My Ebay purchase

[fairfield2002]

I just bought a Mesa 2x12 horizontal rectifier cabinet. Seller said it was vintage 1998 and played only about 5 times. It was advertised as a rectifier cab. I was expecting a full back design with V30's but it instead is a half back design with Black Shadows. Also below the input jack on the back is a sticker with a "4" on it, I assume this means it is a 4 ohm jack correct? Can I plug my Mark IV head with it's 8 ohm output into this jack or am I only to use the 4 ohm output of the head? I'll try to post pics...anybody know how?

 

[MrMarkIII]

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I wouldn't assume anything. Mesa cabs are usually 8 ohms, so maybe your cab was rewired to 4 ohms. But Mesas are not very sensitive to impedance changes, as long as you keep it above 2 ohms. You could try the cab hooked up to a 4 ohm speaker jack, then put the amp on standby and switch the speaker cable to the 8 ohm jack. Then check the sound again, and try to notice any differences.
Always switch to standby (or off) when removing a speaker cable. Never run a tube amp with no speaker load.
Use a clean, loud tone, and keep your guitar's volume all the way up. Often an impedance mis-match will cause distortion (a little, not a lot), even with a clean tone. The sound will often seem "distant" or lacking presence, and the feel is "spongy" and more compressed. Some folks like it that way