Determining Speaker Impedance on customized Carvin 4x12 Cabine

[phenom603]

Background:
I have a Galien Krueger GK 250 ML amp that I purchased about 10 years ago. It came with a Carvin V412-M cabinet that had been customized to work as the external speaker for the GK amp. In case you are not familiar, the GK 250 ML has internal speakers [for practice] and supports a stereo external connection [typically 2 16 ohm 4 x 12 cabinets].

Note: This Carvin cabinet cane with a single speaker jack (1/4 inch phono). The customization was an additional speaker jack allowing the cabinet to connect to the GK external stereo out which are 2 separate connectors. According to the GK 250 user guide, the external speakers must be rated minimum 16 ohm per side. I have used this setup for over 10 years with no issues.

Challenge:
I recently got a custom 50 watt tube amp built for me and want to use the Carvin cabinet. The custom amp has a selector for 4, 8 or 16 ohm output. The amp builder advised me to measure the cabinet speaker impedance before connecting since he has no idea about what mods were made to the cabinet and a mismatch could damage the amp and/or speakers. He also was a bit perplexed as to how a 4x12 cabinet could be configured to come up with 2 separate 16 ohm sides.

This cabinet is front load and disassembling is a last resort to check the mods. I tried measuring the speaker resistance using a digital ohm meter [DC setting, 200 ohm range] and each speaker is reading approximately 8 ohms, HOWEVER, the reading jumps all over the place the longer I keep the leads on the tip and ring of the cable connected to each speaker jack on the cab.

I understand that speakers in general complicate resistance measure due to the coil and that I am really trying to determine impedance. I guess my initial conclusion is 8 ohms a side, BUT this just does not jive with the GK spec of minimum 16 ohms per side and as mentioned, the cabinet has worked great with the GK amp.

It is also my understanding that connecting lower impedance speakers can damage an amp so I am apprehensive about connecting this cabinet to my new amp until I am more confident about the actual cabinet speaker impedance. And, to further complicate matters, the guy who built the amp for me, recently passed away unexpectedly so I have nobody to turn to with questions.

I appreciate any suggestions advice, etc.

Also, does anyone know of any cheap speaker impedance measuring devices that take the whole coil situation into consideration to get an accurate impedance measurement and can connect to my cabinet jack? Seems like something that should be on Shark Tank. Lol.

Apologies for the long post, but trying to answer potential questions before they get asked.

[The Dude]

My recollection is that the GK 250ML will safely do an 8 ohm TOTAL load. The reason they say 16 ohm in the manual is for the guy who uses both the internal and external speakers simultaneously. I have no idea why you would do that. If you have a 2 or 4 12" box attached, you would be best to turn off the internal speakers which can be done simply with the switch on the back of the amp.

I'll surmise that the cabinet you have originally consisted of the typical series parallel combination of 4) 16 ohm speakers and that it was rewired for 2 separate 8 ohm loads (each 2 parallel 16 ohm speakers). This is fine if you are not running the internal speakers of the GK at the same time.

As to running it with the tube amp, I don't know what's available on the amp for outputs, but I'm pretty sure your cabinet is 2) 8 ohm loads, so wire accordingly.

[52 Bill]

Yes, the GK 250ML will run safely into an 8 ohm load. The original version had 16 ohm internal speakers and allowed for external 16 ohm speakers. The series II version had an internal speaker on/off switch which allowed using external 8 ohm loads by turning off the internal speakers.

When measuring speakers with an ohmmeter, the readings of dc resistance will closely approximate the impedance rating, but will usually be slightly lower. If the meter reading keeps changing, check to see that the speaker cone is not moving. If the cone moves from air currents etc., there will be a small voltage created from the movement of the coil in the magnetic field, which will cause the resistance reading to change.

Most 4-12 cabinets that I've seen are 8 ohms. Series/parallel wiring of 4-8 ohm speakers. If the cabinet was modified to run stereo then the two halves could have been wired as 2-8 ohm speakers in series for a 16 ohm load.

[phenom603]

Thanks guys, you have been really helpful.
Just a bit more......

FYI: It is a Series II

Here's more information.

I jut looked at the back of the GK and your information shed some light on something that had always confused me.
The switch on the back says
[ON 16 OHMS MIN EXT. SPKR] [INTERNAL SPEAKERS] [OFF 8 OHM MIN EXT. SPKR]

I thought it was supposed to be a 3 way switch.
I guess the [Internal Speakers] text immediately below the center of the switch threw me off.
I see now that that is just the switch name essentially.

This explains my confusion that the switch was not a 3 way because it only would go left or right, no middle position. Now I see that Left leaves the internal speakers on and requires a 16 Ohm external [internal in parallel with external] and Right turns the internal speakers off off and requires 8 ohm external [external only]. Duh. That only took 10 years to figure out.

My excuse is that I always left it as is and never touched the switch. Also, I think the manual I downloaded was for the Series I.

photo of switch below.


The back of my new Amp has an output selector that has 4, 8 and 16.

So is the consensus that my cabinet has two 8 ohm channels?

If so, With two 8 ohm speakers, would that be a 4 ohm parallel or would it be just straight 8 ohms.
I am not sure if the selector specifies total load or per channel load.
This is modeled after a Marshall JCM 800. Not sure if that might be a clue as to how outputs would typically be designed.

thanks

[The Dude]

With the tube amp, you'll be running mono, so two 8 ohm loads in parallel would be 4 ohms. I'm assuming you don't want to rewire the cabinet so that you can still use it with the GK if you chose. If that's the case you'll need to run 2 separate speaker cables to the cabinet and set the amp for a 4 ohm load.

BTW, welcome to the place!

[phenom603]

Thanks Dude, literally. lol

OK, I hate to be slow, but I get the 4 ohm load for two parallel 8 ohms, but now the GK switch information confuses me concerning the [Internal speakers OFF] position. It indicates Minimum 8 Ohm Speaker so wouldn't that be an 8 Ohm load? I've always run the GK with the switch in this position and both cables connected to the cabinet.

I am all ears....

And thanks for welcoming me as well.

[The Dude]

I'm unclear about what you're unclear about.

I think what you're not getting (maybe?) is that the GK is a stereo amp so we are talking about 2 independent loads on 2 separate amps....unlike the tube amp, which is a mono amp. Does that help?

[phenom603]

The mono versus stereo comment is helpful.

So each output connection on the GK is is a separate amp circuit and looking for an 8 ohm load on each.
Whereas, the mono tube amp single output circuit is sharing the load even though there are 2 cables coming out, therefor, the resulting load is two 8 ohms in a parallel which comes to 4 ohms

[The Dude]

Yep, you got it!

[phenom603]

Thank you very much for all the explanations.
A light just went on.
Reminds me of my college days.
Have a great night.
I'm gonna connect the cabinet tomorrow night and crank it!!!

[loudthud]

This cabinet is front load and disassembling is a last resort to check the mods. I tried measuring the speaker resistance using a digital ohm meter [DC setting, 200 ohm range] and each speaker is reading approximately 8 ohms, HOWEVER, the reading jumps all over the place the longer I keep the leads on the tip and ring of the cable connected to each speaker jack on the cab.

A speaker can also act as a microphone so any sound in the room where you are trying to make a measurement will cause the reading to jump around.

[phenom603]

that makes sense loudthud.
thanks