Dallas Rangemaster clone: Various questions

[pedlar]

Hi!

I am going to build myself a Dallas Rangmaster as my next DIY/Perfboard build using an OC44 (also have a NKT275 I might try).  I am using the schematic from the MusikDing kit "The Range" as a basis / aid for my own layout.

I am going to add the same toggle switch to my build.  However, I have a bit of confusion with the SPDT toggle tone switch and the capacitors used.  From what I can tell by looking at their schematic, the caps are working in an additive method to create the tone.  Eg C2 is always connected to the Base and when the toggle is either up or down, C1 or C3's value is added to C2 and then sent to the Base of the transistor. Is that correct?  So if the toggle is in the position where contacts 2 and 1 are connected, then 9nf is on the signal going to base ("Full range"). And if the toggle is in the position where contacts 2 and 3 are connected, 6nf is on the signal to Base (this would be the original Dallas Rangemaster value, I think). And finally, if the toggle is in the middle position 2.2nf is on the signal to Base ("Treble" setting). Have I got that straight?

MusikDing "The Range" Schematic: http://diy.musikding.de/wp-content/uploads/2013/06/rangeschalt.pdf

What causes me to second guess myself (aside from being an electronics hobbyist newbie ) is their wiring Diagram.  They have annotated "Class" (Classic) beside the wire going to C3 and Full as going to C1.  Is that because they are referring to the actual switch position rather than the wire/switch terminal?

MusikDing "The Range" Wiring diagram: http://diy.musikding.de/wp-content/uploads/2013/06/rangewire.pdf

Document containing original Range Master Schematic: http://www.geofex.com/article_folders/rangemaster/atboost.pdf

Thanks for any help/guidance.

[idy]

I think you've got it. When the switch is "down" it will be connecting the terminal on top to the middle. This is C1, 3.3n (which adds to 2.2 to make 5.5, pretty close to the original 5n.)
So the appearance of the back of the switch is always the reverse of what you would write on the outside of the box.

The middle give you an even thinner sound than stock and the other position fatter. Moves the cut off frequency up or down "about" an octave (?)

[pedlar]

Great, thanks!  I'm sure I'll have more questions on this build soon!

[pedlar]

New question:
There are 2 resistors on this build (R3 and R5 on the MusikDing Build: http://diy.musikding.de/wp-content/uploads/2013/06/rangeschalt.pdf ) that (I believe) should be ideally adjusted so that you get approx -7V on the Transistor Collector.  I was going to use 2 trim pots for this purpose (a 10k trim pot and a 100k trim pot).

I was wondering should I, or is it normal to, add a "safety" resistor with each trim pot so that I can't accidentally set the trim pot too low and maybe destroy the transistor.  Is this a danger?  If so how would I mitigate this risk?  If my idea of  safety resistors is plausible, what values would I use?

Thanks for any advice or help.

[midwayfair]

Is this a danger?

the only possible danger is if you make both resistors 0 and temporarily connect the +9V rail and 0V. Even then it's not a disaster as long as you don't just leave it that way (which would be silly because you wouldn't get any sound anyway).

The resistors do the following:

R5: This sets the DC bias point. C5 bypasses it for all frequencies down to below a hertz, increasing the AC gain to essentially however much the transistor can put out. If you change this resistor, it will have a meaningless impact on the frequency response of the circuit but a large impact on the DC bias of the collector. It's probably best not to deviate too far from the stock value because it probably affects the sound in a way I'm not entirely certain of, but if you want to change a resistor with the least impact and your transistor is at least close to the right gain spec, this is the one to fiddle with.

R4: This provides positive voltage for the base and divides against R3 and some properties of the transistor to set the input impedance. You don't want to change this too much if you want the circuit to stay the same. You'll mess with the loading on the guitar, which will affect the input signal size and tone, and you'd need pretty substantial changes to its value to see any difference.

R3: This is the lower leg of the voltage divider for the base bias. Note that it's pretty small compared to R4. It sets the base voltage fairly low. (The transistor itself will pull the voltage down some, too.) You don't really want to mess with this for the same reasons as leaving R4 alone if you can.

If you can't get it to bias to -7V with a small adjustment to R5, your transistor is probably too high or too low gain, and merely getting the DC bias correct still won't result in the effect being "as intended." Most transistors near the proper gain bucket will get pretty close to -7V without adjustment anyway, so it's your choice if you think the trimpot's worth it I guess.

[pedlar]

Is this a danger?

the only possible danger is if you make both resistors 0 and temporarily connect the +9V rail and 0V. Even then it's not a disaster as long as you don't just leave it that way (which would be silly because you wouldn't get any sound anyway).

