yes, you can use el34's, e34l's, kt77's, kt88's, 6v6's, kt66's and whatever regular guitar amp power tubes that fit in those sockets
the fortin modified rd50h didn't require any bias adjustments and came stock with jj e34l's
not sure if this one needs biasing
better pm Mike
my ex-fortin rd50h had a diezelish gain texture,
I'd compare it with the 50w einstein, but without the diezel wall of midrange and less oversaturated
I wouldn't say it sounds anything like a mesa or marshall
it has the flatter voicing of a tube randall, but the articulation and grainy texture of a real fortin
reminded me of some clips of the bones and natas, but less marshall-like
best sounding high gain amp in the price range, along with the JCA50H, but with better features, finish and construction than jet cities
I had to sell mine to a friend (who's in love with it now), but I intend to order the rd40c 112 combo around christmas
it sounded best with v30's and also with my greenback-based mg music black dogs (weber parts, same as the silver bells, but with aluminum basket)
didn't enjoy it as much with my scumback h75's (gets a little too bassy and not midrangy enough)