mismatched speaker impedance?

[dmartn149]

I have this little amp that I built out of an old reel to reel recorder. It has a single 6aq5 power tube, and the original speaker system measures 3.5 ohms resistance. I'm using a 4ohm speaker now, but I have another speaker I want to use. It is marked 8 ohms but measures 5.6 ohms resistance. So I'm wondering how risky is it to use a speaker with a higher impedance than the OT was designed for? and is there any way to tell if it is getting too stressed before it is ruined?
Thanks

[Enzo]

Before WHAT is stressed, the speaker? Not going to hurt the speaker. Not likely to hurt anything.

Speakers are measured in impedance, not resistance. The DC resistance you read with a meter will usually be a little lower than the rated impedance.

[dmartn149]

Thanks Enzo. I know the resistance will usually read a little lower than the impedance. I'm not worried about the speaker. I'm wondering if it will hurt my amp. I've read that a small mismatch should not hurt an amp, but I'm not sure if that applies to old amps as well as newer ones. I apologise for my ignorance, but the explanations I've found on line have been difficult for me to really understand.

[The Dude]

It's confusing because there are tech specs and then there is real world experience. The two often contradict each other. You could say there is no right answer or maybe 2 right answers. My experience is that a slight mismatch won't hurt anything. I would say you should be just fine. There are 50 year old tube amps around the world that have been run at the wrong impedance since they were built and are still working just fine.

[dmartn149]

Thanks. I thought it would be okay, but wasn't sure.

[Steve A.]

I suspect that much of the misinformation about speaker impedance mismatchs is due to the OT failures in many early Marshall heads which turned out to be caused by faulty impedance selector switches. The Fender amps from the 50's and 60's never had rotary impedance switches and their OTs survived all sorts of speaker mismatches, at least as long as the speaker cable on the heads was not accidentally disconnected with the amp cranked up....

Steve A.

[Enzo]

Agree, when I was on the road in the 1960s and even into the 70s, we never even thought about impedance. it only started to matter when the early solid state amps came along.

The early marshall amps had that facockta plug-in impedance connector. The socket for it got loose, they fell out, guys bent sanded off coathanger into shape and plugged that in. No wonder they failed.

I have a few of those plugs, NOS, if anyone needs one.

[J M Fahey]

The 3.5 ohms DCR speaker is nominal 4 ohms and the 5.6 DCR one is nominal 8 ohms, so you are using an 8 ohm speaker into a 4 ohm tap.
No big deal, it will kill nothing.

[dmartn149]

Thanks everyone. I thought that it would be okay, but figured it was better to ask a stupid question than make a stupid mistake.

Dan

[Jazz P Bass]

Hey. The only 'stupid questions' are the ones that were Not asked.

Rock On.