The resistors do the following:

R5: This sets the DC bias point. C5 bypasses it for all frequencies down to below a hertz, increasing the AC gain to essentially however much the transistor can put out. If you change this resistor, it will have a meaningless impact on the frequency response of the circuit but a large impact on the DC bias of the collector. It's probably best not to deviate too far from the stock value because it probably affects the sound in a way I'm not entirely certain of, but if you want to change a resistor with the least impact and your transistor is at least close to the right gain spec, this is the one to fiddle with.

R4: This provides positive voltage for the base and divides against R3 and some properties of the transistor to set the input impedance. You don't want to change this too much if you want the circuit to stay the same. You'll mess with the loading on the guitar, which will affect the input signal size and tone, and you'd need pretty substantial changes to its value to see any difference.

R3: This is the lower leg of the voltage divider for the base bias. Note that it's pretty small compared to R4. It sets the base voltage fairly low. (The transistor itself will pull the voltage down some, too.) You don't really want to mess with this for the same reasons as leaving R4 alone if you can.

If you can't get it to bias to -7V with a small adjustment to R5, your transistor is probably too high or too low gain, and merely getting the DC bias correct still won't result in the effect being "as intended." Most transistors near the proper gain bucket will get pretty close to -7V without adjustment anyway, so it's your choice if you think the trimpot's worth it I guess.

Great info thanks, I will take it all onboard. I got the idea of having to adjust the values of R3 and R5 (mainly R3) from reading this document about the Dallas Rangemaster: http://www.geofex.com/article_folders/rangemaster/atboost.pdf .  (page 4 details the steps about adjusting).

I still have to do a test to see if the OC44 and nkt275 I have are any good in the gain department.  I guess that will determine if I use the trim pots, but at least I know I can use the trim pots without killing the transistor

Thanks again, really appreciate the information and explanations, it helps me to get a much better idea in my head of what is happening in the circuit.
I will be back with more questions soon no doubt!

[pedlar]

I tested my DSI OC44 and DSI NKT275 using Step 24 from the following article/circuit/formula: http://diy.smallbearelec.com/HowTos/FuzzFaceFAQ/FFFAQ.htm





I was able figure out the pins for the OC44 pretty easily, on mine there is a white dot next to the collector.

Code Select Expand


OC44:
Leakage Current:13.7 μA
Collector Current:508 μA
Base Current: 9.0μA (9v battery, 1m resistor)
Gain/Hfe: (508 - 13.7) \ 9.0 = 54.9Hfe



The NKT275 is not as easy to tell what the pins are, on mine from what I can tell the little metal tab  is closest to the Emitter.   The results of the test seem crazy though:

Code Select Expand


NTK275:
Leakage Current: 226μA
Collector Current: 1494μA
Base Current: 9.0μA (9v battery 1m resistor)
Gain/Hfe: (1494 - 226) \ 9.0 = 140.8Hfe

Isn't leakage way too much or can these numbers be normal in some NKT275's?  I swapped the transistor around in my test circuit but the numbers where really low then (collector current of less than 90).

How do you think either of these will be on the Rangemaster?  I know 70-100 is optimal but the guy in this article: http://www.geofex.com/article_folders/rangemaster/atboost.pdf seems to think you could use one with a value of 45 at the minimum (combined with a 25k boost pot instead of the 10k).

Thanks again for any advice or ideas.

[smallbearelec]

OC44
Leakage Current:13.7 μA
Collector Current:508 μA
Base Current: 9.0μA (9v battery, 1m resistor)
Gain/Hfe: (508 - 13.7) \ 9.0 = 54.9Hfe

NTK275:
Leakage Current: 226μA
Collector Current: 1494μA
Base Current: 9.0μA (9v battery 1m resistor)
Gain/Hfe: (1494 - 226) \ 9.0 = 140.8Hfe

They are both usable devices. Your OC44 will bias to 7.0 volts if you follow the GEOFEX instructions. The measurements for the NKT275 look reasonable; a 3 to 1 spread of gains for these parts is not uncommon. This one is probably better suited for Q2 of a Fuzz Face. Find a similar part in the 70 to 100 buckets for Q1 and build one:

http://diystompboxes.com/barebonesfuzzface/

Where did you get the DSI devices? I once ordered some OC44 from them and was not happy: They quoted a reasonable price, but the parts I received all had gains under 40. They said that they would supply higher gains, but at a much higher price.

[pedlar]

OC44
Leakage Current:13.7 μA
Collector Current:508 μA
Base Current: 9.0μA (9v battery, 1m resistor)
Gain/Hfe: (508 - 13.7) \ 9.0 = 54.9Hfe

NTK275:
Leakage Current: 226μA
Collector Current: 1494μA
Base Current: 9.0μA (9v battery 1m resistor)
Gain/Hfe: (1494 - 226) \ 9.0 = 140.8Hfe

They are both usable devices. Your OC44 will bias to 7.0 volts if you follow the GEOFEX instructions. The measurements for the NKT275 look reasonable; a 3 to 1 spread of gains for these parts is not uncommon. This one is probably better suited for Q2 of a Fuzz Face. Find a similar part in the 70 to 100 buckets for Q1 and build one:

http://diystompboxes.com/barebonesfuzzface/

Where did you get the DSI devices? I once ordered some OC44 from them and was not happy: They quoted a reasonable price, but the parts I received all had gains under 40. They said that they would supply higher gains, but at a much higher price.

