Correlation between tube bias and hiss?

Sounds to me like you need tubes. Could be something else or a combination of problems, but a spare set of inexpensive tubes would be a good place to start. Just remember to check the bias on the tubes RIGHT AWAY after installing the new ones. Sean
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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.

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

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.

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.

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!

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.