SS impedance question

jdbwales

Me again, sorry to bother you clever tech types. I was just wondering as I'm looking at backup SS amps for gigging, what sort of volume drop an amp (specifically Fender FM100h) rated at 'minimum 4 ohms' would have going into a 16ohm 2X12 cab?

In theory I think it should be safe enough as it's an upward mismatch, but it's a 100 watt SS head and the only thing that could potentially put me off buying is a painfully low output from the mismatch - would this be an issue or would we still be looking at a negligible difference in output?

Thank you!

ICBM

It's safe, but you will get a fairly big volume drop compared to the full 100W. A typical Class AB/linear power supply 100W/4-ohm amp will put out about 35W into 16 ohms. On top of that a "35W" solid-state amp will sound a lot less loud than an equivalent valve amp, probably about the same as a 15W valve amp. So if your main valve amp is more powerful than that and you use most of the available power, you may struggle to get the same volume with the Fender. It's likely to be loud enough to get away with unless you play at really high volume though.

What you could do is fit the cab with a switch to run it at 4 ohms - speakers in parallel instead of series.

jdbwales

Thanks ICBM, I'll likely go for it - it's just there for a kind of last ditch emergency scenario anyway. If I was to install a cab switch (sounds easy enough), what part would I need? Some kind of DPDT I assume?

ICBM

jdbwales said:

If I was to install a cab switch (sounds easy enough), what part would I need? Some kind of DPDT I assume?

Yes, a heavy-duty DPDT. I wouldn't use a mini one - while they're technically rated for enough voltage and current they can be fragile, and the last thing you want is a broken switch leaving your cabinet an open circuit.

jdbwales

http://www.maplin.co.uk/p/10a-toggle-switch-dpdt-f-jk30h

Does that look like the right sort of thing or would a flatter switch be better? I imagine it will be for sake of not getting knocked in transport etc.

If so I just need to find a wiring diagram and I'll be good to go - makes sense to install one so I can use the same cab, hauling around another one kind of defeats the purpose...

ICBM

That's the right general type, although you're right - if you can find one with a shorter toggle it will better. If you fit it in another recessed jack plate it shouldn't stick out too far.

The wiring is quite straightforward but I'll take a small bet that you can't find a diagram online with the best scheme! The common way of doing it is this:

https://damacleod.files.wordpress.com/2013/05/impedance-switching.png

But this is inherently unreliable because it runs the series connection through *both* switch poles, so a fault in *either* will leave the cab open-circuit.

What to do is replace the lower link wire with two, in an 'x' between the four lower terminals (insulated from each other). That puts the two switch poles in parallel, so it will take failure in both at the same time to cause a problem. Why this is not more commonly known, I don't know.

jdbwales

Thank you very much as ever ICBM - bloody hero. Much appreciated.