Tayda Rangemaster build bias trouble

[scotheath]

https://www.taydakits.com/instructions/rangemaster/pages/designators-and-components--39

Just finished building the above Rangemaster. The actual board I recieved has an input for a third resistor, R3 but the parts list and schematic has no mention of it, I socket it just in case I get a response from Tayda explaining R3, I'm not expecting one. I built the NPN version and can get pedal working, The issue I'm having is the instructions on site recommend biasing to +7v for the NPN version but I can only get it down to 8.74v with components installed, at 8.74v its more of a distortion pedal and when pot on pedal turned down it gets farty sounding. I put a 3m and then a 470 ohm into R3 with no affect on bias # so R3 has nothing to do with bias pot. Will changing the value of R1 or R2 help. I do not have board boxed up just testing through my test rig. The range I can bias is +9.52v - +8.74v

[niektb]

It's not a difficult schematic so get a multimeter if you have one and test where R3 is located in the schematic (using resistance metering or continuity mode)

[scotheath]

you bet I can definitely trace the circuit, it doesn't appear R3 has anything to do with the bias. I'm just trying to figure out why I cant bias down below 8.74v

[iainpunk]

i bet its a bad solder joint. do you have pictures of the solder side?

cheers

[idy]

Most likely that R is a "pulldown" attached to the input and C1.

If you adjust that bias pot (trimmer), you should almost certainly be able to "bias" all the way to zero. At minimum resistance you will be grounding the base and the base will equal zero. (and no sound... but still...)

Do we know what the gain and leakage of the transistor you are using? Are we sure you are using an NPNP Ge transistor? (Si would bias differently...) And telling us the model # (AC12X, Mullard 123) means almost nothing because they are inconsistent.

[idy]

Sorry my stupid, we are measuring bias at C.  Some recommend a 200k bias trimmer.

Ian is probably right, nothing more common than bad solder joint...

[scotheath]

i bet its a bad solder joint. do you have pictures of the solder side?

cheers

[scotheath]

Most likely that R is a "pulldown" attached to the input and C1.

If you adjust that bias pot (trimmer), you should almost certainly be able to "bias" all the way to zero. At minimum resistance you will be grounding the base and the base will equal zero. (and no sound... but still...)

Do we know what the gain and leakage of the transistor you are using? Are we sure you are using an NPNP Ge transistor? (Si would bias differently...) And telling us the model # (AC12X, Mullard 123) means almost nothing because they are inconsistent.

A Pull down ? has me confused as there is no mention of it on the build page. How does a pulldown work ? what value? what would it do?
I've tried a BC108, BC109 and a 2N3904. The full range of the trimmer pot is 8.74 - 9.52v  only has sound at 8.74v. I used a 50k trimmer as per their parts list for this pcb.
Maybe I'm missing something though to bias positive on bias pad and ground i have clipped to ground wire on the PCB  is that correct?

[idy]

OK, now we know more. And your soldering looks good.

Pull down: the first thing your signal goes through in the circuit, and most audio circuits, is a capacitor. That keeps the DC that is biasing the transistor B "inside" the circuit and not going up the wire to your guitar, making scratchy noises with the guitar pots and going to ground inside the guitar and lousing up the biasing.

But when your circuit is bypassed, the cap is attached to nothing on one end, and it will be charged up on the "inside." So when you hit the footwswitch that voltage built up on the cap will go up the wire and through your guitar to ground and *pop*.
By putting a large value R, 1M or more, the cap can discharge when the circuit is bypassed. Some arrangements just ground the input when bypassed, same idea. The original Rangemaster didn't have this because it had no bypass.

So modern versions of vintage circuits usually add the "pull down."

The sites I see that talk about a bias pot for this usually say 100k or 200k.

But wait: you are using a Silicon transistor instead of a Germanium! That is important. You can't get an NPN Ge?

[scotheath]

OK, now we know more. And your soldering looks good.

Pull down: the first thing your signal goes through in the circuit, and most audio circuits, is a capacitor. That keeps the DC that is biasing the transistor B "inside" the circuit and not going up the wire to your guitar, making scratchy noises with the guitar pots and going to ground inside the guitar and lousing up the biasing.

But when your circuit is bypassed, the cap is attached to nothing on one end, and it will be charged up on the "inside." So when you hit the footwswitch that voltage built up on the cap will go up the wire and through your guitar to ground and *pop*.
By putting a large value R, 1M or more, the cap can discharge when the circuit is bypassed. Some arrangements just ground the input when bypassed, same idea. The original Rangemaster didn't have this because it had no bypass.

So modern versions of vintage circuits usually add the "pull down."

The sites I see that talk about a bias pot for this usually say 100k or 200k.

But wait: you are using a Silicon transistor instead of a Germanium! That is important. You can't get an NPN Ge?

I never looked I just ordered the transistors Tayda has in parts list for this build.
But why can i only bias down to 8.74V ?

[idy]

Because the biasing arrangements for silicon and germanium are different.
I do see Tayda has Si transistors listed in the doc.
I don't know anyone who has succeeded that route. People come around now and then asking about using a switch to select Ge or Si in a Rangemaster, and the answer we seem to come back to is you can't...and why would you want to? All the qualities that make a Rangemaster desirable, the low Z and poor high frequency response, that smooths out the treble, the clipping of the Ge, you loose if you go for Si.

If I was in your shoes and already had this thing built I would either:
1)breadboard, looking at other schematics like LPB, and try to make a Si version. New biasing, new R on the emitter... People talk about "Si Rangemaster"....but mostly in the loose language used in advertising copy and not schematics.
2)buy a Germanium transistor.

