NPN Ge Treble Booster Schematic Questions

[Ben Lyman]

Just wondering if this looks normal to you guys or if there might be a better way. I have it on the breadboard and it works/sounds good.
why is the BOOST control set up like this? It seems to be controlling the voltage, is that okay? Can turning it up all the way risk damage to the tranny?
I don't have any 47uF caps so I put two 22uF caps parallel to make 44uF... in two places... thoughts? yay or nay?
any other issues jump out at you with this scheme?
power filter resistor? 100 ohm? 1k? something else? none?
Thanks!

[Groovenut]

Just wondering if this looks normal to you guys or if there might be a better way. I have it on the breadboard and it works/sounds good.
why is the BOOST control set up like this? It seems to be controlling the voltage, is that okay? Can turning it up all the way risk damage to the tranny?
I don't have any 47uF caps so I put two 22uF caps parallel to make 44uF... in two places... thoughts? yay or nay?
any other issues jump out at you with this scheme?
power filter resistor? 100 ohm? 1k? something else? none?
Thanks!

The AC signal "sees" the DC as a type of virtual ground, there for think of the boost pot like this; when the wiper is closest to the collector you have the most output because you have the most resistance (10k) between the signal and virtual ground, when it's closest to the V+ DC it's the least and or zero (grounded)
The boost pot shouldn't actually control the DC voltage to the Collector. The collector voltage is determined by the current through the transistor times the resistance of the pot (this is also in conjunction with the 470k and 68k bias resistors)

The two 22n caps in parallel is perfectly acceptable... anywhere

I would use a 220R-470R power filter resistor. That will get you below 10Hz filtering. You might want to throw a ceramic 100n in parallel with the 47uF electro for high freq hash filtering help.

The 1N34A is there as a sort of temperature compensation for the germanium transistor.

[smallbearelec]

Just wondering if this looks normal to you guys or if there might be a better way.

Ben--
This is a Rangemaster. Have you read R. G. Keen's article about it on GEOFEX? That's the holy writ. There are a few differences in this schem.

why is the BOOST control set up like this? It seems to be controlling the voltage, is that okay? Can turning it up all the way risk damage to the tranny?

The GEOFEX article explains why the "boost" control is this way. It's really a volume control.

I don't have any 47uF caps so I put two 22uF caps parallel to make 44uF... in two places... thoughts? yay or nay?

It's OK on the breadboard, but Don't do a kludge like that in a soldered build, for you or anyone else...just bad practice.

any other issues jump out at you with this scheme?

I don't like the 1N4002 power protection diode wired to ground. If the power supply is actually connected in reverse, it will usually cause the diode to blow. It's protected the circuit, but then you have a repair job. I use a Schottky diode like a 1N5818 in Series and tolerate the small drop in supply voltage that it introduces.

[Ben Lyman]

Thanks guys, most of that makes sense to me.
Lawrence: is 100n AKA .1uF? and which 47uF do I set it with... power filtering cap or emitter to ground cap?

Steve, I can't get the Geofex page to load, I'll try again later. Thanks!

[Groovenut]

Thanks guys, most of that makes sense to me.
Lawrence: is 100n AKA .1uF? and which 47uF do I set it with... power filtering cap or emitter to ground cap?

Yes 100n=.1uF

Set it parallel to the power filtering cap.

[LightSoundGeometry]

series/parallel circuit with two voltage dividers. we are in chapter 6 right now, just looked at how a wheatstone bridge balances out and you can control something, like a motor for example, with a variable resistor like a photocell.

wire them up like I do, battery input only and leave the fluff out.  batteries last forever these days.

[PRR]

> BOOST control ... seems to be controlling the voltage

No. Look at it some more.

Or, since you have it on your bench, poke it with voltmeter and swing the BOOST knob.

> I put two 22uF caps parallel to make 44uF

In *this* case (many audio cases generally), a little less does no harm, and 2:1 less is not a lot less. Depending what the cap does, you may get a little more hum or a little less bass.

In this case it "looks like" the emitter cap gives 68Hz bass-cut. 140Hz bass-cut would be different but not necessarily bad.

