Neg Ground PNP FF Issues

[Leila VE]

Hi all,

Sorry to beat a dead horse but I can't quite find my problem in old posts.

I built GGG's negative grounded PNP FF and it's having huge oscillation problems:

It squeals when the fuzz is turned up passed 270 degrees (~2:00), it has weird oscillation when the pup selector switch is placed in the neck/middle or bridge/middle positions on a strat but is fine on any position on an ES-335, when run on battery power it sounds like a blanket has been thrown over the amp, and when run on a pedal board (known good connectors - no other pedals on the board) I get loud AC hum but the fuzz sound sounds more "right" than with a battery. I've checked the wiring quite a bit and cannot find any mistakes. The layout looks correct according to JD's recommended one (negative ground, PNP, 3PDT, LED).

Voltages:
9V supply = 8.8 VDC
Q1 C = 8.7
Q1 B = 8.6
Q1 E = 8.4
Q2 C = 8.7
Q2 B = 4
Q2 E = 8.4

Also, the metal can tops of each transistor read the same voltage to ground as the base of each transistor (normal?)!

I've built a few tube amps and am just getting into stompboxes, so I'm a complete newbie to this stuff but not a total newbie with building electronics in general.

So, I see in previous posts that there are a lot of issues with this negatively grounded circuit. But I'm not sure if the voltages I'm seeing on the Q's can be attributed to that or not. Should I just give up and build it positively grounded?

Thanks for any help you guys can offer.

Morgan

[petemoore]

Hi all,

Sorry to beat a dead horse but I can't quite find my problem in old posts.

I built GGG's negative grounded PNP FF and it's having huge oscillation problems:
  I for one don't know what the fix is other than what I Do know...conversion to Pos Gnd Fuzzface for PNP's works when wired correctly, every time I tried, there are numerous problem posts such as yours in the past, but as I say I don't recall a solution for the circuit's problems.
It squeals when the fuzz is turned up passed 270 degrees (~2:00), it has weird oscillation when the pup selector switch is placed in the neck/middle or bridge/middle positions on a strat but is fine on any position on an ES-335, when run on battery power it sounds like a blanket has been thrown over the amp, and when run on a pedal board (known good connectors - no other pedals on the board) I get loud AC hum but the fuzz sound sounds more "right" than with a battery. I've checked the wiring quite a bit and cannot find any mistakes. The layout looks correct according to JD's recommended one (negative ground, PNP, 3PDT, LED).

Voltages:
9V supply = 8.8 VDC
Q1 C = 8.7
Q1 B = 8.6
Q1 E = 8.4
Q2 C = 8.7
Q2 B = 4
Q2 E = 8.4
Also this Feedback loop amp [FF] is not biased, all three pins of Q1 are near supply voltage...Q2 collector should bias around 1/2 of supply voltage, I suspect a wiring error...misvalued resistor...
Also, the metal can tops of each transistor read the same voltage to ground as the base of each transistor (normal?)!
  Reads at GEO reveal bias Info's, the base needs bias voltage of ~>.6v from emitter reading, Q2C should be around 1/2v or 4.5v...I disregard the polarity, as it is the 'interval's or ratio of the bias that needs looked at. 
I've built a few tube amps and am just getting into stompboxes, so I'm a complete newbie to this stuff but not a total newbie with building electronics in general.
  I'd start with a Pos Gnd PNP FF, or get some NPN Ge's and do up a Neg Gnd NPN FF...unless you can figure out whats happening with the circuit you've chosen.

So, I see in previous posts that there are a lot of issues with this negatively grounded circuit. But I'm not sure if the voltages I'm seeing on the Q's can be attributed to that or not. Should I just give up and build it positively grounded?

Thanks for any help you guys can offer.

Morgan

[R.G.]

I tell you three times - negative grounding PNP positive ground effects will sometimes give you intractible oscillation.

Sometimes you get away with it. Sometimes Mother Nature simply says it won't work. I don't have enough experience, skill or insight to tell what separates the ones that work from the ones that don't without building them, so I consider it a bad idea, fraught with peril.

It would be much better to start with a positive ground PNP FF.

As to dead horses, I'll post some wisdom on those.

[puretube]

Welcome to the forum, Morgan! 

[col]

Mine makes a humming noise if the battery voltage falls much below 9v. It doesn't occur if I use an adapter. Apart from that it's fine with both pots on full but I have had similar problems with other pedals like this. Some have been solved by putting a cap across the base and collector of the first transistor. Start with one around 47pf and work your way up to around 1n until the oscillation/squealing disappears. This can adversely affect the tone if the cap gets too big. Also, check your wire lengths inside your box. Shorten them as much as possible and I presume that you are using a metal box to house it in. It needs the box to shield the circuit.

Col