I had earlier posted about severe hum in my GSG and eventually resolved it by replacing the JAN tubes with Russians, or so I thought:
http://www.diytube.com/phpBB2/viewtopic.php?t=5371
Running direct from a line input it was acceptable, but when I put a preamp back in it was horrible again.
I made a couple of more changes this morning and now I can claim unqualified success - I can put my head in the right speaker horn and hear nothing; in the left horn I hear a barely perceptible hiss. And no, I didn't ride a jack-hammer at work this week.
As to what ultimately made the difference, I don't know, but I suspect it was principally lowering the gain of the input stage. For anyone who cares, here's what I did (in order) after I replaced the JAN 6B4Gs:
1. Installed line filter (common mode chokes, full Monty)
2. Class X cap across the IEC terminals
3. Grounded output binding posts (a good safety idea anyway)
4. Two anti-parallel pairs of 1N4 diodes between the circuit ground and chassis ground
5. Changed the 6SL7 cathode resistors from 3K to 5.1K
6. Replaced the 6SL7 with a Sovtek 6SN7
When I first powered it up I was very disappointed - it was absolutely silent and I figured I'd buggered it up for fair. It was only when I put my head in the second horn (couldn't hear anything in the first one) that I realized it might just be that quiet.
Personally, unless you're running a GSG direct, I'd go with the 6SN7 anyway - there's still tons of gain, more than enough with a pre-amp.