What attenuator to get for my amp?

Don't get one that is 15W, you'll want at least 50W so you won't blow the attenuator.

Those amps get a lot of speaker breakup with the original speakers when played without an attenuator.

I played one once (6V6 version), but I'm not that familiar with those. Does it have a jack for the speaker? You might need some simple mods if it doesn't.

 

As @radiocaster said, the speaker breakup will be missed at lower volumes, no matter how high you crank the dial pre-attenuator.

As far as the hardware itself, I recommend a Two Notes Torpedo Captor. It's a flat -20db attenuator + reactive load box + 2 cab sims + DI. It also decouples the load the amp sees from the speaker load, so you can run any speaker with any amp using it. It's an excellent attenuator with a whole bunch of useful features added on.

They also released the Captor X at NAMM which has dual channel output, reverb and compressor or some such other effect, an IR loader and a whole bunch of other gadgets, if you really like the bells and whistles.

 

What's the budget?

At the low end of the scale are the Harley Benton unit from Thomann and the Behringer PS1. The Behringer is a 'tribute' to the next on the list, the Jet City Jettenuator.

These are basic L-Pad + resistor networks. They work very well indeed. Resistors tend not to have 'tone' of their own, but you do miss out on the pushback from the speaker coil and varying impedance with frequency a little. Fletcher-Munson effects come in too. They're not too bad, and an EQ pedal dials out most of it.

Inexpensive, useful, multiple input resistances, neutral sounding, with the above caveats. You will need a speaker jack.

 

Basically for your J-12a all you need is;

Here is a suitable Pot;
https://cpc.farnell.com/monacor/at-62h/high-power-l-pad-mono/dp/LS00547

 

Man, I’m envious! Ive been hunting that exact amp and couldnt find one here!!!

 

I think some DrZ amps put their little baby lite in
back of combo..for situations.

 

Eminence Maverick.

 

The L pad type speaker soaks Carls custom guitars sells would work fine. Or you could build your own for something like $10 bucks.
I bought a fancy pants $25 buck 70watt speaker attenuator a few days ago but am waiting on an enclosure to install it in before wiring up.

 

Pillows and duct tape.