PI question

[ernie_jr]

on the sunn 200s and ampeg b26  both use a pent/triode for the PI (7199 or 6an8) but the circuit design is different.
Any thoughts on which is better? the amp I am planning will be UL using kt 88 or kt 120 with about 560 plate volts with a sunn 200s for one input and ampeg B15 for the other. Both PI's look easy to build.
Thanks for the input,
Ernie

[sluckey]

I'm not familiar with the B26 (maybe you mean B25?) but I've built several Sunn amps a long time ago and I currently own a Sunn Sceptre. I highly recommend the Sunn 6AN8 circuit, but I'm sure the Ampeg 7199 is fine too. But since you are planning a UL KT88 output and a Sunn 200s preamp plus a B15 preamp, sounds like a mostly Sunn build to me. IOW, you're planning a Sunn 200s with an additional B15 preamp. That being the case, I'd stick with the Sunn 6AN8 PI. I know it sounds good, especially for bass.

[ernie_jr]

Thanks Sluckey,
Yes meant B25, thanks. I have built several sunns as well and they have worked well. Was toying with the idea of something different, but will probably stay with the sunn. I have several 6u8's, so I will be using that inplace of the 6an8.
I have also used the Sunn PI with a pentode kt88 with ampeg preamps and that amp sounds terrific for bass.
Thanks again,
Ernie

[PRR]

The only functional difference I see is that Sunn (Dynaco style) direct-couples the cathodyne to the pentode, Ampeg cap-couples.

And these NFB networks give gain=21 for Sunn, 68 for Ampeg.

Either way: 270K pentode load, 2*47K triode loads, 450V total supply available.

The ramifications ripple-through as different DC bias on Sunn's pentode to get its plate down below 1/4 of the cathodyne supply.

Considering that pentode-triodes in today's market may be troubled, I'd lean to Ampeg's biasing. Hard enuff to get a pentode's plate to a specific voltage when you buy tubes by the case from Kentucky or Pennsylvania. Buying re-re-tested old stock, or China clones, the fix bias nails the cathodyne conditions which are less flexible than the pentode.

[ernie_jr]

Thanks PRR.  I am leaning to trying the Ampeg, partly because the power rail set up on their amps fits what I am planning,
(2 pre-amps vs 1 for the Sunn). It makes for less calculating and re-inventing the wheel.
Thanks again to all,
Ernie