In fact the base cap (0.005u) is also in this loop. You can picture the bass-cap effect relative to emitter-cap effect by multiplying by hFE. For quick-think, hFE is 100. So 0.005u at base is like 0.5u at emitter.

The 47u emitter cap is WAY oversize relative to the base cap. Someone just had 47's laying around, knew that sometimes you need that much, and threw one in. 10uFd should be much the same.

The 0.005u against just the 68K gives bass-cut below 500Hz(!). Higher since the 470K and the transistor also load the 0.005u. So whether the emitter cap is good for 68Hz or 140Hz isn't too important.

Parallel caps... we do sometimes do this (in fine finished products) when the required value is distressing. Power amps sometimes do it because they want a value too HUGE to buy in a single can (or fit in the space that Marketing demands), or power-stress causes cap heat which needs more surface/volume ratio. In working pedals (no huge stress), parallel caps doubles your failure rate

> power filter resistor? 100 ohm? 1k?

Simple 1-tube/transistor circuits without any strong requirements: drop-resistor may be 10% (5%-20%) of the plate/collector resistor. That's enough to promise some good, not so much as to cause a large drop.

Obviously if the power is crap and the signal is weak, this may not be enough; if the battery is weak and you need MAX!! output, this may be too much.

[Ben Lyman]

Thanks Paul, but how do I figure out 10% of my collector resistor if it is a 10k pot? just call it 10k?
btw, messing around a bit and settled on a 47k resistor right before the input cap to tame some of the overdrive. I spose I could just turn my guitar knob down to 9 for the same result but I don't want to.

[PRR]

> how do I figure out 10% of my collector resistor if it is a 10k pot? just call it 10k?

The *collector* always sees ALL 10K, so that is what you use.

The variable part of the pot *only* feeds the Audio Out, has no DC/Power function.

[Ben Lyman]

> how do I figure out 10% of my collector resistor if it is a 10k pot? just call it 10k?

The *collector* always sees ALL 10K, so that is what you use.

The variable part of the pot *only* feeds the Audio Out, has no DC/Power function.

THANKS! Having a revelation here!      
So.. my collector is only reading 2.6v, too low for an RM, isn't it?
I guess I need to put trim pots in place of the 470k & 68k resistors?
If one affects the other, how do I know which way and how much to adjust each? Just use my ear until it sounds good?
Still can't get anything from Geofex, maybe I'm not navigating the site properly (computers make me feel old and stupid)

[PRR]

> my collector is only reading 2.6v, too low

It stinks.

Use DC theory, and assume the transistor is acting as a dead-short (not a happy point for audio).

Round-up "3.9K" to 4K because I have not had my second cup.

10K and 4K in series is 14K.

4K out of 14K is 0.28.

0.28 of 9V is 2.57V.

Which is just what you got. (+/- tolerances on everything.)

That transistor is slammed ON. At best it can only handle half of a both-ways audio signal.

And if you *truly* have 470K 68K base bias resistors connected, this is NOT what we expect!

Base voltage should be 1.0V to 1.2V, not the 3+V your 2.6V reading implies.

Look for bad connection to 68K, 47K where should be 470K, etc etc.

[Ben Lyman]

oops... wrong pot! not a 10k but a 50k on my breadboard! changed it and now getting 8v at collector but lower volume. Sounds good, I took off my input resistor, nice clean tone, not much boost though.
Should I mess with the 470k & 68k? or maybe I could add a small resistor b/t 10k pot and collector?

[LightSoundGeometry]

rngemasters really dont boost the signal but add a pleasant tone to the amp . mine sound like %^&*ed wah wah

[Ben Lyman]

Okay, cool, does your "BOOST" volume control turn it all the way off as well? I was under the impression that it started off around unity and went up from there. I am still wondering about what to adjust to get the collector closer to 7v, if that's even what I am supposed to shoot for with this NPN version

[PRR]

> does your "BOOST" volume control turn it all the way off as well?

Look at the plan. Move the pot wiper.

Full "down" (on the drawing) it gets everything the transistor puts into it.

Full "up" the wiper (and output) gets only the sound of a battery bypassed with a fat cap.

Sound of battery+cap is (should be) dead silence.

