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Post by johan on Apr 14, 2007 15:10:12 GMT -5
Hi all, These GE boosters are hot on the boutique pedal market, and cost hundred(s) of dollars. I don't see why. Don't be scared by the schematic, is the easiest pedal besides a passive AB box or so. If you want help building one, just ask. Have been overfocussing on a single stage guitar booster designs and managed to come up with a very basic setup to use Ge transistors in such a booster. In the sense that it doesn't just boost the treble like the Rangemaster. That's why in this as yet still stripped down version, the conponent count is minimal: 3 caps, 1 resistor, 1 pot, 1 transistor. The simplicity of the thing stuns even me, and it seems that just dumping the voltage divider is the trick. You could probably put some resistance across Base - Collector, but haven't tried that yet. Of course the ground doesn't connect to the output network, just to the pot and cap on the emitter. C1 = .022uF C2 = .047uF or higher C3 = 47uFR1 = 100k trimpot or appripriate resistor to bias collector voltage at 4,5V for the optimum transistor operation. Putting the bias over 5V will influence your distortion, make it shred a bit more. R2 = 5K gain pot more detailed report on my blogpage: modman.blogdrive.com/archive/37.htmlthanks for taking the time to read this far j
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Post by JohnH on Apr 14, 2007 16:36:29 GMT -5
johan - thats cool. We need more active electronics on this forum.
An aspect of your circuit that puzzles me is that there is no resistor to feed bias current into the base. Do you get a clean boost, or is it distorted? Have you tried it with a high-value resistor from either base to collector or base to positive?
Another thought, if you want to boost the higher frequencies rather than the lower ones, would be to try lower values of C3.
cheers
John
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Post by ChrisK on Apr 14, 2007 17:03:27 GMT -5
I have a number of questions about this circuit and statements made on the referenced link.
The link states that a low-leakage germanium transistor is used, but for this bias scheme to work (and that indeed is what it is), the emitter/collector (and likely collector/base/emitter) leakage is fundamental to achieving bias via the adjustment of the collector resistor, certainly with the normal biasing mechanisms having "gone missing". Without such "bias", the circuit would at best achieve a half-wave clipped peak amplification once the Vbe threshold was overcome.
Note that the ?quiescent "bias" is set to about 4.5 volts on the collector for best clean signal. This makes sense for the best clean headroom, especially with a transistor with a hfe of 100.
The base is AC terminated via the resistance of the pickup(s) and the input coupling capacitor. It is DC biased by said leakages, and the AC input signal causes an effective variation in base leakage bias and amplification results.
Note that the usual input bias resistors are normally used to "rein in" the substantial hfe variations normal to production lots.
Bias is bias.
Now, if it were actually a PNP or a FET......
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Post by johan on Apr 14, 2007 18:55:12 GMT -5
An aspect of your circuit that puzzles me is that there is no resistor to feed bias current into the base. Do you get a clean boost, or is it distorted? Have you tried it with a high-value resistor from either base to collector or base to positive? John you know your stuff thoroughly or you know the mother circuit maybe. If not you've illustrated my point that there are not too many ways to set up a workable GE single gain stage. Had it setup with a 3M9 resistor between B and C, and later with an extra 1M pot is series which didn't really influence the sound much or in every pickup config. Tim Escobedo does it too: up to 5M in these silicon booster designs It does distort at max gain but in a good way, and as was to be expected it's very sensitive to the guitar volume pot. Back of a bit and you get some solid gritty boost, I assure you. In my measurements I was hoping to get the base more positive than the emitter but the 3M9 didn't seem to influence that. It's hard to measure into the millivolts but if the base really has to be positive, this might just matter? It's a rough circuit. Just wanted to understand the basic GE gain stage. Not so over the hill about germanium myself, rather work with FETs (don't we all?) but have a stash GEs I bought in a silly moment. And as a I wrote, GE pedal are going crazy prices. Not like a want to jump the bandwagon, my blogsite just says DIY for a few bucks. I really think guitar pedals are a joke, but there isn't a guitarist that can do without a solid booster. Another thought, if you want to boost the higher frequencies rather than the lower ones, would be to try lower values of C3. cheers John 22uF is always a better starting point -- but don't want to make it into a Rangemaster or treble booster. That's a know circuit. The link states that a low-leakage germanium transistor is used, but for this bias scheme to work (and that indeed is what it is), the emitter/collector (and likely collector/base/emitter) leakage is fundamental to achieving bias via the adjustment of the collector resistor, certainly with the normal biasing mechanisms having "gone missing". Without such "bias", the circuit would at best achieve a half-wave clipped peak amplification once the Vbe threshold was overcome. ChrisK, what can I say, your comment is really exciting! Ok, there always is some leakage in GE device, even in silicon transistors, but negliable in the latter. That's why they invented them as we know. Are you saying that perhaps a bit higher gain GEs with more leakage might bias even better?? Are you eventually saying I should put an voltage divider on the input? What difference would a PNP make? thanks Johan
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Post by UnklMickey on Apr 14, 2007 20:23:04 GMT -5
John you know your stuff thoroughly or you know the mother circuit maybe. Johan hi Johan, i can't speak to the second part of your statement....only the first. HE DOES.
