Tube Died in Blue Robbie (I think) what Tube makes a good replacement?

So my Blue Robbie is giving me a sort of crunchy radio voice thing now... and sometimes making tone type noises or nothing at all. Blue very graciously offered to fix the preamp at a rate of 85$ an hour plus costs, but I have a feeling i can spend about 20$ for a new tube and pop it in. So the question I have, what would be a good replacement tube to buy? Thanks

Tube Died in Blue Robbie (I think) what Tube makes a good replacement?