Why is the P.I. stage in an AC-30 so sensitive to noise?

[Mark]

The last time I had this issue, was with my Rocket build and it turned out that the board was conducting.

I have same issue in AC-30 and I have all the parts mounted in free air on the valve socket.

Any thoughts?

[pdf64]

I think it’s purely down to it being a high gain power amp, operating open loop. Ok, the ‘higher than typical’ LTP tail loses a couple of dB potential gain, but EL84s have a high amplification factor. Overall it makes a lot out of a little, hence the LTP grids are rather more sensitive than normal, to both wanted and unwanted signals.

[mhuss]

...a high gain power amp, operating open loop.

+1

[Mark]

The thing is, I don’t get this sort of noise from my Trainwreck Rocket clone, though the layout and power supply are totally different.

I’m not a big fan of the AC-30 earthing, I think it could be better.

[mhuss]

Certainly better filtering and grounding would make it quieter.

[Mark]

I think you could double the 8uF caps to 16uF and not hear a difference to the clean sound. I know this as I have tried it.

The serial earth bus is just crazy,