In actual wiring, "knob up" is "wiper down".

Full gain should be fairly high BUT it cuts-back most of the bass and midrange. The "body" of guitar. Output volume may not be "louder", but it should be much brighter. If too bright, dial-back the "boost" to taste.

[Ben Lyman]

Thanks Paul, so much is coming together in my brain right now, I really appreciate everything you've taught me.
I still can't figure out Geofex, I have read a lot of stuff there that I just incidentally stumbled on but nothing on the RM yet.

How do I lower the voltage to the collector on this thing?

I had some spare tinker time tonight so I added a 5.6k resistor from V+ to 10k pot and got the collector at 7.1v
but... the base is .62v and emitter is .51v

hmmm... what should I do?
it sounds really good but as long as I am learning maybe I should make sure I learn it the right way. Thanks!

[ElectricDruid]

I still can't figure out Geofex, I have read a lot of stuff there that I just incidentally stumbled on but nothing on the RM yet.

The link you want is:

http://www.geofex.com/Article_Folders/Rangemaster/atboost.pdf

I built one from this and it's a lovely little box. I added a rotary switch which lets you select six different capacitors for the input cap. Getting the biasing right is the key to getting a nice tone out of it, and that's probably different for different trannies. I tried a few before I got one I liked. Ge trannys are all over the place.

When you get it right, it's a pretty subtle effect, but it makes your guitar sound somehow more vintage!!

HTH,
Tom

[anotherjim]

After moving house earlier this year, the Fuzz Central RM circuit was the first thing I made in my new workshop. Never had a RangeMaster before -  never thought such a simple circuit could do much, at least not without the magic AC30 amp. I was wrong, it was a highly useful way to use up an ancient AC127 that has been in a parts box for 30 years or more! Rather an odd one, it's from an amplifier and is encased in a rectangle of aluminium with bolt holes for heat-sinking. It was made that way since the part number is printed on the heat-sink.

Anyway, I fitted a 1k in series with the power input, so kept the reverse protection diode. The added 1k has no discernible effect on the sound,  makes the reverse diode current "safe" if wrong polarity applied and greatly improves supply noise filtering with the 47uF. I also (always) add a 100nF ceramic across the supply electro cap to catch high frequency noise. The circuit is not especially noisy, but I also used metal film resistors, which the originals did not benefit from.

As Paul noted, the 47uF emitter bypass cap is oversize. I had a surplus of 22uF so used that, but agree 10uF will do too.

I did play with the base bias resistor values and ended up fitting a 1M miniature pot as a variable resistor with 100k in series in place of the 470k, case mounted, so I can always find the sweet-spot. This is a highly interactive circuit, so changes here will change the input frequency response, gain and input impedance somewhat. This is quite audible in my build and time will tell if it needs tweaking occasionally or could have a been a set & forget internal trimmer. I did not fit a reverse Ge compensation diode, but have a socket for one if needed.

I also fitted variable input filtering -  just a pot in series with a larger input cap in parallel with the original. This gives adjustment from treble to all boost without the bypass popping issues of the switched input cap variations (but not as easily repeatable).

[Ben Lyman]

Thanks again guys, I am getting closer!
Tom-  I have had a look at the Geofex page you linked above and now I think I see
what I need to do, gonna give it a shot later when I get the chance.

[Ben Lyman]

Well, maybe my 2n388 wasn't good enough. I read up on testing Ge tranny's and I think I understand but I am having some difficulty understanding how to set my DMM. If I did it right, which I probably didn't, the tranny measured out to 13 (HFE?)

Anyway, I reset all my resistors (as close as possible) to the original values in the schematic and put in one of the tranny's from my recent surplus store pile. It's a 2n169A npn Ge and it sounds good, a little bit distorted when the Boost knob is turned up.

Here's what the voltage readings are

C  6.51
B  0.93
E  0.87

Is there something I should do to get the base up to 1v?
   *It looks like lowering the 470k would do that but how important is getting the base b/t 1-1.2v?
   *Will it make it run a little cleaner when cranked up if I do?
   *Should I call it good and be happy with it?
   *Should I slag off and leave y'all alone?