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Post by JohnH on Apr 14, 2007 20:25:26 GMT -5
I think Chris has put his finger on it, the bias to get the steady state current flowing is via leakage, which is greater in Ge than Si devices. But if it works, it works!
John
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Post by johan on Apr 15, 2007 3:46:57 GMT -5
I think Chris has put his finger on it, the bias to get the steady state current flowing is via leakage, which is greater in Ge than Si devices. I paraphrase to learn: you don't seem to have to put that little bit of voltage on the base, because it already is there because of leakage But if it works, it works! John But we do understand why now -- don't we? The point is once again that what makes old devices sound like no modern device is their flaws. Modern technology is just too accurate. Nobody gotten his breadboard out yet? J
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Post by wolf on Apr 15, 2007 18:03:41 GMT -5
Well I didn't get out the breadboard, but I broke out the drawing board so to speak - MS Paint. I just felt like "neatening up" johan's great circuit. Instead of making this a pedal box, wouldn't it be easier to mount this directly into the guitar? (Sorry for the smudges but I was redrawing a .jpg image)
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Post by ChrisK on Apr 15, 2007 23:09:40 GMT -5
Yo,
I know that we're all ground-adverse in our meta-physical schematification or abstraction thereof, but we should, like show it.
Let's really confuse folk and show the two input terminals and the two output terminals as they would apply to phone jacks.
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Post by wolf on Apr 15, 2007 23:47:04 GMT -5
ChrisK
See above. If you don't see any change, hit the refresh button.
Johan From your site you state: ... connect collector through a biasing resistor to your 9V, connect the emitter to ground through an electrolyte and take the output signal from the collector after an output cap.
Have I drawn the ground at the correct point in the circuit?
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Channelman
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Post by Channelman on Apr 16, 2007 10:29:17 GMT -5
Sorry, but I couldn't resist chipping in on this one.
Ok, the Germanium transistor is 'biased' into conduction by it's own leakage current and the 100k pot is used to set the the collector voltage to 4.5 volts BUT nobody has mentioned the temperature dependence of this parameter of germanium transistors. If anybody builds it, then just set up the 4.5v and then warm up the transistor by holding between the finger and thumb and watch how much it changes. The conventional bias circuit (apart from tolerating hfe variations - as ChrisK points out) also helps fix the bias current at different temperatures.
No mate.....it should be on the negative terminal of the battery.
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Post by sumgai on Apr 16, 2007 13:21:00 GMT -5
Wolf, Ummmm, not quite. Since this is a common-emitter circuit, convention has it that the emitter should be the lead that faces downward in a schematic. Drawing sideways as you have done implies a common-base configuration, and that tends to inhibit proper analysis for a short period.
Additionally, convention also says that we follow the flow of the circuit from left to right, where practical. Displaying the output connection back at the left, in vertical alignment with the input, is misleading to the eye.
And not to gild the lilly, but you added an extra connection - the emitter should not go directly to ground. In effect, the long wire that 'jumps' over the input at the base is ground, and can be eliminated by using the appropriate circuit symbols.
sumgai
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Channelman
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Post by Channelman on Apr 16, 2007 13:30:07 GMT -5
So. 'summing' it up sumgai......you didn't like wolf's redraw
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Post by ChrisK on Apr 16, 2007 14:48:51 GMT -5
Not to mention the contusions and eventual concussion sustained by the electrons.
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Post by wolf on Apr 16, 2007 18:42:00 GMT -5
Well, hi folks.
Channelman I followed your correction and redrew the ground.
And sumgai I followed your suggestions too except about that emitter arrow. I always thought the emitter arrow in an NPN transistor always points outward.
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Post by UnklMickey on Apr 16, 2007 20:56:34 GMT -5
So. 'summing' it up sumgai......you didn't like wolf's redraw *groan* don't you know there is a smite button for people who make bad puns? here, let me show you ...................................... nah, i'll hit the other one. welcome back.