Great, thanks very much for the advice.

I got those components here: http://www.donberg.ie/  (Ireland)  I think they ordered them from Germany?  Maybe they could give you some info on where they get them if you send them an email.  I have never dealt with them before, all this is very new to me!

Thanks again!

[smallbearelec]

I got those components here: http://www.donberg.ie/

I bought directly from DSI; they will sell direct if you order quantity. I looked at the donberg site as well; they sold you usable parts, but IMO they charge a lot for unsorted stock. At the price, you can have something from my store

http://smallbear-electronics.mybigcommerce.com/germanium-transistors/

that has been sorted for gain and audited to make sure it sounds right. I don't know where you are, but we ship anywhere that the postal service delivers.

[pedlar]

Thanks for that smallbear, I have your site bookmarked and will check it out next time I am getting some parts.
----------------------

Could any one tell me what R7 and C4 are doing in this schematic? : http://diy.musikding.de/wp-content/uploads/2013/06/rangeschalt.pdf

I guess C4 is a 100uf Filter cap?  I normally see these as 47uf in other schematics.  If it is a filter cap, is there a specific reason for 100uf?

The R7, I have no clue  .

Thanks!

[Cozybuilder]

Thanks for that smallbear, I have your site bookmarked and will check it out next time I am getting some parts.
----------------------

Could any one tell me what R7 and C4 are doing in this schematic? : http://diy.musikding.de/wp-content/uploads/2013/06/rangeschalt.pdf

I guess C4 is a 100uf Filter cap?  I normally see these as 47uf in other schematics.  If it is a filter cap, is there a specific reason for 100uf?

The R7, I have no clue  .

Thanks!

R7 and C4 form a low-pass filter for your DC. This combination has the cutoff frequency at approx 16 Hz, so that frequencies above that go to ground. See this:

http://www.learningaboutelectronics.com/Articles/Low-pass-filter-calculator.php#answer1

[PRR]

If your power source is dead clean, you do not need R7 C4.

All real sources have crap. Small for a fresh battery or say a 1-Spot; huge for some answering machine wall-wart you found in the GoodWill. An R-C filter will pass DC fairly well (100 ohms is very small compared to the rest of the circuit) but the cap will tend to absorb any rapid variation (hum/buzz).

Usually the 60Hz/120Hz (50Hz/100Hz in much of the world) ripple from turning wall AC into DC is the biggest "crap".

> is there a specific reason for 100uf?

There was a 100uFd cap laying on the bench?

Bigger is better but costs more. As Russ says, 100r and 100uFd trims 16Hz a little, 60Hz significantly, and 120Hz even more. If your supply is not junk, this would probably be ample. Heck, you could use 47uFd and not much different. If you are coming off an UN-filtered battery charger, 1,000uFd would not be enough.

Since you do not know at build-time what power you will give it (today and 13 years from now when the original supply quits), it is good to have a filter, and pointless to fine-calculate an "optimum" value. Got a bargain-pak of 68uFd? Or 470uFd that are not too big? Use 'em.

[pedlar]

Thanks guys, great info!

[pedlar]

I have made up a Circuit Layout.  If anyone could have quick look over to make sure nothing is wrong that would be great.  I am aware that I am wasting tons of space and there may be newb-like (not optimal) mistakes with the layout.  I just want to make sure that it will actually function.  I will also be breadboarding a version of this before I solder anything, to see it working that way too.  (Ps, all the Trace Cuts are just do denote the usable area of the perfboard I have)

I am curious about R2(input jack), is it ok to wire a pull down (antipop) resistor this way? (Ps, the R2 resistor on the LED is supposed to be R3 2.2k, dunno how it got like that.. :/ )



If the image is too small, let me know and I'll upload a bigger version.

Thanks for any help!

[Cozybuilder]

I have made up a Circuit Layout.  If anyone could have quick look over to make sure nothing is wrong that would be great.  I am aware that I am wasting tons of space and there may be newb-like (not optimal) mistakes with the layout.  I just want to make sure that it will actually function.  I will also be breadboarding a version of this before I solder anything, to see it working that way too.  (Ps, all the Trace Cuts are just do denote the usable area of the perfboard I have)

I am curious about R2(input jack), is it ok to wire a pull down (antipop) resistor this way? (Ps, the R2 resistor on the LED is supposed to be R3 2.2k, dunno how it got like that.. :/ )



If the image is too small, let me know and I'll upload a bigger version.

