[quote author="Rossi"]
For some reason my MP-2 has more gain than advertized, about 55 dB instead of 46 dB. [/quote]
First, I'll clarify: I didn't design the A-Designs pre, just added some bells, whistles and do-dats to the MK 2 version along with laying out a new PCB.
Anyway, extra gain: You have a different IP transformer most likely. The later rev. switched this out so I'm assuming you have an early unit which also has this upgrade.
[quote author="Rossi"]
Also, I discovered that the 250 V test point actually measures only 206V and the 6.3V heater voltage is only 6.0V.[/quote]
Your 'lower than design centre' AC input would account for a slight drop in H.T. but the L.T. is regulated with an LT chip and thus wouldn't be affected - except for a dropping out of reg if the input were too low which I doubt it is. Nothing wrong with 6V on the heaters BTW, I usually put them there myself on pretty much everything wanting 6V3. Not sure where you're measuring the 206V H.T. voltage, I'd have to fire up my older computer to see the schemo's. However, I'm not sure that the 'test point' you mentioned shouldn't really read 200V - I'd bet your unit is just fine.
If voltage was an issue then you'd maybe be hearing a buzz noise due to the L.T. reg dropping out and you'd have potentially lower headroom or non-linearities due to some shifted operating points.
F.W.I.W. I swapped in a N.O.S. Mullard EF86 in a unit - microphoncs & noise dropped and sonics went up somewhat.
Hope that helps your peace of mind :wink:
Ciao.