2 transistor fuzz and volume pot clean up ?

[leeyoungun]

Just modified my 90s RI Vox Tone Bender. Replaced all of the caps with "mojo" caps, installed external bias pot and transistor sockets with a pair of NOS OC76-95 and 135 hfe with leakage below 150. Clean up with the guitar volume pot was quite bad before and still remains quite poor with the upgrades. Sounds awesome at full bore, but dialing back the volume pot makes for a gated overdrive tone. What factors come into play with great clean up?

[Schappy]

Is this a 2 transistor circuit?

If it is you usally want Q1 to be around. 70-80 and Q2=100-120.

For the 3 transistor circuit Q3 should be similar to Q2.

[leeyoungun]

Yes 2 transistor circuit. Higher gain transistors shouldn't cause gating with the guitar volume rolled back though?

[Gus]

How did the Vox sound stock?

Did you measure the transistors voltages before and after?

A schematic or link to the circuit ?

[tubelectron]

Hi leeyoungun,

installed external bias pot and transistor sockets with a pair of NOS OC76-95 and 135 hfe with leakage below 150

1 - your biasing may be badly adjusted,
2 - the leakage is far over 150µA (have you checked it with suitable equipement ?),

I think that the 1st point is probably the key to turn.

A+!

[leeyoungun]

The Vox sounded good before, but very unstable in hot temperatures. Gated with the guitar volume turned down. Old transistors were in the 150-160hfe range with leakage over 400. Haven't checked voltages, just biased by ear. Transistor gain and leakage was checked with a Atlas Peak DCA tester. I'm going to test the voltage and try lower gain transistors. Thanks for the replies!

[leeyoungun]

BTW, I followed the Fuzz Central schematic, excluding the values/"improvements" in blue.
http://fuzzcentral.ssguitar.com/vox.php

[tcobretti]

You need to do some research into fuzz face biasing.  That circuit is a fuzz face variant, and it has been discussed here a great deal.  I'd just replace the collector resistor on Q2 with a 10k-20k trim pot and tune by ear (although I've seen people replace the Q1 collector resistor as well).

[petemoore]

  Try different Q's and bias-set for sound - temperature drift, ie set for current temperature.
 Panel mount bias pot.  
A.  Q2 collector resitance = 5k6 [or so] fixed resistor + 10k linear pot.
B.  [or smaller 4k7 fixed/stop resistor +20k pot for off-bias ability]
  A accomodates most of the useful transistor bias points [5k6 through 15k6].
  B allows greater R sweep, say 4k7 through 24k7.

[leeyoungun]

Was a bias issue. Thanks Bruno G. Adjusted the external bias pot with the guitar volume pot backed off and voila. Goes from nice mild overdrive to full on buzzy, saturated fuzz.