Diaz Texas Ranger biasing

[TheGunn]

Hi everybody!
I built Diaz Texas Ranger clone with silicone transistor. It sounds fine until you max the guitar volume pot - then it sounds very farty. The voltage on the collector is 8v with the 68 k resistor. I tried swapping transistors and increasing value of 68k resistor but when it's 100k and voltage at collector is around 7v I still get that farty sound. Anyone knows is it normal or what should be voltage on the collector in this circuit? I suggested myself with Rangemaster bias voltage that should sit in between 6.8-7v. Transistors i used were between 100-300 hfe. Any help appreciated

[plush]

Adding base resistor of some value might help.

[Manfred]

Take the component values from this circuit including the BIAS trimming potentiometer:

[mauman]

Here's the thread with Manfred's analysis and a larger schematic: viewtopic.php?p=275152

[Manfred]

Here's the thread with Manfred's analysis and a larger schematic: viewtopic.php?p=275152

In the post above I replaced the linked image of the schematic by a larger one.

here is a copy of the images with the data I measured later, which I have inserted in the topic of the link above.
Maybe the measured values will help.

ThrobakStrangMaster Transistor Data Measurements.jpg (18.11 KiB) Viewed 995 times

[TheGunn]

I've tried it with 250k bias potentiometer and even with collector voltage around 5v it was still farty (checked with both humbuckers and single coils).

[Manfred]

What transistor are you currently using in the circuit
Can you measure the DC values at the transistor pins when the emitter resistor is set to a value of about 70 kiloohms.

[TheGunn]

I rebuilt it with socket for transistor. I tried 2n3904 and 2n5088 (though I think they are fakes and they are something like 350+ hfe), BC109C, BF257 (hfe between 45-100), BSY56 (hfe 70) and 2n708 (hfe 40). It sounded best with 2n708 but even with bias at 7v if I hit strings harder it still sounds farty (it doesn't sound like overdrive but like misbiased transistor).

[Manfred]

I ran a circuit simulation using the 2n708 transitor.
I recorded the output signal and the signal at the base of the transistor.
Up to an input signal level of less than 600 millivolts peak, the shape output signal is rounded. Above 600 millivolts peak, sharp edges and corners appear in the shape of the output signal.
This happend because at this signal level the transistor goes into saturation, the sinusoidal signal at the base starts to flatten at the positive half-wave.
The output signal contains through the corners and edges and thus also unwanted harmonics that sound unpleasant.
I inserted a 22 kiloohms resistor between input and the 100 Kiloohms resistor, now the signal remained peak rounded to less than 1000 millivolts.

[TheGunn]

Does anyone know if Texan Twang with silicon transistor differs from schematic posted earlier? I heard that it includes NTE123AP transistor. Maybe it was chosen for some reason?

[Manfred]

The NTE123AP is a transistor type with low hFE as well as the 2N708.

SJNTE123AP_DATA_E-1.pdf

(63.32 KiB) Downloaded 26 times

2N708-3.pdf

(45.88 KiB) Downloaded 23 times

[TheGunn]

Lately I'll probably try this circuit out with bc547, 2n2222, 2n3904(this time genuine ) and maybe 2n4401.

[Manfred]

I wrote:

I inserted a 22 kiloohms resistor between input and the 100 Kiloohms resistor, now the signal remained peak rounded to less than 1000 millivolts.

Did you try this?

[TheGunn]

Using 2n708 attack was way less agressive compared to original pedal records and using 22k resistor would even make it worse, so I didn't try it.

[Manfred]

...and using 22k resistor would even make it worse, so I didn't try it.

I am sorry, maybe you misunderstood what I meant.
I meant inserting the 22k resistor between the input and the connection point of the first 100k resistor and the 0.005uF capacitor.
If not, how do you think that resistor would make it worse?

[TheGunn]

I mean that as far as I know adding resistor like this should decrease input signal. And since transistor is lower hfe effect lost some attack. So if I add a resistor between input and pulldown resistor I think that it will lose even more definition. Am I wrong?

[FiveseveN]

as far as I know adding resistor like this should decrease input signal

Right! Because you said

if I hit strings harder it still sounds farty

So the solution is to scale the signal down so it doesn't hit the

[Manfred]

I mean that as far as I know adding resistor like this should decrease input signal. And since transistor is lower hfe effect lost some attack. So if I add a resistor between input and pulldown resistor I think that it will lose even more definition. Am I wrong?

Yes, the signal at the base is reduced by about 36%. Is the definition loss just an assumption of yours or have you tried it in the real circuit?
I think it is a general problem of the rangemaster circuits, that they were designed in a time when the pickups still delivered lower output levels than the modern ones of today.

[TheGunn]

I mean this circuit with 2n708 it was almost no boost and definitely attack was less aggressive. Tried BC547B with 340 hfe and it still sounds unpleasant at maxed guitar volume pot. Maybe I'll try resistor at the input with this transistor.

[Manfred]

It is not possible to simply replace each BJT transistor with another without replacing the components values of the surrounding circuit.
A strongly different Hfe means that the values of the base voltage divider must be adjusted.

Try the following values for using the BC547.

BC547B Try.jpg (23.67 KiB) Viewed 468 times