Blowin It

January 1, 2010
0.000RB_FRANK-WHITE_1IF YOU BLOW A SPEAKER OR IT BECOMES disconnected while playing, is it certain to cause damage to your amp? —T.J., via email

You never want to operate an amp without a speaker load connected. If there is only one speaker in your amp—or the speakers are wired in series—then blowing a speaker could cause the output transformer to arc internally, which can damage its windings. A “no load” condition can cause high-voltage arcing in the output tube sockets, requiring them to be replaced, and the filter caps in the power supply can also be damaged if the speaker load is suddenly lost. If your combo amp has parallel-wired speakers, or if it is driving a cabinet with series/ parallel-wired speakers, as in a Marshall 4x12, then blowing a speaker will typically not cause any damage.

If you suspect that your speaker has blown, stop playing immediately and check the connections to the speaker. You don’t have to turn the amp off right away, as nothing is going to happen unless signal is being fed into the amp. Also, almost all Fender amps as well as many other brands feature a shorting jack to prevent damage to the amp if a speaker is not connected. —Gerald Weber, Kendrick Amplifiers

How important is it to use a speaker cabinet that matches the impedance of the amp? Is it damaging to the amp if the impedances don’t match? —John, via email

The idea behind impedance matching is to have a proper load on the amplifier. If you use too high of a load, say a 16Ω speaker connected to the 8Ω jack, then the amp will lose a little volume and the tubes will compress more. If you do the opposite, however, and have a 2Ω cabinet plugged into the amp’s 4Ω jack, then the tubes will be operating closer to a short circuit, and tube life will be shortened considerably. —Gerald Weber, Kendrick Amplifiers

I have an old Fender Champ and am wondering what will happen if I replace the stock 6V6 tube with a 6L6 or other larger output tube? —Kenny, via email

It will work for a while but eventually the output transformer will overheat and blow. The stock Champ transformer is wound with thin wire that is rated to handle only 30mA of current. Most Champs idle at slightly more than that, so they are always running hot, even with the stock 6V6 tube. —Gerald Weber, Kendrick Amplifiers

How often should you check the bias on a tube amp? —Jack, via email

An output tube will perform its best only when it is properly biased. The problem is that as the tube wears in, its performance characteristics are constantly changing. You could have a new set of tubes installed, and a month later the idle current will have drifted. If the current drifts too low, the amp will lack power and dynamics, and will sound cold. If the current drifts too high, the amp will lack headroom and tube life will suffer. My advice is to learn how to bias your amp, or have a tech do it for you if you don’t feel competent, and check the bias setting often. —Gerald Weber, Kendrick Amplifiers

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