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Embarrassing Tube Amp 101 question

Cholo

Member
Joined
Apr 6, 2002
Messages
35
Oh man, is my face red...I can't seem to find out anything specific on this but here goes. I purchased a used Marshall 1960B straight cab yesterday. I plugged in my 74' LP Custom into my JTM 45 clone head. I was sure to switch the cab from 4 to 16 ohms since my head was already set at 8 ohms. I have since learned that in order to get the best possible performance of the head/cab that you should match impedance. My clone has a selector switch (4,8,16). My question is: can I simply switch the impedance on the back of my head without having the head re-biased or anything like that? Do I need it looked at professionally before switching or is it as simple as it sounds?

PS. It sounded wonderful when I did play it. Very thick with harmonic structure. Oooh baby!
 
B

Banker

Guest
It's as simple as it sounds........change the Ohm selector on the back of the head to match the cab.....job jobbed.
 

goldtop0

Well-known member
Joined
Aug 19, 2003
Messages
8,931
You only need to rebias when changing the power tubes and/or the rectifier tube.
 

ptrickamp222

New member
Joined
Aug 22, 2007
Messages
879
I would always put a ohm meter on the cab to make sure of what ohms it is putting out. A 16 ohm cab should read about 12 ohms. 8 ohm cab around 6. 4 ohm around 3.2. Just to make sure that someone did not wire the cab to a different impedance. Hate to blow something especially if it's vintage. Also will tell you if the cab is electrically in good shape and a speaker is not going south. Just a heads up. And no bias to switch impedance. Always when changing power tubes unless cathode biased.
 
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