Vulcan Rat Creation

[thermionix]

but I think your input impedance will always be lower than R22 (220k) in your last schematic.

It actually will be lower than the half of R22 or R21..
(220k)//(220K)//(3k9x100+25R*)
*B-E junction resistance for 1mA Emitter current - which is negligible here..

P.S.
Many people tend to only count on the resistor(s) leading to circuit's GND and forget than DC supply must also considered GND for AC signals..

Yes, thanks for clarifying.  I was thinking it was R21 in parallel also but I wasn't sure.  Was hoping somebody would comfirm that.

[jfrabat]

Hey, Ben, I liked what I heard on the demo, and I am considering building one of these myself.  Couple of quick questions, though:

• I have 2N2222, 2N7000 and 78L05 transistors (and ONE 7805!) on hand.  Will any of these work as a replacement for the 2N5088's or is this a no go?  I am a complete newb here, so please bear with me...  I have only built 2 pedals so far (well, 3, but the 3rd one is not yet working, so I am not counting that one!).
• Similarly in the diode department; I don't have 1N914's, but I have 1N4148's, 1N4007 and some other one that, to be frank, I don't even remember what it is (looks like 4148's, but with a green painted top
• Last but not least, is this that latest schematic? 
  

Thanks in advanced!

Felipe

PS: Many components are kind of hard to find in Costa Rica, where I live!

[PRR]

> 2N2222, 2N7000 and 78L05 transistors

78L05 is not a transistor, it is a Regulator.

2N7000 is "a transistor" but a MOSFET not a BJT. The circuit would be very different.

In a conservatively designed BJT circuit, try 2N5088. If you are out of '5088s, try 2N2222. It will probably work.

THIS circuit is not "conservatively designed". It depends on the transistor gain (Beta) being in a certain range. A high-gain '2222 has the same gain as a low-gain '5088, so odds are it will "work", at least pass loud signals. At that point, get collector voltage measurements from a working sample. Diddle the 4.7Meg resistors up or down (probably down) to get similar collector voltage. An assortment of 1Meg 2.2Meg 3.3Meg and maybe a couple 470K (half Meg) will be handy. You can series several to add-up to 2.8meg or 4.1meg, whatever makes the circuit happy with the different transistor.

Diodes is diodes. 1N4007 used to cost a lot more than 1N914, because it handles a hundred times the power, but today the prices are all the same near-enough. But FWIW, 1N4148 "is" 1N914 with "better" specs. Anything sold today under the 1N914 number will BE from the 1N4148 production line, just re-marked to make buyers happy.

[jfrabat]

78L05 is not a transistor, it is a Regulator.

You are right; my bad!  So is 7805...

In a conservatively designed BJT circuit, try 2N5088. If you are out of '5088s, try 2N2222. It will probably work.

THIS circuit is not "conservatively designed". It depends on the transistor gain (Beta) being in a certain range. A high-gain '2222 has the same gain as a low-gain '5088, so odds are it will "work", at least pass loud signals.

I will give the 2N2222's a try.  Is there any difference between the 2 types?  I have both of them on hand (well, 2 of the can type, so I will have to use the others for this project), just wondering if one is better...


This is what I call the can type; not sure what they are really called!

At that point, get collector voltage measurements from a working sample. Diddle the 4.7Meg resistors up or down (probably down) to get similar collector voltage. An assortment of 1Meg 2.2Meg 3.3Meg and maybe a couple 470K (half Meg) will be handy. You can series several to add-up to 2.8meg or 4.1meg, whatever makes the circuit happy with the different transistor.

You lost me there...  Let me explain it to see if I am getting the concept.  Breadboard the circuit using the 2N2222, and measure collector voltage.  Based on your the voltage of your prototype, adjust the resistors to get a similar collector voltage to what you are getting (using either larger resistor [or using parallel resistors to increase resistance] or smaller resistors [more likely]).  Is that correct?

Diodes is diodes. 1N4007 used to cost a lot more than 1N914, because it handles a hundred times the power, but today the prices are all the same near-enough. But FWIW, 1N4148 "is" 1N914 with "better" specs. Anything sold today under the 1N914 number will BE from the 1N4148 production line, just re-marked to make buyers happy.

OK, will try to 4007.  Quick question; do you know what these are:



Sorry about the crappy resolution.  There are no letters or numbers, just the band and color...  I have about 10 of them.

Last question,

Is the diagram the latest one?

[Ben Lyman]

Greetings from rainy California Filipe! I would try what PRR says with the 2N2222, I've had plenty of 2N2222 from Radio Shack in the 500-600 HFE range so maybe you'll get lucky.
That schemo you posted was a failed experiment, here's the final, as heard in the video:


Here's a layout that fits a 125B, extra distortion diodes mounted right on a SPST

The 47pF (upper left corner) can be moved around, possibly left out completely

[jfrabat]

Thanks Ben!  I will use that one then.

[Ben Lyman]

cool, let me know how it goes. If it's on the breadboard, you might try experimenting with different tone stacks.

