IMPEDANCES FOR TUBE GUITAR AMPLIFIERS

 

1.) Matched Impedance - It is considered safest to match impedances.

For example;

• 2ohm into 2ohm
• 4ohm into 4ohm
• 8ohm into 8ohm
• 16ohm into 16ohm

2.) Safe Mismatch - You can go out of the amplifier into an equal or higher rated cabinet.

For example;

• 2ohm into 4ohm
• 4ohm into 8ohm
• 8ohm into 16ohm

Note - Running the amp in this manner doubles primary impedance on the output transformer and runs the tubes more "relaxed" or in other words is safer on power tubes than matching the impedance (4 ohm into 4ohm), which is due to the tubes seeing less power. The tubes are not "relaxed" per se technically but they are seeing less power. Over all, matching or mismatching impedances is all about the shortening or extending of tube life.

Tonal effects; An amps distortion characteristic will change and may become darker. IE: there is a slight loss of headroom/over all volume, smoother/warmer top end and a slight low end roll off. You will also notice the amp is smoother/easier to play.

3.) Other Mismatches - You can go out of the amplifier into a lower impedance/lower rated cabinet. 

For example;

• 16ohm into 8ohm
• 8ohm into 4ohm
• 4ohm into 2ohm

Note - Though technically this will not directly damage the output transformer it will stress the power tubes, which can damage the output transformer if a tube shorts. Voodoo Amps does not recommend this mismatch due to increased risk of damage / failures.