PNP Ge fuzz face Volume control issue

[swever]

I've build a positive ground fuzz face using some nos soviet low-hfe germaniums. I've faced one issue – the volume control works simultaneously as a tone control - the lower the volume the less treble there is in the output. I tried changing volume pot from 500k to 250k – it seemed to improve a bit but not completely fixed it, and then it also seemed to loose some of the "magic" (not sure about that  ).

I have been thinking about the possible reasons.

1) I used my self designed pcb layout. Maybe there is just something wrong about the layout.
2) I added a polarity protection diode.
3) I added a power filtering cap (perhaps not needed for battery only operation).
4) There's something about the transistors. The fact that they have low hfe (15-30 per datasheet). That the trannies are not in the range that is considered suitable for ff. They are not "matched" for the circuit.
5) Low quality potentiometer. I used the cheapest Chinese stuff from ebay.

The layout:
https://drive.google.com/open?id=0B_LVUlh-0Wv8SGs1MGx3dHY0QWc



I've got a gut feeling that it's an impedance related thing (again this impedance  ) But since I'm so bad at EE I can't even fully understand what that means, not to mention find a solution. If i remember correctly my SI -ground ff did not have this thing (neither had it any power filtering or protection).

I've read somewhere that a treble bleed cap (between wiper and hot lug, right?) may help, but, as far as I can understand, it's more of crutches than a real solution and may screw up frequency and phase response.

Any ideas are welcome 

[Plexi]

As experience... that tone-functioning on a volume output could be a small shortcut before output to ground.
Check carefully there.
Maybe...the extremely low hfe on Q2 could be: did you try there another PNP with high hfe?

[swever]

Will check both options.

Q2 is quite possibly an extremely low hfe. Datasheet says it's "≥12". In fact I just literally grabbed the first trannies from the box with pnp ge and soldered them to the board

[Plexi]

Oh...huge mistake!
You ALWAYS need sockets here...much more on vircuits where the hfe is an important factor

Sent from my HUAWEI Y600-U351 using Tapatalk

[swever]

Measure twice, cut once.
Not
The good thing is that I can try to desolder the bad boy and hope the heat does not damage it, haha.

[Kipper4]

Measures can be taken to lessen the effect of heat. Clamp needle nose pliers to the transistor legs to sink the heat while desoldering.

[swever]

So, I successfully desoldered Q2 (without killing it  ) and tried replacing it with higher hfe one. In fact I tried several different trannies, including silicon 2n3906 which clearly in not a low hfe device. No difference in volume pot behavior.

I also tried removing the "power filter capacitor" and polarity protection diode - no audible difference whatsoever.

I examined the pcb carefully, both visually and with continuity tester and was not able to find any solder bridges or anything suspicious:





I will try to change the pots to different values and also try higher quality alphas.

Can't think of any other possible reasons or solutions. This thing really drives me to a nonplus...

[patrick398]

I seem to remember having a similar problem when i built a fuzzface, albeit a silicon one. I had a thread going about it but i think it was on another forum (please forgive me) so i'll try and dig it out

[Plexi]

Dont get me wrong: but I clearly see from where the problem comes from in your last pic.
I'm 99% sure that there's some ground short cut.
Try not to cover the pcb with solder next time :-)

[patrick398]

My problem was with the fuzz pot doing a similar thing so not much help i'm afraid.
I've never etched a pcb so am not really in a position to pass comment here but i thought when you etched a board the idea was to have it so that the traces between components were the only conductive parts of the board? Like this...

http://4.bp.blogspot.com/-uY6W30pzN3k/UEuNmRT9J5I/AAAAAAAAAKA/0W6PQYl8YCg/s1600/etched+PCB+not+drilled.JPG

[swever]

Plexi, may I ask you to specify where exactly you think there is a ground short cut? I checked everything again now and I am pretty confident there are no short cuts.
I do tin all my pcb's. Do you think it's a bad practice?

Patrick, I just dug out my old silicon ff. Found out it behaves the same way. Would be great to see that thread.

I tried lowering the volume pot value to 100k and it seems to do the job, actually. It's late here already so I will continue testing that tomorrow.

[swever]

It's a ground plane. It reduces the risk of ground loops and even helps to reduce noise. As a bonus you need to etch less copper with a layout like that.

[patrick398]

Sadly i didn't actually get any useful answers on that so it's probably not worth reading. It's been a long time since i built it as well so i can't remember if i did manage to fix it haha
Good luck!

[swever]

Found this thread with some info on the topic:  http://www.diystompboxes.com/smfforum/index.php?topic=95872.0

[Plexi]

My humble experience: cover the pcb's copper with tin, only made things worst.
More if you have a small separation between ground and all the other traces.
Maybe with a better design, like the one that Patrick shared early it could be usefull.

Did you try conductivity with tester? Maybe it can be confusing now that you have soldered component.
That test is better when you have the pcb with nothing.

I'll edit/mark where I think the problem could be...if it isn't any pcb external shortcut.

[Plexi]

I'm 99% sure that's a ground shortcut.

[swever]

Just checked again - there are no bridges. All the areas you marked in the picture as well as everything else has good clearance. There are no short-cuts in the off-board wiring either.

And yes, as stated earlier, I have checked the board a few times with a continuity tester - first after the etch, then after tinning, then after soldering.

As for tinning, it never caused me any problems - I've etched over a hundred pcb's in the last couple of years and all but two of them were tinned like this one with a soldering iron.


The more I read about this issue the more I realize it's a peculiarity of this circuit. Many are noticing this, including owners of commercial dunlop products and some clones. Apparently the choice of original volume pot value of 500k was arbitrary. 100k seems to work better for me.

[zombiwoof]

I saw a schemo of the Roger Mayer FF on a site years ago (which seemed to disappear later).  I noticed that he used what appeared to be a treble bypass cap on the volume pot, like you would put on a guitar pot, I used a .001uf although I think he used an .002uf, and it worked to retain the highs as you turn down.  I've never seen anyone mention this before.
Al

[swever]

Actually I did see the idea of adding a treble bleed cap to the volume pot somewhere. Will have to try that and see how well it works.

[Plexi]

...but....did it work?