PI Plate values/20K trim pot. wait...there's more

[?Mark]

Hi all,

I've learned gobs since my build and visited with one of my fav, world renowned, local builders,
to see how the pros do things.

I wanted to share my new PI setup. 91k/100k with a 20k trim pot. I can now listen for and find the
sweet spot, where the notes bloom, while adjusting the ac with the NFB disconnected. 100pf fizz cap.

side notes: other tweaks & additions
PPIVM (tweaked to 1% tolerance between pots.)
Zero Loss FX loop(best thing added to date).
Put V2A on spst switch. Cap and 820R to cathode, the 820 to gnd. The Cap gnd has 1meg
to gnd and the gnd side of Cap is switched in or out...NO POP...lots of gain when in.

Went to .05 PI coupling caps.

306VDC B+ on V1
Split V1. CH1 2k7 with .68uf = Fc 86.7Hz
Ch2 1k32 with 1.6uf =Fc 75.5Hz

(check that, 12-17 put on SPDT switch for Shared/
Split. In split mode it provides a 1k32 for V1b based on load line plot and B+. In shared provides a 820 for V1a &V1b
I have a gif. of switch but can't remember how to post them here?!...Help me and I'll post it.)

470k mix resistors, (check that, as of 12-4 went back to 270K)

No CH1 vol bright cap...tried them all, went without. (check that, 12-4 went back to 500pf)
(check that,12-17 put on push pull pot for in or out.)

Might be silly but before ANY of the above changes, I put
.033uf polys on both 32uf cans and .016uf poly on the pre
amp 16uf cap. to help with ESR. I liked them so much
they stayed.

1-1-2016
Everyone says they like the amp but the word shrill comes up now and again.

I've gone thru and put rg174 on all cable to pots and pots intra-connect.
The 68k are off the board and connected directly to the pins via rg174.
For the hell of it, I put FB 100k at pot with rg174 running to board and tap.
The amp has always been quiet even with volume all the way up. I did the shielded
cable move precisely for HF attenuation and I could not be happier with the results.
I looked at the Ducan TS calc. for the difference in slope and cap and went with 36k & 500 very good
100 to 1k Hz bump up.

This is not pat on back for me as I'm only implementing what I'm learning from
all you guys and your shared knowledge. So hats off to all of you!

Everyone now covets the amp and wants a build just like it.

Ha! good for us

Now if I can just get the cheap bastards to pony up the dough
to build another one.

[VelvetGeorge]

Very cool! I think I know who you visited. I recognize the penchant dialing in the PI for note bloom. Let me guess.....shoot for 8V difference between the plates?

Non-electrolytic caps handle higher frequencies better than electrolytic. Pro audio gear often has small bypass caps across them for this reason.

great tips here.

george

[?Mark]

Yeah, Rosa has gotten to know me now...only barked for half a minute
I REALLY like his amps!.

I'm going to post some links that have helped me.
Bear with me if they've already been posted.

If I don't want to or can't break out my Casio fx-450
the below are very handy indeed.

https://www.ampbooks.com/ very helpful!

http://tubes.nekhbet.com/math.shtml

http://www.pronine.ca/rccir.htm

http://www.sengpielaudio.com/calculator ... encies.htm

https://robrobinette.com/Tube_Bias_Calculator.htm

http://chemelec.com/Projects/Reactance/ ... eChart.png

http://www.aikenamps.com/index.php of course...

http://education.lenardaudio.com/en/14_valve_amps.html

http://www.duncanamps.com/tsc/download.html

http://sound.westhost.com/articles/capacitors.htm I'd like to hear comments on this one.

Test equipment on the cheap.
use what ever from whom ever but
a Freq gen and scope, RTA, are a must.

https://www.trueaudio.com/rta_abt1.htm

In order to the know/hear the Freq you are
working with/talking about it, it helped me
to listen to them after calculating them (sort of
like "a picture is worth a thousand words").
having a descent sound system for my LT, Altec,
paints my picture. I can map a range now.
(FYI helps to have a GF with perfect pitch,
she knows exactly what pitch and amplitude
to use to make me take out the trash etc.)

Lots more but these might provide insight and easy
calculation for the lazy at heart, or sometimes B D
when it comes to math, such as myself.