I'm not familiar with Roadsters but that doesn't sound advisable. Is it 100 watts? In some instances you can run a pair of tubes (out of the four, remove either the inner two or outer two) and halve the impedance and be okay.
The Roadster has 4 x 6L6 tubes and a 50/100 switch which could be taking two of those tubes out of the circuit when in 50W mode. I don't know the amp and you should check with someone who does but you can probably run it in 50W mode if the 2 tubes used in that mode are both working. Make sure you are clear on it though otherwise you will burn up your output transfomer. Personally, I would just replace the tubes.