how to lower gain from this schematic

[KenC]

sss333,

What Zo you get for the 5687 SRPP?

Cheers

Ken

[sss333]

If follow exactly the schematic (PAT'S ultimate preamp), the Zo shld be ~100ohm as stated in the diagram.
But i had change a lot setting like:

1) i use b+ ~250V instead of 330v
2) i use 1.5k resistor insted of 1.4k
3) coupling cap i use 0.33uf, instead of 20uf
4) chatode capacitor i use 330uf insted of 6uf

i didn't not measure Zo as i don't know how to measure, and also don't know how to calculate, can you teach me?

[yeo]

hi sss333

don't worry about it. your zo wont' differ much judging by the things you changed.

yeo

[KenC]

sss333,
I also learn from books on this.
Pg 331, Chapter 6, Building Valve Amplifiers  by Morgan Jones will teach you how tomeasure Zo of pre-amp and in Pg 332, he'll teach you how to measure Zo of power amp.

Cheers

Ken

[milen007]

Erwin,

Any progress?

hi kenc

sorry that i reply so late. just have the time to try out. thousands of reasons and lazy butt.

i try using the schematic attached.

however the B+ is only 126vdc though i have not change anything on the psu. maybe the current draw is higher.

it gives a loud hum. just wanna make sure that the schematic are good.

ill try to up the voltage to 138vdc tomorrow and report back

thanks,
Erwin