Thanks for any help!

1) Check the polarity of the electro next to VR2
2) How are the wipers on the 2 VRs connected?
3) How is Q1C connected to -9V?
4) Is VR1 meant for biasing, or for feedback? If feedback, then it should be connected to the emitter, not ground.
5) Are you sure you want to use a power supply on this build with positive ground? It can be done, but needs to be a completely separate supply from your negative grounded pedals.

[Philippe]

building something like a rangemaster is so much easier just using a terminal strip as in the original design & analogman re-creation.  *never could figure out why folks use PCBs or perfboard for such a simple circuit*  R.G. Keen's Geofex has the layout for the terminal strip version...it's very compact, minimalist & showcases your point-to-point wiring skills.

[pedlar]

I have made up a Circuit Layout.  If anyone could have quick look over to make sure nothing is wrong that would be great.  I am aware that I am wasting tons of space and there may be newb-like (not optimal) mistakes with the layout.  I just want to make sure that it will actually function.  I will also be breadboarding a version of this before I solder anything, to see it working that way too.  (Ps, all the Trace Cuts are just do denote the usable area of the perfboard I have)

I am curious about R2(input jack), is it ok to wire a pull down (antipop) resistor this way? (Ps, the R2 resistor on the LED is supposed to be R3 2.2k, dunno how it got like that.. :/ )



If the image is too small, let me know and I'll upload a bigger version.

Thanks for any help!

1) Check the polarity of the electro next to VR2

ah yes, didn't see that one, thanks!

2) How are the wipers on the 2 VRs connected?

I was under the impression that potentiometers only used the wiper and one of the other two lugs to operate as a variable resistor, is this not correct? Or do I just have the wrong pins connected on the Layout?

3) How is Q1C connected to -9V?

The layout is pretty much based off this circuit: http://i1128.photobucket.com/albums/m492/guitarpoet1/Schematic_zps53db4ef4.jpg  To me it looks as though Q1Collector is connected to -9V through the 10kBoost pot.  Although, now I look at it, compared to the schematic I linked, I think I have lugs 2 and 3 the wrong way around, should still work though?  Have I miss-understood something here? 

4) Is VR1 meant for biasing, or for feedback? If feedback, then it should be connected to the emitter, not ground.

Both VRs are for Biasing

5) Are you sure you want to use a power supply on this build with positive ground? It can be done, but needs to be a completely separate supply from your negative grounded pedals.

I am aware of this, thanks for the advice though, I'll most probably just use a battery.
Thank for the assistance, really appreciate it!

building something like a rangemaster is so much easier just using a terminal strip as in the original design & analogman re-creation.  *never could figure out why folks use PCBs or perfboard for such a simple circuit*  R.G. Keen's Geofex has the layout for the terminal strip version...it's very compact, minimalist & showcases your point-to-point wiring skills.

Thanks for the tip.  I don't mind using the perf board.  All this is so new to me, it's all good learning/experience!  I hope to tackle a Terminal circuit soon, it does look quite interesting!

Thanks again!

[Cozybuilder]

2) How are the wipers on the 2 VRs connected?

I was under the impression that potentiometers only used the wiper and one of the other two lugs to operate as a variable resistor, is this not correct? Or do I just have the wrong pins connected on the Layout?

3) How is Q1C connected to -9V?

The layout is pretty much based off this circuit: http://i1128.photobucket.com/albums/m492/guitarpoet1/Schematic_zps53db4ef4.jpg  To me it looks as though Q1Collector is connected to -9V through the 10kBoost pot.  Although, now I look at it, compared to the schematic I linked, I think I have lugs 2 and 3 the wrong way around, should still work though?  Have I miss-understood something here? 

2) Maybe your trim pots are different, but the wiper is usually the leg thats off-line with the others- for VR1 thats the connection on the left, for VR2 its the one on the right. Of course you can wire it to either end leg since its just being used as a variable resistor.

3) You're right, you have 10K between -9V and the collector, with the wiper tapping the signal. Thats fine. You want clockwise turning to move the wiper towards the collector.

I look forward to reading about your breadboard experiments

[pedlar]

2) Maybe your trim pots are different, but the wiper is usually the leg thats off-line with the others- for VR1 thats the connection on the left, for VR2 its the one on the right. Of course you can wire it to either end leg since its just being used as a variable resistor.

3) You're right, you have 10K between -9V and the collector, with the wiper tapping the signal. Thats fine. You want clockwise turning to move the wiper towards the collector.

I look forward to reading about your breadboard experiments

I see what you mean now.  I have revised the layout so hopefully it should make a bit more sense now with the boost pot and VRs etc.  I think I had the 1n004 protection diode the wrong way around in the first layout too?  I have swapped that around now.



Thanks again!