Well, yes - but if the grid never goes positive, you aren't using AB2 anyway.
To me, the biggest difference between the sounds of AB1 and AB2 occurs when the grid voltage goes from negative to positive if the driver isn't robust enough to tolerate the sudden reduction of the input impedance that results. With an OP tube that is linear with positive grid voltage (as most of them seem to be) and a good low-impedance driver that can source the current demanded, there should no change at all in the sound quality when going from AB1 to AB2.
To me, the biggest difference between the sounds of AB1 and AB2 occurs when the grid voltage goes from negative to positive if the driver isn't robust enough to tolerate the sudden reduction of the input impedance that results. With an OP tube that is linear with positive grid voltage (as most of them seem to be) and a good low-impedance driver that can source the current demanded, there should no change at all in the sound quality when going from AB1 to AB2.
Craig405 said:
The only thing I would question is this. MOSFETs are highly capacitive, and 12AX7s don't have much in the way of current sourcing capability. I'd suggest replacing it with a 6FQ7/6CG7/6SN7/12AU7, or if those don't have enough gain, then a 12AT7. Any one of those types can source more current to charge the MOSFET input capacitance.
Also, put a CCS in the tail of that LTP splitter and you won't need unbalanced plate resistors.
I have just finished some tests on my circuit, im pretty pleased with it so far.
Gain up till the source of the MOSFETS is 15,000. This is a lot and will easily be enough, i might unbypass one of the preamp cathodes to reduce this a bit if i dont need it.
I have implemented a crappy CCS on my diff pair (33k resistor), approx 2.2ma, i left the anode resistors at 82k/100k and its very well balanced.
I can get 150v peak-peak with 10mv peak-peak input to drive the 807 G1's with which is pleeenty.
bias had ended up being adjustable from 0-(-31v) at around 20mA.
Bandwidth is 15hz-60khz (-3db).
The noise is the only thing which annoys me, im getting about 2v pk-pk of fuzzy mains hum.. ill look into my grounding later.
Ill be connecting up the power tubes in the next day or so, hopefully no nasty suprises are in store!
Gain up till the source of the MOSFETS is 15,000. This is a lot and will easily be enough, i might unbypass one of the preamp cathodes to reduce this a bit if i dont need it.
I have implemented a crappy CCS on my diff pair (33k resistor), approx 2.2ma, i left the anode resistors at 82k/100k and its very well balanced.
I can get 150v peak-peak with 10mv peak-peak input to drive the 807 G1's with which is pleeenty.
bias had ended up being adjustable from 0-(-31v) at around 20mA.
Bandwidth is 15hz-60khz (-3db).
The noise is the only thing which annoys me, im getting about 2v pk-pk of fuzzy mains hum.. ill look into my grounding later.
Ill be connecting up the power tubes in the next day or so, hopefully no nasty suprises are in store!
Ill have to think back now, i put in about 5mV peak sine wave and got out 100v peak.. cant remember exactly but pretty sure the figure was ballpark.
I just connected the 807 screens to ultra linear taps and the bias current approached 100ma per tube and rising fast (should be 30mA in AB1 like i had it before going ultra linear)!
That was stupid and near fatal haha, will need to read up on effects of ultra linear on the bias point before jumping in like that! (this amp has turned experimental with power tube biasing schemes 🙂 )
I just connected the 807 screens to ultra linear taps and the bias current approached 100ma per tube and rising fast (should be 30mA in AB1 like i had it before going ultra linear)!
That was stupid and near fatal haha, will need to read up on effects of ultra linear on the bias point before jumping in like that! (this amp has turned experimental with power tube biasing schemes 🙂 )
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