transistor biasing - aclam mocker fuzz

[SprinkleSpraycan]

hi all. i breadboarded this beauty of a fuzz and need a little help.
D1 and D2 are 1n4001
Q1-3 are BC238

Sounds amazing, but i would love for the swing of the bias control to go from 3v to 8v. Im measuring this at the collector to ground.

Stock parts gets me to 2v58 - 6v83.
I replaced r9 with a 2k7. this got me pretty close. 3v06 - 7v24.

Ive done what reading i can about the swing of a transistor but im lost. how to i get to 8v without also moving away from the 3v? the way it is sounds good but id like to know how to take it all the way to the finish line.

[GGBB]

Bias is affected by R9 (which also affects Q2 gain) and R10. But it is set via the bias control, R7, and R8 which create a voltage divider, so you need to adjust R7 and R8. It's very unlikely you'll be able to hit exactly 3.0v-8.0v (unless you use trimpots for R7 and R8 - and even then power supply fluctuations may apply) so aim for <3v to >8v.

[SprinkleSpraycan]

Bias is affected by R9 (which also affects Q2 gain) and R10. But it is set via the bias control, R7, and R8 which create a voltage divider, so you need to adjust R7 and R8.

Affected and set are two keywords I've not had highlighted to me before. Just that alone is a helpful breakthrough.

How does that differ from transistors that have a resistor between collector and base?

[GGBB]

How does that differ from transistors that have a resistor between collector and base?

It's the same - that resistor would affect bias but it's not really how you set the bias.

[JustinFun]

Off topic from the original question but on-topic for the schematic - I thought the OA200 diodes had a voltage drop more typical of other silicon rectifier diodes - 1.1v or so? Is 0.6v really correct?

[GGBB]

Always check the device data sheet including relevant graphs. I found 0.9V "typical" for OA200. Diode Vf is sensitive to If - the lower the current, the lower the Vf. So 0.6 might be accurate for this particular circuit - with no DC bias on them the diodes only see AC signal current. Diode data sheets usually include a graph of If vs Vf.

[SprinkleSpraycan]

Off topic from the original question but on-topic for the schematic - I thought the OA200 diodes had a voltage drop more typical of other silicon rectifier diodes - 1.1v or so? Is 0.6v really correct?

That detail was already apart of the schematic. I settled on 1n4001 simply from auditioning diodes on hand.

[Rob Strand]

3V to 8V mod.

Gets 8V without messing with the circuit too much.

[R.G.]

Good commentary Gord!!

>> and good comment, Rob. Cheaper and more expedient addition with one resistor.

@SprinkleSpraycan ::
Bipolar transistors with an emitter resistor (like this) set the current through the collector-emitter by the voltage on the base.

What follows here is an exercise in using addition and subtraction of voltages along with use of Ohm's law to get the voltage and current across resistors once you calculate the voltages and currents. Ohm's law is incredibly useful for this kind of work. The only other factoid needed for these calculations is to know that for high gain (hfe > 100) bipolars with an emitter resistor, the transistor will make the emitter voltage be one silicon diode drop (0.5 to 0.7V) lower than the base voltage.  That's all the magic insight needed for the stabilized bias circuit - which this is. The results are accurate to the extent that resistor tolerances let them be, and the accuracy of exactly where Vbe is in the range.

For a 1.2K emitter resistor, the current through the collector will be approximately (Vbase - 0.5V) / 1.2K.

The conditions you're asking for - collector at 3V to 8V require (9V-3V)/3.3K = 1.8ma through the collector resistor at 3V and (9-8)/3.3K = 0.303ma at the 8V end.
That is the same current (very nearly) that the emitter sees if it's a high gain transistor; so the emitter voltage at 3V on the collector is 1.2K * 1.8ma = 2.16V. The base voltage to get there is about 0.5V higher, or 2.66V.
At 8V on the collector, 303uA flows through the 1.2K emitter resistor, making the emitter voltage be 1200 * 303uA = 0.3636V, and the base voltage has to be 0.5+0.3636 = 0.8636V.
So your biasing setup has to provide the base with 0.8636V to 2.66V.
The base to ground resistor R8 is 3.3K. Ohm's Law again: at 0.8636V, the current in R8 is 0.8636/3300 = 262 uA; for 2.66V, the current it needs is 0.216/3300= 806uA.
So the upper resistor, R7 plus the pot setting has to be no less than (9V-2.66V)/806uA = 7866 ohms and no more than (9V - 0.8636V)/262uA = 31054 ohms.
So to get a range of collector voltages at the collector of 3V to 8V, you need R7 to be 7866 to 31054. Since R7 is already 5600, you could leave it at that. The pot needs to be bigger. Subtracting 5600 from 31054 gives 25454 ohms. A 25K pot would do the trick, probably. Biggest sources of error will be if the transistor Vbe  is really 0.6V or 0.7V at these currents.

[pacealot]

I feel compelled to interject that, for the Mocker, which is based on the original 7-series Vox amps' distortion circuit, Aclam chose for some reason to model the second transistor stage after an error in one version of the schematic. All the 7-series preamps I've been able to see detailed photos of have had 68k at R7 (sans bias trimpot) and 47k at R8 (putting 3.3k there was the original schematic error). Perhaps this combination sounded good to the folks at Aclam, but it doesn't represent the correct bias of that stage in the original amps upon which the Mocker is modelled. Just something to consider.

