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Microphone Month 3

Can I swap 5881 or KT66 Power Tubes in Place of 6L6 Power Tubes?

Q: I have a guitar amp that uses 6L6 power tubes, and I would like to try KT-66 and 5881 power tubes to hear what difference it makes in tone and volume. Can I do this?

A: We do not recommend that you try this sort of thing unless you are positive you know what you are doing. We are not responsible for damage to your amp or tubes that may result!

First, the easy part: you amp’s warranty will likely be voided by such a swap. If damage results, you are on your own for fixing it.

Second, another easy part: These are all octal power tubes, so from a pin/tube socket standpoint they are compatible. However, KT66s are physically larger and may not fit in your amp if space is limited.

Now, from an electrical standpoint: These tubes can all be considered variants in the 6L6 family — there are many more as well, including a number of 6L6 variaties. The big question is how much plate voltage your amp runs and how much plate voltage the tube in question can handle. 5581 tubes, for example, usually have a lower plate voltage tolerance than 6L6 tubes. In many cases, 6L6s can be safely substituted for 5881s, but not vice versa. In a few cases, where the plate voltage tolerance is high enough, 5881s can be used in amps designed for 6L6s. There can still be other technical issues to deal with such as bias settings, bias resistor values, power transformer issues, and more.

Our advice: before you try such a swap, talk to the manufacturer of your amp and of the tubes you intend to use or consult with a qualified amp technician who can verify that the combination you want to try is viable.

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