Ilya said:First, the PSU heaters filter bank caps values are very large. I've browsed through several PSU circuits for tube gear and I've never found heater regulater filters to be that large. Is there any special purpose for that?
Two channels draw 3-4A of current. That's a lot of work for a cap to smooth out the ripple to a level that is flat enough for the regulator. 10000uF minimum for this reason (C16-17). I learned later that the post-regulator cap doesn't actually have to be this big. It has negligible ripple to smooth out and only really acts as an unnecessary reservoir. 1000uF should be enough. 22000 is overkill for all of them, I admit.
There is little to no PSRR in this Drive-1 design. This DC heater rig was a precaution to not insert any unnecessary hum to the system.
Ilya said:Second, the heaters supply is floated to the B+. What's the benefit of this?
Tubes have a maximum heater-cathode voltage difference. See data sheets. Stage V2A is cathode follower where the cathode is at +100V potential (I forget the exact value). Heater needs to be biased closer to this value for long and reliable tube life.
Ilya said:Third, is there any reason (sound wise) to make a tube regulated PSU instead of solid state? I remember Winston O'B speaking regarding that subject - that some semiconductors throw out a lot of hash and that we're essentially listening to the PSU. Any thoughts on that matter are highly appreciated.
There is one major source for noise in this zener referenced pass transistor regulator. That's the zener itself. This noise is completely eradicated with two separate RC networks on the preamp side (R33/C15 and R36/17).
There is some truth to us "listening to a PSU". If the PSU is not ideal, has not so much capacitance and no regulator (or some inferior tube regulator rig), the PSU will audibly "dip" on large current surges, and even let through significant ripple. This is very common on guitar amps because of the power stages. It can happen on plain preamps as well, especially with extremely simple PSU's like a plain CRC with very little capacitance. The practical effect is more distortion.
Drive-1 PSU on the other hand is WAY too stiff. It will always deliver solid B+. It's so clean it's boring. Think mastering equipment transparency. That was not my design goal, but an over eager design "accident". I didn't think it through. Next time I will attempt something designed for distortion I will make a really sh*tty PSU.