Anybody doesn't mind checking my layout for errors?

[swever]

I've asked about converting from NPN to PNP in the unofficial facebook group and a great guy named Mark (Mark, are you here too?) kindly offered his vision of the conversion after asking me for the schematic. Since I am learning to design pcb layouts (using diylc for now) I made an attempt to do just that.

All the images – my layouts (the "original" and then two versions of the modded board arranged for PNP and NPN respectively), two versions of the schematic, and Mark's hand-drawn mod suggestion are available here: http://imgur.com/a/SB6vv

The blue rectangles are JST-XH connectors that I want to try for all off-board connections this time. I know my layout is clumsy and it uses way too many jumpers (some on the copper side). But then it's my second attempt ever and it's quite compact. What worries me more is that I am not sure it's correct. I guess I have spent too much time on it to me able to spot any errors anymore. That's why I'd be grateful if you guys would have a look.

[Fancy Lime]

Hi Alexander,

sorry I don't quite get what you are trying to do. Do you just want to convert the circuit to use PNP transistors? For that you do not need to change the layout. All you need to do is flip all polarized devices (diodes and polarized caps) and the positive and negative leads from the power supply to the board. And of course install the PNP transistors the right way around. It is important to note that everything before the input coupling cap and everything after the output coupling cap  (or 3 output coupling caps in this case) needs to stay as it was in the NPN version (else you cannot use a negative ground power supply). So the schematic changes quite a bit, but the layout can stay exactly the same. The only thing that is strange in the layout to me and that might need changing is the Volume pot. It seems only the 2nd and 3rd leg of the volume pot are connected to anything, and the 1st is not. That would sort of work in conjunction with a low inpt impedance of a following device (~100k) but thats probably not what you want. You would want to connect the 1st leg of Volume to the negative lead of the power supply (for a negative ground device).

Unless I misunderstood what you are trying to do. What PNPs would you like to use? 2N2907A?

Cheers,
Andy

[swever]

What I was trying to achive is a negative ground circuit with PNP transistors. The idea for this type of conversion was suggested by the guy from facebook group. The last two hand drawn schematics are his work.

That's why some of the tracks from the "original" version of the layout (that follows the "original" schematic) are replaced with jumpers to allow to configure it for both pnp and npn, while staying negative ground.

You are absolutely correct about the volume pot. Lug 1 must go to ground. This is going to be another jumper 

I was planning to use some soviet NOS germaniums that I grabbeed locally at a flea market not too long ago dirt cheap, namely МП25(MP25), МП39(MP39, МП41(MP41)

[Fancy Lime]

As far as I can tell, the handwritten schematics seem to be correct but they are unnecessarily complicated if you want to use the same layout for NPN and PNP variants. I will try to explain things a bit using the parts numbering of second of the printed schematics (the black one). The hand-written schematic flips everything between C1 and C3/C9. That means that C4 is connected to negative ground in NPN and PNP versions but all the other parts in there that were connected to V+ are now connected to V- and vice versa. If you want to use the same layout for NPN and PNP you can also connect C4 to V+. Everything that would "normally" be connected to negative ground and is DC decoupled from the signal path via a cap, can just as well be connected to V+. To a pure AC signal, V- and V+ are the same thing. That means that you can simply flip the whole circuit between the input and output caps, in your case between C1 and just before the volume pot.

The thing with the whole ground conversion is, that *zen on* there is no ground *zen off*. What we call "ground" is just a reference voltage that we need for different devices to communicate because voltage is not defined as an absolute value, only as a relative one. Relative to "ground". But we only need that between devices. Inside an amplifier or a stomp box (which is also a kind of amplifier in that sense), we get rid of this reference voltage by using the input coupling cap. Then we define a new reference between the two rails (V+ and V-). This is called biasing. In fact, we need to do this for each separate stage inside the amplifier. only when the signal comes out the other end, we need to reference it again to the same reference voltage as on the input because the rails do not exist outside. For everything to work we also need to reference the internal reference (the rails) to the external reference ("real ground"). The easiest way to do that is to just tie one of the rails to ground, usually negative. The only important thing is that we do this the same way for all devices that are supposed to be powered by the same power supply. So its best to think that "ground" only exists outside the input and output coupling caps. In between, there are only rails. And anything DC decoupled can be tied to any constant voltage, V+, V-, V1/2, anything. For a AC signal, these are all equal because V+ and V- are connected via the battery or power supply, which from an AC perspective look like a gigantic capacitor.

So in practice: Flip the whole circuit. That means, what is the V+ connector to the NPN board becomes the V- connector to the PNP beard and vice versa. Flip all polarized devices and install the transistors appropriately. The ground connection of the input and output jacks as well as the 1st leg of the volume pot go to V-. Thats all there is to it.

If I were you, I would not make one board that can take PNP and NPN and needs switching. That makes things a lot more complicated because there are many polarized caps. And I don't see the benefit of it. I would just make two versions if I wanted to A-B compare NPN and PNP.

BTW, I really like the idea of a "soviet mammoth" (Шерстистый мамонт, maybe?), especially because Siberia is where the last Mammoths survived until less than 4000 years ago.

Cheers,
Andy

[swever]

Sooo after a couple of months I finally found time to get back to this project. Sickness leave does have it's perks 
Fancy, I think you are completely right. No need to over-complicate. Especially now that I have some high gain NPN germaniums.

I've tried tracing my pcb layout, found one error and fixed it. It seems to me it should be correct now.

https://i.imgur.com/17Zn8ne.png

I'd be happy if you guys had a look before I print and etch this.