Myths, Legends and the Mojo of the Dallas Rangemaster

Discussion regarding early stompbox technology: 1960-1975 Please keep discussion focused and contribute what info you have...
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MoreCowbell
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Post by MoreCowbell »

DougH wrote:Basically I left Rc at 10k and Re at 4.7k (I guess I should have used 3.9k but I don't think that makes a major difference). I subbed the 68k with a pot. I didn't have any 100ks so I used a 50k and fixed resistor in series to dial in different Vc's.
Ive built several like that (100k trim, 180k V - base bias, but with a 3k9) and they sound great. I personally like a .0068 input and .022 output cap, and i'd recommend a 10pf-47pf cap to ground on the front end. I've also has good results replacing the 68k with a 56k and 20k pot in series to make the bias externally adjustable. I've only done that using the 470k v-base bias resistor however.

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Post by DougH »

I like .0068u on the input too. I will play some more with the output cap to make sure it's where I want it. Did you install the shunt cap to kill RF?

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Post by MoreCowbell »

DougH wrote:I like .0068u on the input too. I will play some more with the output cap to make sure it's where I want it. Did you install the shunt cap to kill RF?
Dual purpose...kills rf and bleeds just a tiny bit of the "ice-pickyness" (great word, eh ?) off without killing the RM vibe.

BTW Doug...it's nice to see you getting back to enjoying your hobby. I can definitely relate.....

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Post by markm »

Question with the biasing of the RM for Doug.......
With the 6V and 180K, was there alot of distortion from the circuit or was it fairly clean sounding?
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Post by MoreCowbell »

markm wrote:Question with the biasing of the RM for Doug.......
With the 6V and 180K, was there alot of distortion from the circuit or was it fairly clean sounding?
Mark...
I typically bias them in the 6-6.5v range as well...i notice a bit more clarity (less "hair" on it) without sacrificing much sustain. At 7.2v, it seems like it "hits a ceiling" and compresses a bit more.

as always, ymmv.

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Post by markm »

Okay, that seems it would make sense.
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Post by DougH »

markm wrote:Question with the biasing of the RM for Doug.......
With the 6V and 180K, was there alot of distortion from the circuit or was it fairly clean sounding?
It wasn't as distorted, but I suspect part of that is due to the transistor itself. I'm going to try these circuit values with one of the AC188's I have. Then I'll have a direct comparison with the other RM I already have boxed up.

FWIW, I tried this again last night (with the mullard/180k/4.7k/etc) and it passed the morning-after/honeymoon test. The moment I turned it on it was obvious that this really is something. My parts should be here in a day or two and I'm going to box it up.

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Post by markm »

Well, maybe it's time for me to start looking into this circuit again. :D
Thanks Doug.
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Post by DougH »

MoreCowbell wrote:[
BTW Doug...it's nice to see you getting back to enjoying your hobby. I can definitely relate.....
Thanks Andy. I haven't really gotten away from building. Just gotten away from the internet and some of the forums somewhat. I've dropped about 3 or 4 forums I used to visit. Now it's just this and ax84. I have a couple ideas I'm trying to get into production, but the parts ordering and business side of things bores me to tears, so it's going slow. I also just built an amp I really like. I'm doing the woodwork for the cabinet right now.

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Post by markm »

any pics?! :D
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Post by DougH »

I'm going to put up some amp pics when I finish it.

edit: ...If it turns out good.

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Post by Vinnie »

Please let's make a short revision for anyone interested into get the working point of a voltage divider biased transistor:

a) Vb = VccR2/R1+R2
b) Ie = (Vb - Vbe)/Re = Ic ; Vbe is tipically 0,3v for a Ge trany
c) Vc = Vcc - IcRc

If R1 = 470K, R2 = 68K, Rc = 10K and Re = 3K9 then:
Vc = - 6.85 --> GOT MY MOJO WORKING! :lol:

If you bias the transistor far away from this value, the transistor is working on the non linear region (moving near the saturation or cut regions), which can be a good experiment except for anyboby searching for a clean boost (why does the people search a clean boost in a rangemaster? :lol: )

By the way for R1 = 180K and Vcc=9.5 ; Vc is - 3.59 ;
I use to place a 4n7/6n8 input switch and a 2K7 resistor in series with a 2K2 trim pot at the emiter in my rangemasters.

