VJ combo, 4 ohm jack, help to find cabinet?

[blitzkrieg bop]

I notice that the speaker of my valve junior combo is plugged into a 4 ohm jack. So it seems that I can plug it into any 4 ohm speaker cabinet, however, I can't seem to find many (either 4x12's or bass cabs). Does anybody know where I can find some 4ohm speaker cabs, preferably 1x12 or 2x12, or anything really, though a 4x12 is a bit big for what I use it for?

[b3_john]

If you don't like 4 ohm loads (I do), there are mods out there specifically for the VJ combo to tap an 8 ohm or 16 ohm jack. My VJ head has separate 4/8/16 ohm output jacks. Check 18watt.com or google "valve junior mods". Or you could rewire two 8 ohm 12s in parallel to get a 4 ohm load.

 

Now I don't advocate you plug that 4 ohm output into a higher ohm cabinet but I've been known to do it (as recently as this afternoon :wink:). As a rule I would never do this the other direction (and definitely never with either a 2 ohm output or load).

 

Then again I'd think nothing of running a 30 watt amp "feathered just a bit" into a 20 watt speaker trying to get a certain sound, so clearly I can't be trusted with this stuff.

 

John

[blitzkrieg bop]

Thanks for the input!

 

Can I hook up an 8 ohm speaker to the 4 ohm jack, or is this bad? I'm not too sure how speaker impedence works. Will this ruin the speaker or amp?

[Junior 1]

You can use an 8 ohm speaker with a 4 ohm load. You can plug in a cab with a higher ohm rating, but never a lower rating. I've used a 12 inch 8 ohm cab with my VJ combo for a while. You lose very little power with a simple amp like the VJ combo, when you don't match the ohms properly.

[b3_john]

Its "bad" and "you should never do that". There, we got that out of the way. I do it all the time, but for me its a calculated risk. I'm not responsible if you blow your amp. YMMV.

 

As I said above, I always (well, almost always) use a larger speaker load than the amps rated output load. So an 8 ohm speaker plugged into the 4 ohm amp output jack, (again generally speaking) NOT the other way around. More load than than the amp expects, not less. And NEVER do this with a 2 ohm load or output unless you know what you are doing and why.

 

Also, I think the mod to add an 8 ohm output jack for that amp is very simple. Edit: I was almost right, there is a very simple mod to add a SPDT (Single Pole, Double Throw) switch to change the output impedance of your Valve Jr. combo's existing 4 ohm output jack to either 4 ohm or 8 ohm. This is because the output transformer is already tapped for 4 or 8 ohms (and 16 ohms too, though that might require installing a different switch and/or another jack). See Epiphone Valve Junior mods page [euthymia.org] and scroll down to the "Output Impedance Switch" section at the bottom. If you aren't electronics savvy or don't understand how or why you'd want to discharge a capacitor in a high voltage circuit before working on it, any competent tech with a drill could do this mod for you in about 5 minutes.

 

John

[Zuben]

John,

 

If you have a 2X12 cab, open it up, rewire it. Series and you'll have a 4 ohm load. First check the ohm of each speaker. Many series 8 ohm cabs are 16 ohm speakers or 4 ohm speaker in parallel. Most will be 8 ohm or 4 ohm. Altec did make a lot of 16 ohm horns.

 

I use to build a stage PA speaker with 2 15" a horn and 8 8" full range that I crossed over to midrange. It was one of my most popular cabinets. Some are still in use.

 

I could wire them for 8 or 4 ohm.

 

Get a good Ohm meter and play around with different wiring.

 

Peace

[bennyguitfiddle]

careful ohm impedence is different than

ohms resistance and you can't measure it

with a basic multi meter.

 

But the formulas are the same concept

 

if you wire series you add the ohms

2-8 ohm speakers in series is 16 ohms

 

parallel is a different formula

_________1___________

1/X + 1/X + 1/X + 1/X

 

so lets say you have 4-16 ohm speaker

 

X=16 ohms 1/16 = 0.0625

0.0625

0.0625

0.0625

0.0625

+__________

0.2500

 

1/0.25 = 4 ohms

[b3_john]

Zub, yes, I personally prefer a pair of 8 ohm vintage 12s (20 watt) in parallel to get a 4 ohm load (provided the amp will support it). I've never been a fan of two 12s in series to get 16 ohms because I think it affects the tone, but I'd do it if I had to (like an amp that didn't have a 2 ohm or 4 ohm output).

 

John