Need help biasing Q2 in a basic wah circuit

Started by Tone-Analyst, January 28, 2016, 09:41:59 PM

Previous topic - Next topic

Tone-Analyst

I decided to pop the lid on my old Blubber Crying Baby and check the transistor voltages. Q1 has textbook values but Q2's base voltage is lower than the emitter.

Q1
B +0.69v
C +4.44v
E +0.10v

Q2
B +2.74v  (should be 3.4v - 3.6v)
C +8.91v
E +3.27v

I bypassed the 470k resistor just before the base of Q2 with a pot, I'm down to 200k total to get the base of Q2 to reach 3.6v and pass above the emitter voltage, which also crept up as the base voltage went up.

Should I play with the 1k collector resistor or the 10k emitter resistor ?
Thx!!

Schematic:

http://www.aronnelson.com/gallery/main.php?g2_view=core.DownloadItem&g2_itemId=47120&g2_serialNumber=2

PRR

Your volt-meter has a 1 Meg input.

With Q2 base resistor at 470K (0.47 Meg), a 1 Meg load causes a large drop *while* the meter is connected.

If you have two meters, watch Q2 Emitter as you poke Base. E goes down, then comes back up.

I don't see a problem with those voltages, though some Wah-guru may have tips.
  • SUPPORTER

Transmogrifox

I'm suspecting your measurement at the base is being strongly influenced by your meter's internal resistance.  It wouldn't be possible for B to be less than E in this circuit and still have 3.27V on E unless something was severely damaged.

Because everything else in the circuit measures just about perfect I would say the problem is your meter.  My bet is it's 1Meg input.  Against the 470k to Q2 base this makes a significant drop to ground.

I'll bet if you measure from Q1C to Q2B you will see something less than 100mV drop.  Then subtract that from Q1C meaduurement and that will be closer towhat it actually is.
trans·mog·ri·fy
tr.v. trans·mog·ri·fied, trans·mog·ri·fy·ing, trans·mog·ri·fies To change into a different shape or form, especially one that is fantastic or bizarre.