Vox ToneBender

[phillip]

In the original pre-Fuzz Face Vox ToneBender circuit (the one that came with OC76 and SFT transistors) there's an 820-ohm resistor from Lug 1 of the Attack pot to ground.  Does this resistor prevent the Attack pot from grounding out and causing oscillation?

I've built a few Silicon Fuzz Faces over the years and some of them have oscillated when the Fuzz pot was turned all the way up (i.e. minimal resistance).  Would an 820-ohm resistor help cure oscillation in such instances?

TIA!
Phillip

[petemoore]

Probably sets the minimum gain, maybe so the effect keeps working even with the pot turned all the way down?

[Gus]

Take a look at the 3 tran or rocket.  The SF and VTB gave me the idea of the bias pot.

  The 820 is to bias up the Si tone bender not to gate.  I made it a pot  for fine adjustment.

Look for the C D SF schematic at effectronics.

 I saw the use of the 820 ohm in a VTB schematic I think R.G. drew.  I also looked at a friends texas square face that used a 2K gain pot.

Did some thinking read some books from the 60's about transistors looked at hfe vs Ic curves....................

 The 820 will not affect AC gain it is always bypassed by the cap connected to the wiper of the gate pot.

 FF are layout and power supply bypass cap sensitive, add a battery bypass, cap check you wiring.  Sometimes dpdt switches allow some signal leakage from out to in.

[phillip]

It's interesting because the layout that I'm using is more or less a duplicate of the original Dallas-Arbiter circuit board, with the addition of an input pulldown resistor.  Other than that, they're the same.  I've always used a 2K pot on all my FF builds...but that might only be a positive thing on the Germanium models...maybe it's not such a good idea in Silicon versions.

I wonder if the original Silicon D-A Fuzz Faces had oscillation problems?

Phillip

[Lonestarjohnny]

Phillip. I got one old D/A FuzzFace and it never has, I also have a old Classic fuzz by Roger Mayer and the Mayer is a bit more agressive than the D/A, but I have never had them to oscillate on me. they will change tone with a change in temp. though.
JD

[phillip]

JD, is your D-A Fuzz Face a Silicon or Germanium?

Phillip

[Lonestarjohnny]

Germ. Phillip,
JD

[brian wenz]

Hello Hello-
   Stock Si Fuzz Faces  [Dallas Arbiter] have oscillation problems on high fuzz settings.
Brian.

[RDV]

Stock Si Fuzz Faces[Dallas Arbiter] have oscillation problems on high fuzz settings.

Yes, cause they used 2 relatively high-gain transistors(of the same type) together. It seems strange to me that they never figured out that all they had to do was pick a lower gain transistor for Q1. Hell, I figured that one out in my first month at DIYing. I make a mean little Si FF with a 2N4401 or 2N2222 or similar @ Q1, & a 2N5089 or similar at Q2. I mean, it wouldn't have cost them a dime other than having to stock another tranny. You don't even have to gain match them, it's already done for you.

RDV

[phillip]

Hmm I have a few 2N2222 with gains of around 120, and some 2N2222A with gains of 200 or so...I bet those would be the ticket.

I ordered some PN2369A from Mouser, which Jack Orman used in his YAFF.  The data sheet says that their gains range from 40 - 120, but all 30 that I checked had gains of between 50 to 53...every one of them!

At least I now know that even the original D-A Silicon circuits had the same osciallation problem

Phillip

[brian wenz]

Hello RDV--
  Yeah, they wanted to make sure that WE had something to figure out!
Brian.

[RDV]

Hello RDV--
  Yeah, they wanted to make sure that WE had something to figure out!

There is a certain charm to the sound right before the oscillation too though. Allows you to go right to the edge of the cliff, so to speak.

RDV

[phillip]

Yeah sometimes I can't believe that they used such high gain transistors in the Fuzz Face circuit, especially the BC108C and BC109C.  Even the BC183L is pretty gainy.  I've tried the BC108B, which has pretty much the same gain range as the 2N4401 (200 - 250 or so), and it sounds pretty good...maybe still a little too gainy.  The only one that I haven't been able to get ahold of and experiment with is the BC209C.  One day I'm gonna track some of those down...

BTW, I just popped in some AC188s that I got from Pete Roberts into my Ge. Axis Face and they sound great.  Very smooth sounding, and they don't have that "fizzy" sound that some Ge. transistors have.

Phillip

[brian wenz]

Hello Hello--
     What interests me is that the original Vox [and Sola] Tonebender used two different germs in it's circuit [SFT 337 and SFT 363E].  Looks like SOMEBODY knew what they were doing back then!
Brian.

[phillip]

Whoever was putting those Vox ToneBenders together had the right idea, and since it predates the Fuzz Face, I wonder why the Dallas-Arbiter guys didn't pick up on the idea too?  I guess they were trying to save a few pennies, or to make assembly faster, or both  I guess having only one transistor type in one parts bin streamlined things a bit...at least until the Silicon years when they used 5 or 6 different types.

Phillip

[brett]

Hmm I have a few 2N2222 with gains of around 120, and some 2N2222A with gains of 200 or so...I bet those would be the ticket.

Have a look back at the thread on "piggybacking" transistors.  In a Fuzzface, it's really good to get hFE on Q1 at about 70 for optimum input impedance.  Try 2 x 2n5089s or similar high gain trannies, with their bases soldered together, a 15k resistor between their emitters and the collector of one cut off.  Treat the little package as a singe tranny for Q1 - I'm sure you'll love it.

The concept is shown here, but with "tunable" hFE for Q1 and Q2. http://members.optusnet.com.au/~jethro.dog/mainpic6.jpg