Correlation between tube bias and hiss?

Albertk, is your ST-70 stock, with no mods or updates of parts? How old are the tubes? Their are a lot of things that could be wrong here, but should be fairly simple to fix as long as you can isolate the real problem. It could be a combination of things as well. The hiss may not have anything to do at all with the fact that the tubes don't bias. Give me a bit more to work with if you can.

All good stuff above. It wouldn't be a bad idea to get a set of Svetlana or Electro Harmonix EL 34's. The 5Ar4/Gz34 is a problematic tube at times depending on the brand that is in there. I would get a new Sovtek. They are much better than the Chinese junk that was all you could get for a while. The 7199's are getting tough to get. You could try the Sovteks or try to get some JAN/Phillips mil spec NOS which is about the best of this type. Sometimes what people describe as a hiss can sometimes be a tube socket for one of the 7199's. I suspect that the Cap in the bias supply could be going or maybe a weak 5Ar4 could cause the bias problem if new tubes don't fix it. Clean the tube sockets. If you don't do anything else to the amp you need to get rid of the Selinium rectifyer, and repalce it with a diode if it hasn't been done already! These crap out in a spectacular way and take other stuff with them, not to mention the toxic cloud of selinium gas that they fill the room with when the blow. I don't think I have any of the info I would need any more to help you with the other readings. I used to play around with these amps and you can get them to sound pretty good, just don't put a fortune into it. Their are some modern tube amps out there you can get used for under a grand that do outperform even a realy well modded ST 70.