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Post by UnklMickey on Apr 16, 2007 21:19:54 GMT -5
...Since this is a common-emitter circuit, convention has it that the emitter should be the lead that faces downward in a schematic. ... ...And sumgai I followed your suggestions too except about that emitter arrow. I always thought the emitter arrow in an NPN transistor always points outward. if i might be so bold, as to translate here. Wolf, you are right about the direction of the arrow. Sumgai was talking about the rotation of the entire symbol so that the emitter is the terminal is the closest of the 3, to the bottom of the page. there are exceptions, but this convention usually works best for a common emitter, or a common collector configuration. i wonder if the input impedance of this circuit might be exceedingly low, particularly if the hfe of the transistor is low and/or R 2 is fully bypassed by C 3
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Post by wolf on Apr 17, 2007 0:49:20 GMT -5
Well, I studied electronics way back when. Basically the rule was the base of the transistor is always at the bottom of the diagram.
Anyway, I think all corrections have been made (unless I messed up something on my last edit).
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Post by sumgai on Apr 17, 2007 3:24:46 GMT -5
wolf, Doesn't matter when you studied electronics, the 'rules' haven't changed that much. In schematic drawings, the IEEE 'style guidelines' state that the common element should be the downward facing lead. The reason is simple - you are to put the input side of the equation to the left of the drawing, and the output side to the right. But since both sides of the equation must have a common reference point (in this case, the common element of the transistor), it must be shown as 'shared' between the two sides. Laying a common-emitter circuit on its side, so that the base is down, misleads one to first believe that he or she is looking at a common-base circuit. It so happens that I am not a member of the IEEE, so I can't breach their website in order to find a link to these style guidelines. Perhaps Chris........... Of course, if you're in a country that doesn't adhere to the IEEE standards....... And yes, the mnemonic for remembering which way the arrow goes is: NPN - Not Pointed i NHTH sumgai
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Post by sumgai on Apr 17, 2007 3:29:06 GMT -5
Look, up in the Internet Cloud....... It's a Bird! It's a Plane!! It's...... Channelman!!!!!! Welcome back, young feller! Hope you can stay around for a spell. sumgai
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Channelman
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Post by Channelman on Apr 17, 2007 4:20:56 GMT -5
Welcome back, young feller! Thanks for the welcome back sumgai......you must be mixing me up with someone else though. The Beatles have a song about me......starts off with, "When I get older......". Havn't been called "Young feller" for quite a while. wolfNPN emitter points outwards PNP emitter points inwards
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Post by sumgai on Apr 17, 2007 11:48:01 GMT -5
Channelman, I consider the day a wondrous one indeed, if I don't suffer at least one mix up! ;D
sumgai
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Post by ChrisK on Apr 17, 2007 16:17:31 GMT -5
Waaaaaay, back.
I haven't seen common base transistor usage ('cept for RF) since a few years after the invention of agriculture.
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Post by wolf on Apr 17, 2007 18:33:06 GMT -5
Well gee whiz, I didn't mean for my comments to bring this discussion to a screeching halt, hence I deleted them.
Anyone built one of these things yet?
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Post by sumgai on Apr 18, 2007 16:18:27 GMT -5
wolf, Deleting things like this can disrupt the flow for future readers. Pleaese restore them, or something like them, in order to avoid confusion. In the future (and for everyone, not just wolf), please use the Strikethrough tag to show that you've "changed your mind". (That's the fourth button from the left, top row.) Sorry, don't feel the need to do that just now. sumgai
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Post by wolf on Apr 19, 2007 0:02:17 GMT -5
sumgai Well since nobody responded to that posting, I don't think it is going to disturb the flow of anything. But be that as it may (and I doubt if it ever was), I'll be sure to keep that message board practice in mind.
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Post by ChrisK on Apr 19, 2007 11:56:43 GMT -5
I once built a simple, dead germanium guitar booster.
When I was in college, a professor turned the tables on everyone and put an AC circuit analysis final exam question into common-base form (he'd only taught the common emitter parameters).
Everyone choked except for one older guy (me, I was 23) who'd actually seen and used that archaic model in the past.
Translation is.
If you understand the model of things, the equations (and the memory thereof) are moot, since they are self-extracting and patently obvious.
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Post by dd842 on Apr 19, 2007 11:58:48 GMT -5
I once built a simple, dead germanium guitar booster. LOL +1 to you Chris
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Post by johan on Apr 20, 2007 9:18:55 GMT -5
I once built a simple, dead germanium guitar booster. maybe the standard biasing network you think is missing from my schematic killed yours ;D ? No really Chris, please explain, what you meant by your initial comments and show me how you would set it up, even if you don't have the time to breadboard it yourself. thanks J
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momo
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Post by momo on Apr 22, 2007 10:15:50 GMT -5
I built the simple Dallas Ragemaster using the original schematic and Germanium PNP's as the original, so its positive ground and will not work with regular negative ground supplies,but this is still THE booster to have...Toni Iommi, Clapton Beano album. Its tone goes from slight treble boost to glassy razor sharp high end, just really great for soloing and cutting thru the jungle! Cheers
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