My SOHA II has developed the infamous hiss problem
However, it's only in one channel......
1. I'm positive, the hiss problem was not there when the amp was 1st built
2. The problem appeared when I accidently caught the headphone cable and knocked the amp off the shelf it was sitting on. It didn't fall far as the shelf is quite low and the amp was tethered with the power and signal cables, but immediately afterwards I noticed the noise.
3. I suspected I'd damaged the valve but swapping them around beween channels or leaving them out, or changing them for a different type doesn't affect the noise.
4. It's not volume dependant - i.e it's not affected by moving the pot, and is only heard once the E12 relay engages.
5. It's still present if the inputs are disconnected.
6. It's not noise in my ear, if I reverse the headphones, the hiss moves to my other ear.
7. I can only hear it with my Grado's. It may or may not be present with the Sennheisers but if it is, it's just below my hearing threshold.
There's a possibility I might be able to do some checks with a friend's oscilliscope. I'll keep you posted.