How do I find out how many Ohms my amp is?

[my_shadow22]

I have a old Gibson GA-8 that I thought was 16ohm. It has a vintage 10 inch Jensen speaker in the cab but I've been using a Celestion Blue with it recently that sounds killer! Anyway, I wanted to wire them together and I just tested the ohms on the Jensen and it tests as 4 ohms. Not that I'm freaked out or anything but I want to find out what the deal is. Can I do this with a multimeter?

p.s.-If you can answer this, you probably know the answer to this too, is it ok to wire speakers of different impedances together? I wired the Jensen and Blue together and they sounded great with the Gibson. I tried it with a Fender Vibrolux and when I flipped the standby switch there was all this crackling and static. What's up? THANKS!

[John Phillips]

It's quite difficult to determine the correct output impedance if there's no way of telling from the amp itself.

You need to measure the maximum power output into different load impedances - the one that produces the most power is the correct match. You need loads of 4, 8 and 16 ohms and a multimeter that reads AC voltage. Crank up the amp full while measuring the voltage across the load. Power = voltage-squared divided by impedance. Start with the 8-ohm load and calculate the power. Then do the same with the 4 - if you get more power, it's a 4-ohm amp. If you get less, try the 16. In each case, run the amp for the minimum time to get a good reading.

If in doubt, you can assume 8 ohms - since it is within a safe mismatch no matter what the amp is (unless it's 2 ohms, extremely unlikely for an amp like this). It's not a good idea to run a 4-ohm load from a 16-ohm amp, or the other way round.


Yes, it is OK to mix impedances, but you need to know a little more about what you're doing - the resulting impedance will not be a 'standard' value, but it will be within safe mismatch range from at least one impedance setting. Also, power distribution is not equal. In parallel, the speaker with the lower impedance draws more power; in series, the higher one.

eg - a 16 and an 8 ohm speaker together in parallel give a total impedance of 5.33 ohms (safe from either 4 or 8 ohm outputs) and the 8 ohm speaker takes 2/3 of the total power.


The crackling from the Vibrolux was probably not caused by anything to do with the speakers, unless there was a bad contact in the speaker jacks. Even at that, I would expect just an intermittent signal, not crackling. Possibly something else has been disturbed. Does it do it with the normal speakers connected now?

[my_shadow22]

Awesome, thank you very much for the reply!!! I'm going to see what it is tonight.

The mismatched speakers still sound great with the Gibson but still crackle with the Fender? There is no problem with the original speakers. I'm going to re-wire the speakers to see if that helps. Thanks again!