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.

Albertk, just went through a tube problem in my Atma-sphere MK-60 2.2, and sounds very similar to the problems that you describe! However, with out more info (as Maxgain wisely suggests) a diagnosis of your problem is a shot in the dark, at best!

It could be a problem with old output tubes or the hiss may be due to a malfunctioning bias circuit. It would seem that the bias supply can't supply a high enough bias voltage to increase the current through the cathode resistor. It's not uncommon to have old resistors drift so far in value as to limit the bias voltage adjust range. The easiest thing for a non-tech to do would be to obtain new output tubes (preferably a complete tube compliment). Having these on hand will allow quick troubleshooting and reduced down time if a tube problem does occur. With a set of good tubes, you can determine if the new tubes are able to bias properly. If not, then a trip to a tech is in order. Good luck.

Agreed...retube. You will not really have a problem if the old tubes are okay because you will eventually need the new set regardless. The intermittent hiss could come from other parts, but it is next to impossible to replace them easily. Another option (cheaper) is to switch the tubes from side to side that can be switched. If a tube is the problem and you were able to move it to the other channel, then the intermittent hiss should also switch channels. Obviously, if the hiss remains in the original channel after switching, then most likely you have eliminated those tubes you could move as probable culprits. I am not familiar with your amp and am basing this suggestion on the tube configuration of my cj MV75a1. Tube sockets can also be problematic in cases of intermittent distortions.

Maxgain
I bought the amp on EBay and as far as I know it is all stock. I don't know the age of the tubes, but believe they are at least 3 years old. I have a schematic diagram and a voltmeter if getting some more readings would help.
Albertk