[SprinkleSpraycan]

Since R7 is already 5600, you could leave it at that. The pot needs to be bigger. Subtracting 5600 from 31054 gives 25454 ohms. A 25K pot would do the trick, probably. Biggest sources of error will be if the transistor Vbe  is really 0.6V or 0.7V at these currents.

I had tried this already, however out of "just try stuff" and a hunch. A B25k got me to a desirable 8.6v but the sweep is now awful. Your comment made me glad to know I wasn't on the wrong track. So I ditched the linear taper and replaced it with an audio taper. Now the sweep works like a dream.

Robs suggestion is also very effective. Certainly not an idea I would have had. I will play for awhile and see what works best.

I can't thank you enough for the thorough answer.

[SprinkleSpraycan]

I feel compelled to interject that, for the Mocker, which is based on the original 7-series Vox amps' distortion circuit, Aclam chose for some reason to model the second transistor stage after an error in one version of the schematic. All the 7-series preamps I've been able to see detailed photos of have had 68k at R7 (sans bias trimpot) and 47k at R8 (putting 3.3k there was the original schematic error). Perhaps this combination sounded good to the folks at Aclam, but it doesn't represent the correct bias of that stage in the original amps upon which the Mocker is modelled. Just something to consider.

Cool factoid! I'll play with that as well. If for no other reason than historical accuracy.

[pacealot]

What I found when I played with it is that the significant lowering of gain from using the 3.3k resistor was actually useful for less intense (i.e. more, dare I say, "Beatley") fuzz sounds, but I also didn't get a terribly useful range of bias from the Aclam trimpot arrangement. And when I realised that it thoroughly compromised the circuit's usefulness as a clean preamp, I went back to the stock resistor values and began playing instead with the "fuzz only" components (those turned on by the original amps' footswitch) for biasing/gating, and I found those to be a lot more useful for tweaking the fuzz characteristics. A good example is in the Creepy Fingers Lonely Heart fuzz (Sergeant Fuzz in PPCB parlance) – the "biasing" or semi-controlled gating is done at the emitter of Q2 rather than at its base. Something else to play with, potentially...

[Rob Strand]

had tried this already, however out of "just try stuff" and a hunch. A B25k got me to a desirable 8.6v but the sweep is now awful. Your comment made me glad to know I wasn't on the wrong track. So I ditched the linear taper and replaced it with an audio taper. Now the sweep works like a dream.

Robs suggestion is also very effective. Certainly not an idea I would have had. I will play for awhile and see what works best.

The 25k A taper is going to work better.   The addition of the 10k resistor was only a way of getting 8V without changing the pot.

[Rob Strand]

I feel compelled to interject that, for the Mocker, which is based on the original 7-series Vox amps' distortion circuit, Aclam chose for some reason to model the second transistor stage after an error in one version of the schematic. All the 7-series preamps I've been able to see detailed photos of have had 68k at R7 (sans bias trimpot) and 47k at R8 (putting 3.3k there was the original schematic error). Perhaps this combination sounded good to the folks at Aclam, but it doesn't represent the correct bias of that stage in the original amps upon which the Mocker is modelled. Just something to consider.

Another possible issue is the power supply voltage.  What is the supply on the real amp?   My estimates show about 6V !! and biasing to about 1.8V with the 68k/47k base resistors.

[pacealot]

Another possible issue is the power supply voltage.  What is the supply on the real amp?   My estimates show about 6V !! and biasing to about 1.8V with the 68k/47k base resistors.

I've never had a real one in my hands to read voltages, but my understanding thus far has been that the first two transistor stages generally run around 9 to 10VDC, which is one reason that section has been modelled a few times now as a standalone fuzz. But you may be onto something with your calculated voltages. In any event I'd imagine that makes the voltage divider at the base of Q2 possibly even more fiddly?

I've been meaning to build the whole preamp and run it at ~24VDC to get some "real world" data, but I just haven't been able to as of yet.

[pacealot]

Just to make things easier to follow, these are the two schematics I have for the original amp circuit:





The second one seems to be the factory schemo, but contains the spurious 3.3k value.

[R.G.]

[...] is based on the original 7-series Vox amps' distortion circuit, Aclam chose for some reason to model the second transistor stage after an error in one version of the schematic.

I'll have to go look at my documentation. I have made PCBs that repro the entire Vox  7-Series preamp. The reports from builders that have played actual 700s say it sounds right. I can't remember off the top of my head what the values in the diode pair driver are.

[PRR]

I thought the OA200 diodes had a voltage drop more typical of other silicon rectifier diodes - 1.1v or so? Is 0.6v really correct?

0.6V to 0.7V is the rule of thumb. A *rectifier* specced at over 1V is being abused (rectifiers often are).

The OA200 is a nominal 100mA part. In this circuit it can't pass much over 1ma. 60mV/decade, we expect 120mV below our 0.6-0.7V benchmark. That is what the spec sheet says.

[Rob Strand]

0.6V to 0.7V is the rule of thumb. A *rectifier* specced at over 1V is being abused (rectifiers often are).

The OA200 is a nominal 100mA part. In this circuit it can't pass much over 1ma. 60mV/decade, we expect 120mV below our 0.6-0.7V benchmark. That is what the spec sheet says.

The 0.61V/0.62V on the schematic is likely to correspond to a multimeter measurement.    Multimeters often use a test current of 1mA for diode ; not always a precise test current.

Both the plots and the tabulated data for OA200 predict about 0.65V @ 1mA.