sourcing a proper transistor for first pedal build, Austin Treble Blaster (Dallas Rangemaster) by Geo FX

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e.r.stevens
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Post by e.r.stevens »

I decided to attempt my first guitar pedal, using the GEO Fex pdf of the Austin Treble Blaster: http://www.geofex.com/Article_Folders/R ... tboost.pdf
for their clone take on the Dallas Rangemaster. I'm having a hard time finding a transistor from the ones the pdf mentions that have a gain between 65-100. Also in datasheets it's hard to pin down how the gain is listed, some clarification on this would be nice.

Looking for anything close that would be suitable. Seems the transistor needs a tuning process to get it to a sweet spot for the pedal to sound right so any transistor that can be set to in the recommended range would be appreciated.

Also would be nice if i can source the transistor from a website where I can also get some 24awg solid core wire to make jumpers. I believe I have everything else mentioned in the parts list in the pdf from Mouser and had the 47uf caps on hand (mouser did not have them for some reason?)

Also any advice in general would be appreciated, feeling a little lost in the sauce right now.

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mauman
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Post by mauman »

Welcome! According to the Geofex build doc you linked to, you're looking for a germanium PNP transistor with beta/Hfe between 75 and 100 (ideally) or 50-150 (should still work,) along with some hookup wire. The brand of transistor doesn't matter as much as the beta. If you're in the US, you can find both items at stompboxparts.com, they have some MP42B PNP Ge transistors in the 70 Hfe range, and 24 gauge stranded hookup wire (I like stranded better than solid core, it doesn't break as quickly when it's flexed.) Or pedalhackerelectronics.com has several different Ge PNP transistors in the correct beta range, you may want to get a few and see which one works for you (no hookup wire here though.) I've ordered items from both of these suppliers and have had good results. The Geofex build doc has instructions for biasing ("tuning") whatever transistor you get.

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Post by FiveseveN »

e.r.stevens wrote: 31 Mar 2022, 21:15 (mouser did not have them for some reason?)
That document was written in 1998, the particular model it references may not even be manufactured anymore. Mouser still offers more than 130 equivalent caps.
Ignorance more frequently begets confidence than does knowledge. (Charles Darwin)

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e.r.stevens
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Post by e.r.stevens »

mauman wrote: 01 Apr 2022, 04:26 Welcome! According to the Geofex build doc you linked to, you're looking for a germanium PNP transistor with beta/Hfe between 75 and 100 (ideally) or 50-150 (should still work,) along with some hookup wire. The brand of transistor doesn't matter as much as the beta. If you're in the US, you can find both items at stompboxparts.com, they have some MP42B PNP Ge transistors in the 70 Hfe range, and 24 gauge stranded hookup wire (I like stranded better than solid core, it doesn't break as quickly when it's flexed.) Or pedalhackerelectronics.com has several different Ge PNP transistors in the correct beta range, you may want to get a few and see which one works for you (no hookup wire here though.) I've ordered items from both of these suppliers and have had good results. The Geofex build doc has instructions for biasing ("tuning") whatever transistor you get.
So I'm going to grab the MP42B PNP at 60-70 HFE and see how that sounds and then try a MP16B from the other site at 40-100 HFE.

Also felt kind of uncertain about what style of jacks to get. GIven how old the PDF is, wasn't sure if there was something better I should have grabbed, I got two different jacks: https://www.mouser.com/ProductDetail/Am ... cycode=USD
and:
https://www.mouser.com/ProductDetail/Ne ... cycode=USD

will these jacks work? or be a pain to install them? I think everything else should be ok though.

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Post by mauman »

You can make either one of these jacks work. You probably don't need the switching contacts for what you're doing but you can just ignore those. Tip for signal and Shield (= sleeve for us old Telecom guys) for ground. If you use the input or output jack as a switch for an internal battery, that's the Ring lead of a TRS jack to battery negative. Some of my favorite jack styles for pedals include Switchcraft 111x (enclosed) and #11 (open) for TS and 112bx/#12 for TRS. Other people like similar models from Neutrik and Lumberg. Let us know how it goes!

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Post by e.r.stevens »

So I have the MP42B PNP transistor and trying to confirm the E, B, C on it. I see a dot on one side of the can that seems to have a backwards C around the dot but not sure what this identifies. Someone on another forum said that a dot would identify the Collector, Base being in the middle and the other lead being the Emitter but wanted to confirm this before putting it in. I was also reading if a multimeter can measure forward voltage, you can find out which lead is which this way.
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Post by e.r.stevens »

I'm in the middle of the tuning process, adjusting the resistors to get around -6.6 to -7.2 V measuring at the collector and ground and the directions specifically in the PDF (attached above, pg. 4 of 7) state to apply battery voltage but doesn't say where to do this exactly? Feel like the answer is obvious to people who know electronics and circuits well and wanted to know for sure where to apply the voltage on the terminal strip. Does the positive lead from the battery go to the power wire on Lug 5? and the ground battery wire to Lug 1, if i had to guess? Any insight would be appreciated.

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Post by cspar »

The way you suggested is backwards.

In the schematic the bypass switch connects the battery's negative terminal to the circuit.

In the build document this is the "power wire" that connects to L5.

Here's the battery part of the build document instructions;
14. Battery connector (-, black) wire to the bypass switch
15. Battery connector (+, red) wire to the input jack stereo signal lug

The sleeve of the stereo jack connects the battery's positive terminal to ground as long as your using a mono instrument cable.

Here's some pinouts of Russian transistors. The MP42b is in the top left corner.

К is Collector, Б is Base and Э is Emitter.
image002.gif
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Post by e.r.stevens »

Thanks for the insight and info. I swapped the Emitter and Collector to be on the correct Lugs and also swapped the voltage polarity to the circuit and still getting -1.4 volts though when measuring from the collector and ground. I'm not sure what else to check for, could the 47uf caps be messing with the voltage? I didn't order new ones, the ones I had on hand are Shoei 16v caps, they measured out close to 47uf so I thought they were fine but thought maybe the 16v rating was a bit low maybe? I could take the 470 K resistor out from L5 and L3 and make sure that is ok? Other than that I don't know what else to do.

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Post by cspar »

Reread the build instructions, especially the part titled "Turn On, Tune In". There is a method there for biasing if you haven't tested the hfe and leakage of the transistor.

The 68k resistor is the one that you want to adjust for the best results.

Germanium transistors can be finicky depending on the leakage and hfe.

People tend to use trim pots to set the bias. Not usually to measure the value like suggested in the build document but as a permanent part of the design.

There are times that you might need to rebias the pedal like changing power supplies if you're not using a battery or just to swap transistors if you're using sockets.

If your intent is to not have a trim pot in the final build it's easiest to find the required resistor values for biasing on a breadboard before using any solder.

If you're fine with an internal trim pot have a look at this page which has a wonderful analysis of the circuit.

https://www.electrosmash.com/dallas-rangemaster

The schematic with the input cap mod at the bottom of the page shows how to add a bias pot.
"Just because the forgoing circuits have produced results there is no reason experimenting won't lead to added results." L. E. Darling, in his article containing the earliest published vacuum tube synthesizer circuit, Popular Science Jan 1920

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