minimum ohm ratings plus or minus ?

Jonathan Mitguard · Post by **Jonathan Mitguard** » 11 Apr 2010 7:50 am

Lately I've been using a Princeton reverb II with a stock 12. I have historically used JBL K 130s and have two of them. I want to try a 15 with the Princeton They are rated at 4 OHMS. Of course the Princeton has a minimum of 8 ohms. I know I don't want to plug a 4 OHM into this tube amp but. I measures the impedance of the speakers and they appear to be closer to 7 OHMS. Is this in an acceptable range for the Princeton. Could I just add a resistor in the line to make the desired OHM rating?

Post by **Dave Mudgett** » 11 Apr 2010 8:07 am

If that 7 Ohms you measured was from an Ohmmeter reading, you measured the DC resistance, not the complex impedance. A speaker with a nominal 8 Ohm impedance typically has a DC resistance of about 6-7 Ohms, so what you have is almost certainly an 8 Ohm speaker. The complex impedance includes capacitive and inductive reactance, and varies as a function of frequency.

I suggest you do not add any resistors - this would be a bad idea in any case. Up to a 100% mismatch in the lower impedance direction is generally considered safe for old blackface or silverface Fender amps because their output transformers were pretty robust. Using a too-high impedance speaker is more risky than using a too-low impedance speaker.

There's a concurrent thread about load impedances and running an extension cab from a Deluxe Reverb, where some of these issues were discussed in more detail - http://bb.steelguitarforum.com/viewtopic.php?t=181272

Jonathan Mitguard · Post by **Jonathan Mitguard** » 11 Apr 2010 9:52 am

Yes I just bought the multi meter yesterday and figured I could have potential to miss interpret readings. Both the JBLs say 4 OHMs on the back but both have been re-coned. Each time the technician said that JBL no longer made 4 OHM parts but that they would do their own rigging (my word)to make a 4 OHM load. It sounds like you think it would be OK to use one of the 15s even if it was 4 ohms. I don't typically play very loud. And thanks so much for responding quickly.

Post by **Dave Mudgett** » 11 Apr 2010 10:07 am

I'm not sure what 'rigging' they would do to turn an 8 Ohm speaker into a 4 Ohm speaker. Either they reconed it 8 Ohms or 4 Ohms. If it reads 7 Ohms DC Resistance, I'd say it's an 8 Ohm speaker.

If you used two 8 Ohm speakers in parallel, the total nominal impedance would be 4 Ohms. Perhaps that's what they were suggesting. Or maybe they just told you they were 'rigging' it to 4 Ohms, and just reconed it 8 Ohms with the recone parts they had.

As long as the output transformer is robust and in good shape, I think it should handle a 4 Ohm load OK, especially if you're not pushing the amp. Using a mismatched load either way in a tube amp with an output transformer reduces the output efficiency, so one tends to push such an amp harder to get the same output. But it sounds to me like your JBLs are 8 Ohms anyway.

Gary Jones · Post by **Gary Jones** » 11 Apr 2010 12:09 pm

The rule of thumb for measuring Speaker impedance by directly measuring DC resistance is to multiply by 1.3 , so an 8 ohm speaker should measure somewhere around 6.15 Ohms DC.

I have seen this question about using resistors to change speaker impedance before, and what most people don't take into account is that if you have a resistor in parallel with your speaker, the resistor will receive as much power as the speaker. So if you are pushing 40 Watts, the speaker will receive 20 Watts, and the resistor will receive 20 Watts. Not only are you throwing away half your power (that will be dissipated as heat from the resistor), but you need to use a resistor that can handle 20 Watts. That would have to be a pretty big resistor.

As the power of your amplifier goes up, the power that the resistor has to be able to handle also goes up, and of course, the more power you are wasting.

Using resistors to change speaker impedance is a really bad idea.

Gary Jones

Jonathan Mitguard · Post by **Jonathan Mitguard** » 11 Apr 2010 1:54 pm

That is great info Gary, The resistor is a bad idea. It didn't seem right anyway which is, of course, why I asked. Am I correct in assuming the coil in the speaker is where the impedance occurs? I also assume that is the part that JBL no longer supplied back then and the tech was going to use some other coil or make one. Apparently if he did it was an 8 OHM after all and I have been setting my amps for all these years to 4 ohms and aint blown nothing. I think I am pretty easy on my amps, though should I start a heavy metal band, perhaps I'll see some smokin' transformers.

Gary Jones · Post by **Gary Jones** » 11 Apr 2010 2:12 pm

Johnathan, this article will explain it better than I ever could.
http://www.transparentsound.com/measure ... ngdahl.pdf

It is not uncommon to find D-130's that have been re-coned to something other than their original impedance. It seems a lot of them were 16 Ohms originally, and that was not as desirable as the 8 or 4 Ohm varieties.

I have seen good re-coning jobs, and terrible jobs on D-130's, but I suppose the only real measure is how good they sound in any application.

Gary Jones