Hiwatts..

[Mike Sulzer]

I think the filter capacitor is cheaper.

Wait, i have an idea. If this could otherwise be achieved via a voltage divider from HT to ground, maybe this was done for noise supression. Any ripple present at the HT would also show up at the grid via the divider, correct? Cathode followers generally have good PSRR. Maybe this was enough of a reason to choose this topology. I would think the performance would be more consistent over the life of the tube versus the life of an EXTRA rc filtering stage stage. Its just an idea, what do you guys think?

[Shadrock2]

The Hiwatt circuit in question not only provides bias for the PI but also provide an active presence boost and cut to enhance the presence effect.

[SoulFetish]

I think the filter capacitor is cheaper.

I think your right. I dont think you would design it this way if you were designing from scratch. But, here are some factors which may have been at play:
This was a madification to an existing commercial design. Depending on when he wanted to impliment these changes, he may have been designing this circuit around existing hardware and stock which had already been purchased. It may have been cheaper to make these changes with this design than adding an RC stage, particcularly when we know how much importance he placed on component layout and lead dress.

[Malcolm Irving]

... . Any ripple present at the HT would also show up at the grid via the divider ...

This applies to the divider on the grid side of the CF as well. Any ripple on the grid of the CF would come out on the cathode, with a gain of almost 1. It is noticeable that the divider on the grid of the CF gets its HT from a highly smoothed power supply node.

[SoulFetish]

This applies to the divider on the grid side of the CF as well. Any ripple on the grid of the CF would come out on the cathode, with a gain of almost 1. It is noticeable that the divider on the grid of the CF gets its HT from a highly smoothed power supply node.

Great point!

[mhuss]

Try scoping across the 100k resistor to see how much current is flowing through it. (Connection into the scope needs to be floating.)

Of course there's current flowing through the 100k, into the virtual ground of the cathode output.

In the older circuit, the signal was going through this triode, but it was still an 'almost 1' gain factor.

Whatever the electronic theory reasoning, this PI stays cleaner longer, and subjectively sounds much more dynamic and less squashed, even when clipping. I believe this was a design goal. Perhaps this is because the bias point does not shift as much with a heavy signal load.

As far as cost, Dave Reeves was not a modern penny-pinching MBA. ;-) He wanted to build what was, in his view, the best amplifier possible.

[Mike Sulzer]

Of course there's current flowing through the 100k, into the virtual ground of the cathode output.

In the older circuit, the signal was going through this triode, but it was still an 'almost 1' gain factor.

Whatever the electronic theory reasoning, this PI stays cleaner longer, and subjectively sounds much more dynamic and less squashed, even when clipping. I believe this was a design goal. Perhaps this is because the bias point does not shift as much with a heavy signal load.

As far as cost, Dave Reeves was not a modern penny-pinching MBA. ;-) He wanted to build what was, in his view, the best amplifier possible.

This is the theory section. We are supposed to figure out why things work as they do, not just throw up our hands and say "This circuit is just so much better!". "much more dynamic and less squashed, even when clipping": If so, it sure would be great to know why.

[d95err]

The Hiwatt circuit in question not only provides bias for the PI but also provide an active presence boost and cut to enhance the presence effect.

I can't see how the voltage reference stage affects the presence circuit. You could use the same circuit for virtually any type of phase inverter. Also, the treble cut part of the circuit is not really active; it's just a simple passive filter.

[Shadrock2]

I can't see how the voltage reference stage affects the presence circuit. You could use the same circuit for virtually any type of phase inverter.

D9... I believe you are 99% correct. I posted that because the OP had circled the previous amp stage as well as the voltage reference. Still... it seems possible to me that ac signal can get through to the voltage reference cathode via the presence network and the 22n blocking cap. That would be due to the relatively low 100k grid leak of the PI verses the 220k cathode R of the V reference. Admittedly it is hard to judge the relative size of the ac signals involved.

[mhuss]

This is a job for Spice or one of his buds. Any takers?