Vox Treble Bass Booster transistor voltage

[Chugs]

Hey, I am making a Repeat Percussion and Vox V8401 Treble Bass Booster for a friend. Does anyone know what collector and base voltages I should shoot for on the transistor in the Treble Bass Booster?

[LucifersTrip]

bump...nobody?


http://analogguru.an.ohost.de/001/schematics/Vox_TrebleBassBoost.gif

[PRR]

There's nothing to "shoot for". Bias is solid by design.

If collector is not 1.5V-1.6V, it's wired wrong.

[LucifersTrip]

There's nothing to "shoot for". Bias is solid by design.

If collector is not 1.5V-1.6V, it's wired wrong.

Is that known info or did you make the calculation?  What should the base voltage be?


thanx much

[Chugs]

In the end I used a BC108 hfe300 and got 1.6V on the collector.

[LucifersTrip]

I breadboarded this last night and PRR was right on.  (I guess I should've realized that this is nothing more than the first part of many fuzzes)

With a tran with 228 hfe, C was 1.56v and B was .5v (similar to Q1 of say, a Fuzz Face)

I tried various other gains:

hfe/ C voltage
42 / 2.20
70 / 1.88
200 / 1.72
228 / 1.56
304 / 1.62

I got similar sounds and sweeps from all 5, but there was a  vol drop with the 42 & 70 and the 304 was a bit harsher

The first 4 above are vintage metal can types. The last is a modern 2N3904. I have to admit, I thought the collector voltage would be < 1.56.

Other than this treble/bass booster, I've only built just treble boosters, so I don't know what is expected from the sweep here.

For me, the useable part of the sweep for the bass is the last ~10% and for the treble the last ~25%. Everything in between, there is a volume drop with no great treble or bass boost.  Full bass & full treble do sound great.

Can anyone tell me what the sweep is like on this one?


thanx

[LucifersTrip]

TOTAL VICTORY!

Because of the not-so-great sweep I noted above, I tried different pots. My first choice was a 50K (since it would be the most common error) and it gives a PERFECT sweep from low to high. A 100K also worked well, but I'm stickin' with the 50...

[stringsthings]

There's nothing to "shoot for". Bias is solid by design.

If collector is not 1.5V-1.6V, it's wired wrong.

i've worked thru the math for voltage-divider biased C-E amplifiers ( with no resistor between Collector and Base ) and i understand that ... i can't find any material on this particular circuit .... i know that Base has to be one diode drop above Emitter ... 0.6 to 0.7V .... where do i take the calculations from that point to get Vc?

[PRR]

> What should the base voltage be?

If the Collector is right, the values right, then the base voltage IS right.

For the reason stringsthings gave: "I know that Base has to be one diode drop above Emitter". If it isn't near there, then the transistor is NOT in a useful zone (useful for amplification).

> i know that Base has to be one diode drop above Emitter ... 0.6 to 0.7V

True, for "typical" current.

But look at the collector load: 180K. Quite high. What is the current? We don't know exact, but we can take some extremes and set limits on it. Minimum could be zero (but that's unlikely since it passes signal). Maximum could be 9V/180K or 0.05mA (but that would be slammed and it would never be clean). Assume "in the middle" from zero to 0.05mA. Say 0.025mA.

The transistor is specced to be a 100mA part. Doesn't need to be that big. Isn't an exotic spec. Was probably just hanging around the designer's bench. 2N5232 specs are not detailed, but 2N3904 is now common, is 100mA, Aron has them, and the specs. Vbe is 0.6V at 0.1mA to 0.7 at 3mA. 0.025mA is off the graph but probably like 0.55Vbe.

Pencil "0.55Vbe" for now. After the first run-through we'll have a better idea of current, can re-check our assumed Vbe.

> where do i take the calculations from that point

It is a 430K:220K voltage divider, probably.

IF the amp has a bias-point (if it "works"), we know the tap is 0.55V.

What voltage do we need at the top of 430K:220K to get 0.55V?

0.55V * (430K+220K)/220K

I work it out to be 1.625V.

This assumes the Base draws ZERO current from the voltage divider.

There IS base current. How much? Do we have to account for it?

2N5232 spec is given as "beta: 250-500". This is similar to 2N3904 beta. But such numbers are usually at 1mA. Beta may be less at the low 0.025mA here. The 2N3904 data shows hFE "typical" very flat from 0.1mA to 10mA. It may be (must be) down some at low-low current (and the hard-data says so), but maybe not by much. Let us take "250" as the current gain. If collector current is 0.025mA then base current is 0.025mA/250= 0.000,1mA. This must flow in the 430K resistor. 0.000,1mA*430K is 0.043V added drop.

So now the collector voltage which makes the base happy is 1.625V+0.043V or 1.668V. If hFE were higher, it would be less. But even if hFE were infinite we'd be back to the 1.625V assumption. (Base current is much less than divider current.) So collector voltage CAN'T vary by much. It has got to be in the 1.6V-1.7V range.

(I musta counted too fast when I figured 1.5V-1.6V.)

OK if collector is near 1.6V and supply is 9V, then there is 9V-1.6V= 7.4V across the 180K collector load. This current must be 7.4V/18K= 0.04mA. That's somewhat higher than our "in the middle" estimate. Go back and squint/extrapolate Vbe for 0.04mA. Might be nearer 0.58V. Voltage-divider means to get 0.58V at base the collector must sit near 1.69V. The base current is higher but still small compared to divider current. We might pencil 1.7V as our expected voltage.

EDIT: Oh!! I forgot the 100K collector to ground! This is an odd feature (it makes the clipping less asymmetrical). There's a couple ways to account for its effect. Obviously if collector is anywhere near 1.6V, then there is 1.6V/100K or 0.016mA in the 100K. That means 0.016mA less in transistor. So the "0.04mA" is more like 0.024mA. Vbe is lower, maybe 0.056V. Collector may be 1.65V, or a wee bit more. No big difference.

> got 1.6V on the collector
> C was 1.56v

That data-sheet on 2N3904 is very old. I suspect many new-production devices have a little lower Vbe at a given current. Vbe also varies with device area, and some old metal-can devices may be large switches, lower Vbe at a given current.

The Vbe (and collector voltage) will also vary with temperature. 0.002V per degree C at the base, so over 0.005V at collector. With a sharp meter, you can calibrate it like a thermometer. When it is hot enough to boil spit, the collector voltage may be 1.16V. Since the idle collector voltage is also the maximum signal swing, you get a bit less output when it is that hot.

> hfe/ C voltage
> 42 / 2.20
> 70 / 1.88
> 200 / 1.72
> 228 / 1.56
> 304 / 1.62

We see that for parts hFE>100 it always comes into the 1.5V-1.7V range. The differences may be Vbe (device area) more than hFE. Or the low-low-current hFE may be less than whatever condition you tested hFE at.

For parts with hFE<100 the base-current in 430K starts to make a difference, the collector must sit higher to get everything happy. But note that almost 8:1 change in hFE is not even 1.4 change in collector voltage. This is a very stable circuit.

[stringsthings]

It is a 430K:220K voltage divider, probably.

IF the amp has a bias-point (if it "works"), we know the tap is 0.55V.

What voltage do we need at the top of 430K:220K to get 0.55V?

0.55V * (430K+220K)/220K

I work it out to be 1.625V.

This assumes the Base draws ZERO current from the voltage divider...

there's where i was getting off track .... many thanks! ...