Hiwatt... different PI styles...

[eddie25]

Does anyone have any opinions on the tonal or performance aspects of the two different ‘PI driver’ (probably not the correct term in both cases) setups in the early Hiwatt circuits? One seems like just a voltage reference for the PI grid with no signal going through, and the other a cathode follower actually passing all the signal to the PI, (most people seem to reference the former). I'm talking about the triode right before the PI in most schematics.

Is there a preference amongst Hiwatt enthusiasts? There doesn’t seem to be a lot of discussion on this difference among Hiwatts. Maybe it's totally negligible?

Thanks!

[Gaz]

The Vintage Amps forum has a lot of discussion:

http://vintageamps.com/plexiboard/viewf ... 085815e77b

CF PI is better, other one is lame

[gingertube]

I prefer the second of your examples but look at the Hiwatt Forum linked above for what others think too.

The main difference in the 2 schemes you have above is the presence control in the second one (the one I prefer). This is a HI Frequency cut/boost, do nothing at centre setting control which works via feedback. It is one of the "iconic" Hiwatt circuits.
Most presence controls simply boost by cutting the HIGHs out of the main global feeback loop.

Cheers,
Ian