.

Specifying disaster prevention devices is always a crap shoot. Before you can do it right, you have to define the disaster, and crystal-ball technology isn't all that good these days. The key provisions to look for are (1) lowest possible capacitance and (2) largest possible power or energy rating. These two things directly contradict one another, so picking a device is best done by going cross-eyed over descriptions in the catalog, then picking a middle of the road choice with enough voltage. If it's too much capacitance and affects your treble too much, go back for a smaller one. I wish I could bound that more closely, but to do that, I'd have to know the operating conditions in your amp, the primary and leakage inductances of the OT, and whether or not the transients to be eaten are single events or caused by some quirk at signal frequency. Notice that this process (whip in what you hope is enough protection) is the same process followed to "design" catch diodes. 8-)

Transients at signal frequency are bad because high frequencies make the protectors eat many transients, and the average power burns them out. MOVs and transzorbs, etc. can eat a huge power surge if it happens once, but if it's repetitive, they only last until the average power overheats them. That ought to be rare, but then, we are worrying about disasters. 8-|

[quasi-mojo]

I've used the 1.5KE400CA before, a 400 volt part. 400V is the highest voltage they make, and you need a rating about twice your B+, so you'd have to string them in series.

FYI, LittleFuse makes a 600v part.