A: The answer depends on the specific amp you’re using. A few — but definitely not all — 6V6 amps can handle 6L6 or even EL-34 power tubes. However, do not assume that your amp can do so. Prior to making a change like this, it is mandatory to verify with the manufacturer that the amp can handle the larger tubes. It is also essential to find out whether the amp needs to be rebiased to support the new tubes. Finally, make sure you know which power tubes are compatible. For example, some 6L6 power tubes can put out 25 watts and others can put out 30 watts, with different demands on the amplifier.
PROBABLY REDUNDANT BUT ABSOLUTELY NECESSARY WARNING: Do not install anything but manufacturer-approved tubes in your amplifier. To ignore this warning is to risk damage to or destruction of your amplifier.