It has a ton of volume boost available so any kind of tone stack should probably work. I just went with the simple low pass filter because i'm a minimalist and it's an easy one to fit into a layout.

The SWTC II is a good read: http://www.muzique.com/lab/swtc.htm

A Big Muff Pi tone stack might be cool.

Or the tone knob plus presence knob found in the Crunch Box

[jfrabat]

Actually, having volume, gain, tone and presence (4 knobs) would work just fine, because I only got big boxes left, so aethetically, it would make more sense to have a big box for 4 knobs and a switch.  Just help me with my lack of knowledge (I just got started a month ago with building pedals; unfortunately, I am now hooked!):

In the diagram above, the tone stack is the filter, right? (C10, R17 & R18, right?)

[Kipper4]

R17 R18 C13

[PRR]

> the can type; not sure what they are really called!

"Metal can".

The other is "plastic".

Back around 1970, we thought the metal was a better seal against moisture. But not perfect. Meanwhile the plastic (epoxy) technique got very much better. Today the epoxy is widely preferred even in very tough environments. Metal can is nearly obsolete. (That particular "metal can" may be epoxy in a metal shell. True METAL has a metal base plate and glass seals.)



If you have to play on Mars, get expert advice. For any likely place you would play, any case type is totally fine.

True metal case is likely to be "vintage". But 2N2222 kinda marks the point in history when transistor technology "matured". The '2222 and all later types work very predictably and all about the same, unlike older types which could be a bit squirrelly.

[Ben Lyman]

By the way, I didn't include my power filter/protection in the schematic..
You can see all the parts in the layout but here's what it looks like. 

[antonis]

@Ben: Better raise C1 to 100nF (preferably ceramic disc..) 'cause a 333kHz LPF is quite useless.. 
You could also halve C2 to 47μF for a hum-free supply..
(IMHO, ripple filtering should be still adequate..)

[rutabaga bob]

Ben!  Nice demo!

[Ben Lyman]

Thanks Larry!

Antonis, thanks, that is a typo, it's s'posed to be this:


But now you got me curious about using a 47uF for C2.. why is 47u better than 100u?
How about I also raise R2 to 100r?

[jfrabat]

So, combining the tone stack of the Crunch Box with the pedal would look something more or less like this?



Please give it a once over and see if you spot any obvious errors.  Also, please check if the pots values make sense (I used what I found for the Crunch Box).  Keep in mind I dont fully understand this yet...  Yet being the key word here!

I still need to define which values I need to on the power filter/protection...

This...



or...



I got some time, though, as I will not start work on this pedal this week anyway.

Felipe

[antonis]

But now you got me curious about using a 47uF for C2.. why is 47u better than 100u?
How about I also raise R2 to 100r?

Also raising R2 to 100R (from 47) shouldn't have any effect on LP filter 'cause you've lower C2 to 47μF (from 100) - so you've kept the same RC product..
Additionally, raising R2 should result in higher voltage drop from PS to +9V point (added to Schotky's Fvd..) - which is negligible in this case but it's a theoretical discussion..

The propose of using a 47μF cap with the 47R resistor is to form a more effective HP filter (at about 72Hz instead of 35Hz) for "hum" rejection (50-60 Hz).
The higher the cut-off frequency the better the filter effectiveness (6db/octave) but you also need a relatively big capacitor for power supply ripple smoothing so, as almost everywhere, it's a matter of compromise..
My bad...
We deal with LP filter so disregard the above absurdity..


P.S.
You probably experience a significant Volume drop between S1 open & closed mode - if so, a better arrangement with a series resistor at S1-Off mode should result in no need for Volume pot adjustment...

[Ben Lyman]

..We deal with LP filter so disregard the above...
Ahh yes, I thought maybe such 

Filipe: either of those power filters work and do the same thing, I always go with the top one.
As antonis said, there is less voltage drop going through the 47 ohm resistor as opposed to using the larger 100r.
If you don't have a 1N5817 schottky diode, there are other diodes/methods but I was taught to do it this way and it hasn't failed me yet.

For the tone stack, I don't know what that will sound like as you've pictured it. Probably breadboard it first.
Look into the way the crunch box tone pot is set up, it's different than the Rat filter which we have here.

One final thing, try leaving out the extra diodes+2.2nF until you listen to the pedal and then decide if you really want more distortion.
I only added them because it's so easy but probably not really important in a pedal like this. you can always build a tubescreamer to go in front of it anyway 

[jfrabat]

I think I'll start breadboarding yours and then play with it a bit and see if I can blend a bit of the Crunchbox into it.  I am not confident enough in my skills to try to do it from scratch.  I have barely started making pedals, and of the 2 I have finished, one works wonderfully, but the other, not so much...  And the one I got in progress I will most likely have to restart!

[jfrabat]

Started breadboarding...  missing one cap, which I have to get (100p), but something is obviusly not connected right, as I am getting no sound yet.

[Ben Lyman]

I don't think there is a 100p in this... do you mean the 47p? That one is not important.

Check the transistor pin outs. Maybe they're backwards?