Should I use a shorting speaker jack for saftey?

[Gingertube]

For open circuit speaker protection I use "tether" resistors on each primary connection of the output tranny. 360K 4 Watts (actually 2 x 180K 2 Watt in series) from the primary connection to 0V on each of the push pull sides.

That way what is done on the secondary side is entirely immaterial.

Many ways to skin the cat.

Cheers,
Ian

[Pedro Vecino]

As a curiosity, some new Rivera models uses a partial protection method. When connecting the speaker jack, the power tubes cathodes are assigned to ground. Without it, remain disconnected.
I say "partial" because if on the other side nothing is connected this protection does not work.

[ReadyTeddy]

Is there anything wrong with using diodes from each plate to ground so that the diodes turn on if the plate voltage goes below ground, but when the plate is above ground, the diodes do not affect the circuit in any material way? Not only will the diodes prevent the plate voltage from going negative, but when the diode is on, it draws current which helps to clamp the other end of the primary winding, which should help the other plate from seeing too high a voltage. I've seen them used in some Fender schematics, and it struck me as a good solution to the problem.

[trobbins]

The diodes are just one way of the many available that have been used to assist over-voltage protection of the OT - they need a pretty high PIV (ie. usually series 1N4007) as they are prone to failing themselves, but there in lies a possible weakness that they aren't actually protecting that primary half-winding.

[ReadyTeddy]

The diodes are just one way of the many available that have been used to assist over-voltage protection of the OT - they need a pretty high PIV (ie. usually series 1N4007) as they are prone to failing themselves, but there in lies a possible weakness that they aren't actually protecting that primary half-winding.

To the extent that the windings are well coupled, i.e., the leakage inductance is small, then the other half winding will be protected. If the diodes are prone to failing, then they've been improperly specified--any part has the capacity to fail or be very reliable depending on how well the part has been matched to its application. There are any number of diodes that would be very reliable in this application.

[trobbins]

Sorry, the comment was meant to represent an historical view, as there have been recollections by a few about such diodes failing. That could have been poor design at the time - using diodes with too low a PIV - perhaps from an expectation of only seeing a certain voltage spike level (eg. 1-1.5kV spikes).

The leap of faith is when energy is in one half-winding, and clamping the other half winding is being used as the protection mechanism for the first half-winding. Imho it is better to clamp across each half winding, as that avoids the leap of faith.

[mhuss]

I think the worst case is an intermittent cable that keeps opening and reconnecting. This is where the amp side protections (resistors or diodes) are better than a closed circuit jack. I use the 10x resistor on the output jack method when building amps, but the other methods mentioned also work well.