Hi, mates. I recently got an EKO all-tube amplifier for guitar, 50 watts. As you may know, this is an old vintage Italian amp, which has 2 clean channels and no distortion, but the clean channels sound beautiful.

Here's my concern: It doesn't say the speaker OHM value to match a cabinet.

I have a Marshall 4x10 cabinet rated at 16 OHMs and it works ok, but it belongs to another amp. I have a 4x8 Hartke Piggyback guitar cabinet that runs at 4 OHMs. How do I gauge the OHM rating of an amp out jack? I have a multi tester, will that do? How to I work it?

Please, help me out.

Any ideas?

 

Here, maybe this will help you...

http://www.sengpielaudio.com/calculator-InputOutputImpedance.htm

http://www.sengpielaudio.com/calculator-AmplifierLoudspeakerAndOhm.htm

 

Thanks a lot, bro. There seems to be no shortcut to nailing impedance and ohms. However, I found this article:

http://www.prestonelectronics.com/audio/Impedance.htm

And the long story short is:

Wrong impedance might undermine the output and quality sound of the amplifier until

a. Too low the nominal impedance on a solid-state amp will cause it to overheat (It's happened to me before).
b. Too high the nominal impedance on a tube amp will blow the transformer and/or the power tubes.

Am I right?

Based on this principle, if my 16 ohm cabinet worked fine on my all-tube EKO Amp, running a 4 ohm cab will not blow it. At worst, it will sound weak.

Am I on the right track?

 

Not that I am correct ( I am not deep into schematics and deeper engineering)

but

14V drop across 4 ohms is 3.5A V=IR

P = IV so power = ~ 49 W

to that puts it right there.



If you go with 8 ohms

14V drop across 8 ohms is 1.75A V=IR

P = IV so power = ~ 24.5 W ( WRONG!!!)


So the correct OHMS is 4


Again I am just a simpleton when it comes to electronics ( basic electronics skills here)

 

Could you specify the exact reference of your amp?

 

That's the thing. No schematics or anything. It is an old amp that has been restored. Here's an image of it placed between the 16 ohm cab and the Marshall head.

 

There is an Italian guy whose net name is Stefano267 you can find him at different DIY forums.
He's done a remarkable restauration of this exact same amp.
You can find his article here: http://www.guitariste.com/forums/amplification,restoration-of-an-eko-viscount-reverb-amplifier,313977,30.html
Look for the word ohm to fond the resistance/impedence of the speaker, or you could send home a PM.

Or you could simply contact support at Eko : http://www.ekoguitars.it/en/cont/4850/contact-us

 

Thanks a lot, Olymoon. This is a lot of help. Thank you all guys for your support.

 

You're welcome, I hope that you'll solve the problem.

 

From the link

"and at approximately 10% THD giving 30W/4Ohm, I consider this its maximum power output."

If you can't afford an Oscilloscope then you need to know other numbers such as the wattage then some very basic math and you arrive at 49 watts= 4 OHM

So the answer was 4 OHM like I originaly posted even before checking out Stefano267 amp project.


So technically the answer is 4 OHM, however when is anything right when it comes to music? LOL

Look at Eddie Van Halen for example, hooking up a variac transformer to his amp which he turned down to around 90 volts instead which then starved the tubes thus driving up the preamp and getting the famous brown sound.


There really are no rules when it comes to sound and music. Then only rule that applies is this one... Can it be recorded properly and still sound the same?

 

dan you're spot on.
like you i don't know heaps about amplifier internals but as you said increasing resistance (less ohm) increases current (assuming stable voltage)
and matching speaker resistance is less a specific value and more a range of safe values.
pushing these limits can have awesome results, going to far can end in tears.

mercury, depending on the resistance of each speaker in your marshall cab, you may be able to rewire to increase the resistance.