fuzzrite PNP germanium with negative ground, possible?

[Frances Rhodes]

hi everyone

i want to build a germanium fuzzrite, but i want people to be able to use it with a daisy chain (eventhough i hate them things...) without any use of a charge pump (cause i really don't like those!!), so i'm wondering if making a PNP negative ground version was possible.
i took the schematics for the fur's rite and drew it on eagle, and i did the same thing that was done on the PNP fuzz face and PNP rangemaster to make them negative ground.


one thing bothers me, though: on the silicon version, there is a single collector resistor on Q2, but on the germanium version there are 2, with a cap (C6 on my drawing) in between going to ground, which is positive in that case. so i don't know if i shall replace that one ground with a +9V or leave it become negative ground.
shall i leave the schematic like this or do i have to change that ground for C6?
or, that whole idea to make a PNP fuzzrite negative ground won't work for some reason i'm not seeing?

cheers!

[tonyharker]

Why not use NPN Germanium transistors, and reverse the polarity of the caps and the supply?

[Frances Rhodes]

well, because all i can find is PNP germaniums.
but otherwise, that would have been the best solution, by far!

[nepalnt21]

small bear electronics - they have the transistors in question

[Frances Rhodes]

yes, i know where to find npn germaniums online but the shipping overseas will be at least 5 times the cost of 2 transistors, and i don't want to buy a whole lot of them in order to save on postal fees, that's why i'm trying to convert this circuit from positive to negative ground.
if i'm sure i won't fry the transistor if i make a mistake, then i'll try and breadboard it, but i can't risk to kill my transistors cause they're f***ing expensive here and i don't have many of them.

[R.G.]

This is one of those ever-recurring questions - can/how-can I run PNP germanium distortion pedals with negative ground instead of positive ground.

The simplest and brute-force way is to just wire the negative ground wire to the circuit as a "negative power supply" and wire the "positive ground" as to the +9V. This can work. But I recommend you not do it. I've spent quite a lot of time trying to make this work for all situations, and the bottom line is that reversing power and ground works sometimes, but not always, and there's no good way to predict whether it will work or not. When you can't make this work, the higher the circuit gain, the more likely it is to oscillate or just break into full-bore hissing.

You can make some of the cases work with big bypass capacitors on the power supply, 1000uF or more; some cases can be helped by careful wiring and grounding and high frequency ceramic bypass caps. Some cases can be helped by tinkering the resistors on the transistors, by inserting small resistors in the bases or emitters - basically, all the high frequency radio circuit tricks to stop oscillation.

The problem is that simply reversing power and ground works often enough for people with little experience to get one right, the go tell the whole internet that it always works. It doesn't.

To fix this, the most conceptually straightforward way is to use NPN germaniums. You've already discounted this as ecomomically unpleasant.

The next most simple thing is to just provide a separate power supply that's correct for the pedal, and not try to use it hooked to the same negative ground power that all the other pedals are using. This can be  one of the better AC to DC power adapters designed to be quiet with pedals; these can be under US$20 in many places. The cheap, noisy ones are not work any amount of money as they will hum and whine in your pedal's sound. You could build your own power supply regulator from a "wall wart" you get in a second hand store and put a regulator in the pedal, things like that.

The deluxe solution is to use a charge-pump power supply inverter chip. This would take in +9v and ground and MAKE -9v on the same ground. I have designed this into commercial pedals and it can work very well. More complicated, but probably cheaper than a separate power supply.

[pinkjimiphoton]

everything rg said in spades. if ya DO get it to work, and i have, often when ya turn your guitar knob all the way down it will break into a LOUD oscillation that changes pitch with your knob.

i've put 22,000mF caps in across the rails. helps some things, but still not stable, and the damn cap is bigger than the circuit!!

charge pump is the way to go, but make sure its not like a 7660 or something, the whine from them is audible and annoying,. i dunno what the current darlings are for these DIPS, but i like the staunch old 1044 in most cases. can't handle a lot of current, but no cp whining.

most of the chinese kits they sell on ebay are pnp with npn wiring. they work .... or not... 50% of the time fromm my experiments with them, but the q's tend to blow out really easy with the reverse ground deal too for some reason. way above my paygrade.

[Frances Rhodes]

hi R.G.

thanks a lot for that detailed answer!!
i knew that reversing power doesn't always work, but i didn't know in what way. but as long as i don't blow everything i might as well try and learn. but if i'm not sure i won't risk it.

the thing is i intend to build this pedal for someone else and i don't put battery snaps in my pedals for many reasons, and some of them guitar player don't understand why they should use a dedicated PSU for some pedals (some even don't understand why they shouldn't buy cheap unregulated wall wart...), that's why i want to try to build negative ground whenever it is possible.

for this circuit i didn't just change every ground to +9V and vice-versa. i figured that some grounds needed to stay grounds and not become a +9V, for pull-down resistors, that go to ground whether it is positive or negative, the "filter" resistor R5, and the volume pot also (lug 1 to ground in either case). and C6

i read a few things about charge pumps (mostly bad things about switch mode power supplies that hiss), and that their PCB needed to be carefully designed or it could cause a lot of noise, that's why i was skeptical about trying them, since it's for high-gain pedals that may already be a bit noisy.
i thought of building pedals that plug directly to main power, with a small transformer inside, but that would drastically make them bigger as all the transformers i found were rather big, even for very small output power.
but if you say charge-pump is the deluxe solution then it changes a lot from my point of view!!

would i need to build it on its own PCB or "daughter board", apart from the circuit?
and it's good to know the 7660 chip is bad compared to the MAX1044

EDIT: i just notices that my drawing showed NPN transistors labelled PNP, i'll update the schematic so that it shows PNP transistors and no one gets confused!

[Frances Rhodes]

here is a new drawing showing pots and PNP transistor symbols

[italianguy63]

I do tons of charge pump powered PNP pedals.

My go-to charge pump chips are the TC1044S and the 7660S.  The "S" designation means they are high frequency-- and DO NOT whine because the frequency is higher than you hear.

MC

[italianguy63]

If you choose to go the Charge Pump avenue...

Here:  This will help you.





MC

[Frances Rhodes]

thanks a lot!
i'll try that!