Do you have a bias adjustment, or the capability of changing the bias resistor to accommodate new tubes? If not, you will need tubes that are rated for the proper current range of your amp. Depending on the rating of the tubes, the current can be excessive and can do damage to tubes and/or amp, or they can draw too little current and sound harsh. I believe that your amp has the switch to choose between 6L6 or EL34, but no bias adjust so the tubes need to be within a certain current range to work properly. The MB acceptable tube range is about the same as the GT tube range of 4 to 7. As long as the tubes are a "mid-range" rated tube, they will be OK to use. It's best to check the current draw of the tubes when installing if you're not sure. Do you like to take chances? :wink: