Ah yes,
That's a problem I found when doing the +6dB gain mod! Because the output stage gain is rising +6dB, the noisefloor (including any ground noise) is also raised +6dB. I did a simple fix- I wired a piece of insulated wire diagonally across the board like this:
To find the optimum place to connect the wire, I connected one end to the ground trace where the centre-tap of the transformer enters the board, and with the unit powered up, and monitoring the output with both controls up until I could hear the hum, touched the other end of the wire to parts of the ground trace on the other side of the board. When I touched it onto the far side of the board, the hum reduced significantly (>12dB lower!)
This may not work for your setup, because I have a feeling it depends how you have your other grounds organised in the unit, but this helped my friends unit a great deal- it only needed to be done after the +6dB gain mod.
The "Gain Groups" of the transistors are organised by letter- no letter is unsorted, A is low gain, B is medium gain, and C is high gain (can't remember the actual hfe group ratings...
Handy page explaining transistor markings
...but it's something like A= <100, B= 100 to 500 C= >500
I think I remember Jakob saying that the circuit is designed to work with any hfe range transistors- they're all working at quite low gains anyway, and are all bias stabilised.
(BTW, make sure you don't confuse the
hfe gain of an individual transistor with the
actual gain of a transistor amplifier stage- the hfe is a measure of current gain Ic/Ib. The AC gain of the transistor is set by the collector load and emitter resistor. Some circuits rely on a higher hfe transistor, but this is often more to do with impedance transformation and current amplification and overcoming feedback than actual "stage gain" and "voltage gain" for AC)
Mark
