One of my ST-70s is a stock unit in which I recently replaced the driver board. The new board is simply the stock circuit with new/improved materials.
The unit is dead quiet and sounds great. However, prior to adding the new board - and even now with the new board installed - after about an hour of warmup I get a rather loud hum/buzz (if pressed I'd have to say 'buzz' rather than 'hum') through both speakers. This noise starts out intermittent and then becomes constant. The odd thing is that the noise stops when I touch the volume control on my preamp (a Van Alstine Super PAS-3i). It is also worse when I have the volume on the preamp set to its minimum position. Touching the amp itself lowers the level of the noise only somewhat. The other odd thing is that - while I initially suspected the preamp - the noise does not manifest itself when the preamp is connected to my DIYTube ST-35 amp.
Now, obviously, there is some sort of ground problem like a poor connection that only makes itself known as the amp heats up. Given that it is in both channels, I'm suspecting the quad cap (replaced with an 80-40-30-20 about a year ago) or the main ground point next to the quad cap.
Any thoughts/advice? Sound like I'm on the right track?
Thanks,
Joe