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Post by The Rotagilla »

For whatever reason I never gave treble boosters much thought until I played a 65 Amps Colour Boost a few weeks ago. That thing just dished out tone. Great sustain/feedback at reasonable volume. Definitely on the short list of projects.

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Post by DougH »

The values I used were:

R1=180k, R2=39k, Rc=10k, Re=4.7k

Plugging through Ohm's law gives you Vc= ~6.5V for those values.

I measured ~6.2V, on my breadboard. Pretty close and the difference is probably due to parts tolerances.

FWIW, I preferred the sound of it with Re=4.7k too. I liked the slight increase in bass response.

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Post by The Rotagilla »

Well curiosity finally won yesterday and I built a RM. I used the General Guitar Gadgets layout and values. With no tweaking and an untested Mullard OC71, I got -6.5V for the bias. The stock .0047 cap was a little too bright for me as was a .0068 (using a Tele through a Carr Rambler) so I think I settled on a .022 in the end. It's an interesting little circuit and I may tweak it some more. This weekend I'm taking it down to a friend's shop that has a Louis Electric Bluesbreaker.

I do have a question though. Can you guys hear the difference between transistors? The OC71 I used came in a grab bag. Also in there were a couple other OC71's,OC75's, OC44's, CV7004's and a few others but all were germaniums. With the exception of some being louder than others I could not hear what I would call a significant tonal difference. It seemed as long as I stuck a germanium in there, the RM was happy. Maybe biasing would have straightened out the volume differences but I would not kill myself trying to track down the magical mojo germanium for this circuit.

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Post by DougH »

I've tried AC128's, AC188's, other Ge's that I can't remember the p/n of, and currently a Mullard NPN (CV something...). They all sound different. Some are just slight differences, others are more pronounced. So far the Mullard sounds best by far, more clarity and "punch", hard to describe but pretty noticeable.

But you can certainly build a good sounding RM with other transistors too. My other RM has an AC188. IMO it doesn't sound as good as the NPN version I'm building now, but I gigged with it few yrs in a former band and was real happy with it.

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Post by DougH »

Just a quick update-

I subbed the .0068u input cap with a .0082u. This gives the same Fc of the HPF as the original RM circuit with 470k/68k/3.9k and .0047u input cap (all things being equal with a transistor with hfe of 89). This removed the last bit of edginess and it sounds very warm and sweet compared to other settings. Also subbed the .01u output cap with a .022u which adds a slight amt of bottom without getting mushy (thanks for the suggestion, Andy).

I also subbed an AC128 and made it a negative ground circuit, just to compare the xsistors. This CV7112 sounds a lot better, more clarity and presence. The AC128, while not bad, sounded fizzy in comparison.

One more thing I tried- I compared my circuit (with CV7112 Mullard) with the original circuit values- 470K/68k/3.9k and 7v bias. All I can say is there's no comparison... The original values sounded cold, sterile, and thin in comparison.

This circuit is definitely not something you just want to slap together without thinking about. A little tweaking pays off in big dividends.

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Post by The Rotagilla »

Image

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Post by MoreCowbell »

DougH wrote:
MoreCowbell wrote:[
BTW Doug...it's nice to see you getting back to enjoying your hobby. I can definitely relate.....
Thanks Andy. I haven't really gotten away from building. Just gotten away from the internet and some of the forums somewhat. I've dropped about 3 or 4 forums I used to visit. Now it's just this and ax84. I have a couple ideas I'm trying to get into production, but the parts ordering and business side of things bores me to tears, so it's going slow. I also just built an amp I really like. I'm doing the woodwork for the cabinet right now.
Well, you know that if there's anything I can help with, just ask.

Agreed on the biz side of things. Add to that the fact that I dont really enjoy building the same thing over and over.... :)

If you get the time / urge, i'd love to hear a sample or two of the amp....

Have a great day !

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Post by DougH »

I posted a couple build pics and a schematic of the NPN Rangemaster I just built, based on this discussion.

It's in the diystomp gallery, here: http://www.aronnelson.com/gallery/v/DougH/NPN_RM/

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