Amplifiers contain voltages high enough to Kill you even after the amp has been powered Off !!! If you don't have enough electronic experience to identify and discharge the Filter Capacitors which contain these Deadly voltages, take your amp to a Qualified tech. If you don't know a qualified tech in your area, post a request in the alt.guitar.amps newsgroup, the folks there will be happy to help you.
Another option is to swap your 6L6's with EL34's. EL34's have a darker tone, the break up alot earlier than the 6L6, and their overdriven sound is radically different than a 6L6. If you're looking for a big change in tone, then this may be just what you're looking for. But EL34's ARE NOT direct replacements for the 6L6. You'll need to make a slight modification to your amp so it can use them.
1. Replace the 470 ohm resistor that's between pins 4 and 6 of each output tube socket with a 1000 ohm, 2 watt resistor.
2. Disconnect pin 1 only of the 1500ohm resistor that's between pins 1 and 5 of each output tube socket.
3. Disconnect the wires on pin 1 of the output tube socket.
4. Reconnect the wires that were on pin 1, to the end of the 1500-ohm resisors that are now standing free on each output tube socket.
5. Connect pin 1 to pin 8 of each output tube socket with a solid wire.
6. Increase the value of the resistor that's in series with the bias diode to reduce the bias voltage until you obtain proper bias. This will vary from amp to amp.