We can substitute 6CA7 tubes in amps originally fitted for 6L6s; however, the price paid is some crossover distortion. To cure this, simply feed a sine wave generator into the input, and adjust the bias control (like the one on Fender amps found just beside the choke and power transformer) until the crossover distortion disappears. The advantage of using 6CA7s is a somewhat crisper, more defined treble, as well as bass that's a little bit tighter. You can also substitute 6550s with no bias modifications, but you must then add a fan for cooling. 6550s last longer than 6L6s due to their special construction, and do not get microphonic.
Feature by Gerald Michaud
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