Can I plug Blues Jr. Into my 4x12 cab?

zosozep7

Silver Supporting Member
Messages
8,040
Hey all! I have a 4x12 cabinet left over from my B52 amp. Can I plug my Fender Blues Jr. into this cabinet? Will it sound ok?



:munch
 

guitargeezer

Silver Supporting Member
Messages
1,150
"Really? I always thought that amps could drive loads ABOVE their load without trouble, but not below.

Am I wrong? Is it the other way around?"



Trust me...or, rather, trust Billm. He is the most knowledgeable Blues Junior expert on the planet. Seriously.
 

dk123123dk

Member
Messages
3,885
I would stick with an 8 ohm speaker cab. You can risk it with a 4 ohm cab, or a 16 ohm cab if you want, but you would be stressing the transformer. Perhaps the transformer can handle the additional stress, but it can't be good for extended periods of time.

dk
 

strat56

Member
Messages
1,167
I think I read or heard somewhere that with solid state amps it was ok to go with higher resistance but with tubes it was ok to go lower. I could be wrong though, it's happened before.
 

Bottle_Rocket

Member
Messages
176
The lower the ohms of the cab the less resistance it offers. So an 8ohm amp run into 4 ohms will cause the amo to put out too much power for the amp to handle and may cause the tranny to blow up.
On the other hand an 8 ohm amp run into a 16 ohm load will lower the power the amp puts out. You wont blow the transformer but may prematurely burn up your power tubes.
 

GCDEF

Supporting Member
Messages
27,458
From the manual

M . SPEAKER CONNECTOR - This jack provides a connection for the internal speaker OR any quality 8–ohm external speaker cabinet that you choose.

The manual is pretty specific there. I'd believe it.
 

Billm

Member
Messages
574
OK, let me explain.

The output transformer's main job is to convert the impedance of the output tubes to the impedance of the speaker for maximum efficiency. When there's a mismatch, there's a reduction in efficiency. Most any tube amp can withstand a 100 percent mismatch, so an 8 ohm nominal rating can drive a 4 ohm load or a 16 without problems. One notable exception: don't try to go lower on amps with 2-ohm output.

When the impedance gets too high on a tube amp, you get the possibility of flyback--arcs that can damage the output transformer or the output tubes. The BJr and many other modern amps have anti-flyback diodes in the output circuit to short out these high-voltage pulses.

When the impedance gets too low, the tubes and output transformer run warm. But it takes a lot of current, like playing loud into a dead short, to damage either.

Fender used the stock Blues Junior chassis in the Two Tone, a big cabinet with a 10 and a 12, both 8 ohm speakers, connected in parallel, a 4 ohm load.

I drive a second 8 ohm cab all the time from my Blues Juniors. So do many of my customers.
 


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