Tube Amp Archive

While browsing Ebay tonight, I noticed a pair of 8 ohm Alnico EV SRO 12" speakers...The pics show each 8ohm speaker hooked up to a meter, one measuring 8.6 ohms, the other 8.9...

I have a pair of the same speakers, and mine show 5.7 and 5.9...

I was under the impression that an 8 ohm speaker should read between 5.5-6.0 ...In fact I've never had an 8 ohm speaker measure above 6.5...

Do you guys think that the 8.6 & 8.9 ohm readings indicate a problem?

Maybe they had an impedance meter not a DMM?

Didn't think of that...just went back and looked, and it appears to be a DMM set to read ohms...here's the link, he wants an awful lot of money for 'em.

http://www.ebay.com/itm/Electro-Voice-S ... 4d1b0658b9

Circuit Specialists CS12010 is an ordinary ($30) DMM, so more than likely if there is a problem it's with the measurement.

Cheap meters are not compensated for the series resistance added by: wires / selector switch contacts / probe tips which may not make perfect contact.

To boot, he's not measuring straight at the speaker terminals but somewhere out of the picture the red/black wires are connected to green crocodile clips.

All adds up, never down.

The proper measuring method is to short the meter probes, take a reading (which will NOT be 0 by any means) , then measure the speaker and substract bthe first value.

I have often measured up to 1.2 ohms "loss" in cheap meters, specially if old, copper oxidation adds to the problem.

Cheap meters out of the box measure between 0.3 and 0.6 ohms, go figure.

There's also another possibility: that those speakers have been reconed sometime in their 30/40 years life, doubly so because the oldest ones did not stand that much, just 60W RMS.

A reconer might have used an aftermarket coil, which exist in up to 7.5 ohms DCR, or a handwound one with slightly thinner wire than needed.

I bought an expensive-ish, true RMS meter (Meterman 38XR). It's great at measuring everything EXCEPT estimating the resistance of coils. When trying to measure speaker resistance, the value never settles. Same with the resistance of OT primary coils. I have to use one of my cheap meters for these jobs (as I don't have an inductance meter).

Why?

xtian wrote:I bought an expensive-ish, true RMS meter (Meterman 38XR). It's great at measuring everything EXCEPT estimating the resistance of coils. When trying to measure speaker resistance, the value never settles. Same with the resistance of OT primary coils. I have to use one of my cheap meters for these jobs (as I don't have an inductance meter).

Why?

I wonder if the auto-range feature has trouble with inductance? Can you lock the range?

Robert

For some things, you really just need a Fluke! Even something like this, if it's working. http://www.ebay.com/itm/Fluke-8600A-Dig ... 4d2496d7fe AFAIK it takes a regular IEC cord.

I have a Tektronix CDM250 Digital Multimeter picked up on ebay university lab surplus, works great.

Phil_S wrote:For some things, you really just need a Fluke!

+1 - Buy the good stuff - they are accurate and tough.

Advertisements that never made it: "If it works, it's a Fluke!"

The auto range on my Fluke 89 had a problem with measuring the resistance of an inductive part one time (it may have been a pickup).
Just needed to put it on to manual range selection.

Be careful with these...the majority of these old "SRO's" suffered from
the speaker's frame failing...the relatively weak "spokes" would crack
and break under the wieght of that magnet.....That's among the reasons why they went to the "EVM" style cast frames(aside from Alnico being almost unavialable). They do sound great , though and are very efficient.

Looks like one speaker is 8.6, but the other i am not seeing the decimal and it looks like 89. ????

Wow, that cracking speaker frame problem is good to know, never would have thought of that happening...looking at one of mine now and holding it, they are stupid heavy.... The idea of hauling a Twin Reverb around loaded with a pair of these is comical, I imagine most of the frame cracks happening when the guitarist finally reaches his destination after carrying it, and sets it down hard in disgust while holding on to his new hernia...The cost of tone is high!

xtian wrote:I bought an expensive-ish, true RMS meter (Meterman 38XR). It's great at measuring everything EXCEPT estimating the resistance of coils. When trying to measure speaker resistance, the value never settles. Same with the resistance of OT primary coils. I have to use one of my cheap meters for these jobs (as I don't have an inductance meter).

Why?

I have a Meterman 37XR.

Great meter!