PNP to NPN rules?

bumblebee · Post by **bumblebee** » 24 Oct 2007, 02:54

EDIT:
say i wanted to change a NPN circuit to PNP is all i have to do is swap the polarity of the electros put in the pnp's and make it positive ground?

i messed with it on paper and it looks right but thought i'd ask anyways.

Post by **modman** » 24 Oct 2007, 08:57

Positive vs negative ground is something different from the transistor type and doesn't always correspond. Old FFs had positive ground and PNP, but a negative ground PNP FF is also an option.

What do you want to do? Do you want to try an NPN in the circuit or make it negative ground or both?

bumblebee · Post by **bumblebee** » 24 Oct 2007, 09:03

ii wana make a vezx WM pnp - ground and another one pnp + ground for some more experimentation with this circuit.

heres the layout and schem i been using

schem

layout

R.G. · Post by **R.G.** » 25 Oct 2007, 13:14

bumblebee wrote:ii wana make a vezx WM pnp - ground and another one pnp + ground for some more experimentation with this circuit.
heres the layout and schem i been using

So what you really want to do is mess with the same circuit in both positive ground and reversed ground.

I only repeat this here because you may not have seen my posts on this before. There are some PNP/reversed ground circuits that have excess noise or oscillation that cannot be cured (at least by me) without changing them back to positive ground.

Theory says that which power supply terminal you call ground doesn't matter. In practice, there is a real power supply impedance that does matter. There is also something else that matters that I have not found.

I say this from experience. I have done a lot of work with normal and inverted grounds. Much of the time it works fine. Sometimes it does not. I've had a few times when the "does not" could not be made to work by any combination of standard and esoteric power supply bypassing and anti-oscillation tinkering. And I've had a lot of experience at making things quit oscillating, unfortunately.

Experimentation is good. You might find the magic secret of why some inverted grounds oscillate and why some of the oscillations are hard cases that don't yield to fixing. It's good that you are approaching this with open eyes and considering it to be an experiment, not blindly presuming that reversed ground will work.

bajaman · Post by **bajaman** » 25 Oct 2007, 19:52

And I've had a lot of experience at making things quit oscillating, unfortunately.

me too

An amplifier will always oscillate, but an oscillator will not always oscillate

Good points RG - every so often there is something that denies logical or rational explanation.
I recently restored an old Maestro 15w valve amp combo - it was found in a chicken coup, so you can imagine it's state.
I rebuilt it completely - fitted a restored (by me) alnico 12 inch speaker, and fired it up - beauuuuuutiful tone BUT annoying hum, which varied in intensity depending on where the 2 volume controls were set to.
I followed the original Gibson wiring exactly, so I was a bit surprised to hear the hum. I spent hours trying different earthing schemes without success until while using an alligator clip fly lead, I found the perfect earth connection - no hum when the controls set anywhere - bingo!!
So, I cut a length of cable and soldered it in the same place - result: hum again

I finally discovered that the only thing that worked was the alligator clip fly lead, so I cut the clips off and soldered this lead in place - result: no hum

I subsequently measured the resistance of the fly lead - less than 0.2 ohms - still cannot figure out why it worked and yet a similr resistance hookup wire failed.
Sorry, if i hijacked this thread
cheers
bajaman

R.G. · Post by **R.G.** » 25 Oct 2007, 23:19

Not a hijack. More of a correlative support statement.

Not believing in elves, ghosts, or any faeries other than Maxwell's Demon, I have a hard time with not knowing, but this is one that has stumped me a few times.

There is an explanation there somewhere. I believe it's got to do with power supply impedance and grounding, but in spite of being accused as a star-ground bigot and cleaning the power supply to a fair-the-well, I have not been able to stop a few hard cases. These same hard cases quit instantly when reverted to the non-inverted ground.

Wanting not to spend the rest of my allotted time chasing this wraith, I just warn people what they're getting into. One of them will solve it one day, no doubt.

bumblebee · Post by **bumblebee** » 27 Oct 2007, 00:23

thanks for the tips, i'll just try a few variations and see what comes of it.
oscillations not a bad thing imo so long as its not over the top motor boating along with it. i had a motorboat face once and i kinda liked it like that.
i do a fair bit of noise jams between songs so oscillation and feedbacks always welcome.