Tonebender Problems

[p1_ind]

I built a Tonebender MKII but the sound is just not right.
At minimum fuzz it has a lot of fuzz
and as I turn the control up I get tons of
feedback and the fuzz becomes very gated.

The only mods from the circuit that I did was
using a .1uf output cap and 500k volume pot.

I took some voltage readings at the transistors.
|     Q1     |     Q2    |     Q3    |
C| 7.3V    | 103mV  |  8.6V     |
B| 48mV   |  53mv   | 103mV  |
E|  0V       |  0V       |  76mV   |

I have checked the circuit at least 20 times for a problem
but can't find one.

Please help,
Adam

[Fret Wire]

What schematic and layout did you use? What trannys? PNP or NPN?  Were they measured for leakage and gain? Grounded pos or neg? Did you attempt to bias? Make any resistor, cap, or pot substitutions?   Voltages look off.

[p1_ind]

I used the schematic from Fuzz Central
which is the same as at other sites.
But used my own layout.

I used Germanium PNP transistors with pos. ground.

I change the collector resistor on Q2 to 47k,
change the output cap to .1uf, and changed
the volume pot to 500k.

I tried changing the 8.2k on the collector of Q3
to 10k but it only changed a few millivolts.
And I changed the 47k on the collector of Q2 back to
100k with almost no change as well.

I also replace Q2 and Q3 with almost no change in voltage
readings.

Adam

[petemoore]

I always used and liked having a trimpot on the second Q of a FF/ 3rd transistor of a Tonebender collector. It makes it much easier to bias and rebias after transistor swaps.
 The voltages look off enough to warrant going to look for a resistance value, wiring error, or misoriented transistor. Also polarized caps must be oriented properly.
 I would use the techniques in the debugging page.
 Q3's voltage at collector should be around 4.5v to 5.5v, since yours is reading 8.+ volts there, looking for errors around that transistor is probably a good place to start.

[nightingale]

sounds like a biasing issue to me~
hth,

[petemoore]

The small V difference between B/E on Q3...seems like there should be a greater difference there.

[Gus]

The first stage seems to have a low leakage Ge.  You can change the bias a little by adjusting the 100k base to ground resistor.  The input transistor uses leakage bias it does not have enough leakage to turn the transistor on to give you about 5V at the collector.  Try a 220k.  You might want to try a bias network something like the hot silicon at Doug H's site.

For the 3rd transistor try a resistor in parallel  with the fuzz pot start at 2.2k then go up or down  then try a resistor in series with the "bottom" of the pot to ground.  This will show you another way of adjusting bias in a FF circuit.

Circuits that use leakage current are not predictable that is why I tend to build with Si transistors.  Ges can sound very good but can be a problem.  

Gus

[p1_ind]

I replaced Q2 and Q3 with Si PNP transistors and the voltage on Q2 came up and the bias on Q3 adjusted properly but Q1 is not quite right.
I will replace it tomorrow and see if it helps.

If so maybe I may order a set of measured Germanium trannys from Small Bear or if the all Silicon sounds good maybe just leave it alone.

Thank you everyone for there help.

Adam

[Fret Wire]

Throwing in unmeasured ge trannys sometimes works ok, especially if you use trim pots. Unlike a rangemaster, this has 3 trannys, so your chances for pot luck success go way down. When it doesnt work, biasing can be a bionic headache.

The 3 tranny set from Small Bear will be gain matched for the Tonebender, and include bias resistor specs for those particular trannys. You won't be disappointed in the sound.