Output safety resistor

[Aurora]

I'm working on a appx 80W amp....On some occations, I've seen references to a sort of output "safety"resistor, - a 60-70 ohms resistor across the output, as a sort of safeguard against no load situations... what's your thouhgts on this??

[tubeswell]

Handy for those 'in case' moments (if someone forgets to plug the speaker in or the speaker coil goes open).

A permanent resistance across the secondary doesn't do any harm if the resistor is sufficiently high resistance to not affect the reflected load, nor divert output power away from the speaker (as a permanent resistor would be in parallel with the speaker load).

If you're worried about the purity of it, leave the resistor out, or wire up a switching jack so the resistor is only connected when a speaker isn't plugged in

[TUBEDUDE]

But be aware using the switched jack will not protect anything if the disconnect happens at the speaker end, or if you have an open voice coil.

[pdf64]

I think it very beneficial to hardwire a safety resistor across the OT secondary, typical value in commercial amps seems to be about 25x the nominal load, eg Traynor YBA1 http://www.0rigami.com/vb/traynor_bassm ... _yba1b.pdf Vox AC50 https://el34world.com/charts/Schematics ... ac50_1.pdf
Too low a value will tend to damp speaker bass resonance a bit.
Consider that the <1kohm feedback potential divider on many Fender BF/SF models may act as such a safety resistor, and help to mitigate an open secondary.

[Stevem]

I use a 250 ohm resistor so it's well above loading down the amp any in normal use.