Maybe someone here will come up with a recipe. I can't find one.

[scotheath]

Because the biasing arrangements for silicon and germanium are different.
I do see Tayda has Si transistors listed in the doc.
I don't know anyone who has succeeded that route. People come around now and then asking about using a switch to select Ge or Si in a Rangemaster, and the answer we seem to come back to is you can't...and why would you want to? All the qualities that make a Rangemaster desirable, the low Z and poor high frequency response, that smooths out the treble, the clipping of the Ge, you loose if you go for Si.

If I was in your shoes and already had this thing built I would either:
1)breadboard, looking at other schematics like LPB, and try to make a Si version. New biasing, new R on the emitter... People talk about "Si Rangemaster"....but mostly in the loose language used in advertising copy and not schematics.
2)buy a Germanium transistor.

Maybe someone here will come up with a recipe. I can't find one.

Thank you for the help, I socketed the transistor and will look for a germanium. But still have to ask with that change will it change the fact that the trimmer pots range is only 8.74 to 9.52v?

[idy]

Assuming everything is right on your build...(boy, who wants to bet?)...
you are asking how a transistor works. The two biasing resitors (with no transistor in place) would create a voltage (68/470) x 9... that's 1.3 volts. The transistor "turns on" when the base (in this case, a silicon transistor) is .6 v above the emitter, and stays there. When it turns on current is trickling in the base, and depending on how much current, the C to E is turning on, opening and allowing current to flow. So we want the voltage at the C to be about 7v? If the transistor is turned on, that number falls (that C is getting closer to ground.) If the transistor is turned off the C is going up.. its just got the pot between it and 9v.

You can read about this stuff, hope I am helping. Transistor with the signal coming off the C are inverting: the more current you but into the base, the lower the voltage on the C, the lower the current into the base, the higher the C goes. So maybe a bigger trimmer (less current) would not help.

Also in Si circuits the Emitter resistor goes down to like 360 ohms on an LPB (still 10k on the Collector, like your pot) but doesn't have a bypass cap. The bypass keeps the DC voltage where you need it but allows the signal (down to some low frequency) to swing close to ground.  Madbean's "donut" is an LPB that has a 220n in parallel with the 360ohm E resistor.

That's the most I can explain, that's the level of clarity I have. Others know more but maybe don't care to explain for the umpteenth time that Si doesn't just plug in to a Ge circuit. It is interesting and if you breadboard you might find something.

[idy]

and do we even know if the transistor is in the right way? That's kind of easy to get confused, since the board can be used either pop or non...and the transistors they recommend have different pinouts. Here showing both sides of the board would help. As would checking the voltages on all three pins of the transistor.

[scotheath]

and do we even know if the transistor is in the right way? That's kind of easy to get confused, since the board can be used either pop or non...and the transistors they recommend have different pinouts. Here showing both sides of the board would help. As would checking the voltages on all three pins of the transistor.

Thanks for all the info, lots of it way over my head I've always just purchased PCB from Madbean and Tayda and ordered parts from Tayda following the parts list and haven't had any problem I couldn't figure out till now, But this also explains the last build I tried that failed, a Tonebender from Tayda with their parts, I also used a si transistor in it and built it NPN, almost the same results but worse couldn't bias down far enough to get anything but farty noise.
With the RM I socketed the transistor and yes I checked pin out and even flipped, just to make sure. I haven't checked voltages at pins though, I will when I get home but will look for some germanium transistors and hopefully get it working along with the Tonebender
Again thanks for all the input.

[iainpunk]

what are the voltages on the other transistor pins?

cheers

[scotheath]

what are the voltages on the other transistor pins?

cheers

With BC109 biased as low as can go at 8.75v. E=.31, B=.87, C=8.75v
                  biased as high as will go at 8.52v. E=0 , B=0, C=9.52v.
Again I'm not really sure what any of that tells me except data sheet says

[idy]

That part of the data sheet tells you what voltage will fry the transistor, maximum voltages when measured between the 3 pins.

So you see that when a little voltage is on the base (it will never rise more than about .6v above the emitter, but more voltage will force more current through. So a little odd that it is less than .6 different...) the transistor opens a little bit and the Collector falls, but only by a tiny bit. If the trimmer was larger, then less current would go through the trimmer (at one end of travel) and more would got through the Base and open the transistor further, and maybe fix your problem.

You are using higher and lower to refer to bias. Best is to think, more current on Base = transistor on, lower V on Collector. Less current on Base = transistor off, Collector is at supply voltage.

I can't tell you how changing the Emitter R would change things...

[idy]

Kind of annoying, yes the parts list shows a 50k trimmer but the schematic shows....
100k.

Maybe using a 100k or 200k trimmer would help. Bigger trimmer, more juice going into base, lower Collector voltage. Just saying.

[iainpunk]

what are the voltages on the other transistor pins?

cheers

With BC109 biased as low as can go at 8.75v. E=.31, B=.87, C=8.75v
                  biased as high as will go at 8.52v. E=0 , B=0, C=9.52v.
Again I'm not really sure what any of that tells me except data sheet says

what those voltages say it that there is not enough current entering the Base of the transistor. i would question the value of the  trim pot, those voltage would make absolute perfect sense if the trim pot was 50k instead of 100k, you might want to check that value. you could put a 56k in series with the existing trimmer, by bodging the board, or you could get yourself a new 100k trimmer.

cheers