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Author Topic: HELP! One knob Silicon Pep Box?  (Read 6211 times)

Ben79

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HELP! One knob Silicon Pep Box?
« on: March 25, 2015, 06:29:43 PM »
I'm building a Pep Box but I'm cramming it onto a small perf board with 2 other fuzzes (Jerkulator and Bazz Fuss) so I'm trying to simplify each so it only has a volume control and gain can be set by the guitar's volume.

If I want to lose the 'effect' pot from this (and run it at max gain), would I replace it with a 10k fixed resistor and connect the - of the 25uf cap with the base of Q2 and have the 10k running to ground from that junction?  



Thanks for any help.

Roly

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Re: HELP! One knob Silicon Pep Box?
« Reply #1 on: March 26, 2015, 04:52:38 AM »
AAAAaarrrggghhhh...   :o

Where did this come from?   ::)

Deadly problem: If the pot marked "?" is turned right down the Base end, the battery is then connected directly across the second transistor Base-Emitter junction, and you can kiss goodbye to the second BC107 (poof!).

(and I'm not too sure about the 56k in parallel with the 500k Output pot either ... in fact the whole darn thing looks a tad dubious.  8|   If I had my way there would be a license test before anybody was allowed to post circuits on the net.  :trouble )


Quote from: Ben79
I want to lose the 'effect' pot from this (and run it at max gain)

1. Make the "?" pot a 150k fixed resistor from +9V to the 2nd Base,

2. change the 100 ohm pot-ground to 10k Base-ground,

3. connect the -ve end of the 25uF from the first Collector directly to the second Base.

That should get you close.  If you want to tweek the voicing then trying resistors for "?" in the range 100k to 220k should cover the available tones.

HTH
If you say theory and practice don't agree you haven't applied enough theory.

Ben79

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Re: HELP! One knob Silicon Pep Box?
« Reply #2 on: March 26, 2015, 05:45:46 AM »
Many thanks once again for your quick and brilliant help Roly.

The schematic actually lives on the Musikding.de forum but I've been using the build documents supplied by Ian Sherwan at Ghost Effects for his Pep Box kit as a reference and he mentions not to set the 500k bias trimmer too low or too high.  I think that resistor in parallel with the volume pot is just there because it was seen in the original pedal which must have been made when the factory only had access to 500k pots?

I'm pleased with myself as I had a go at working out what the pot was doing and reasoned that it was just tapping off signal to ground so the higher the resistance to ground, the higher the volume, the more intense the clipping.  Last night I soldered in a fixed 10k to ground and it turns out I was right.  Haven't tested it yet though.  Was my analysis correct?

I've put these 3 simple circuits on one small perf and now I'm wondering how to implement them.  I'm thinking 3 dpdt toggles like a miniature pedal board and a volume control on each.  I should probably audition all options before deciding on the order they run in.  Or I could use a 2p3t rotary which would be neat but then they wouldn't be stackable.  I'm also thinking a master input attenuator would be a good idea so the overall gain of the pedal can be set.  Any other implementation ideas?  All 3 in parallel with mix pots/sliders? :loco

 
« Last Edit: March 26, 2015, 05:58:47 AM by Ben79 »

Roly

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Re: HELP! One knob Silicon Pep Box?
« Reply #3 on: March 26, 2015, 06:52:40 AM »
ThanUThanUThanU ... your a great audience here tonight.

The only effect of the 56k in parallel with the output control is to reduce the gain of the driving stage by reducing its AC load resistance.  It just doesn't seem very ... erm ... well thought out(?).


Quote from: Ben79
Was my analysis correct?

Yes.  +1 Internet.




Quote from: Ben79
I should probably audition all options before deciding on the order they run in.

Absolutely.

Quote from: Ben79
Any other implementation ideas?

As I keep saying, a major point for building your own over buying commercial is that if you don't like it the way it is you can leap in and give it a rewiring it will never forget.  You are in charge instead of being the hapless victim of the compromises forced by some company bean-counter.
If you say theory and practice don't agree you haven't applied enough theory.

Ben79

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Re: HELP! One knob Silicon Pep Box?
« Reply #4 on: March 26, 2015, 07:29:17 AM »
Quote
You are in charge instead of being the hapless victim of the compromises forced by some company bean-counter.

Amen!  I'm gonna see if Noam Chomsky will endorse this pedal when it's finished. 

J M Fahey

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Re: HELP! One knob Silicon Pep Box?
« Reply #5 on: March 27, 2015, 07:51:57 AM »
1) that Fuzz pedal is HORRIBLE
2) the 56k resistor was added so there is a brutal Bass cut (notice the 3300pF coupling cap) so everything below 900Hz disappears.
Then, why didn't they use the proper 50k pots?

From the mind of the "designer":
Quote
a) the amps we are making have 500k volume pots
b) guitars have 500k volume pots
c) if amps and guitars have 500k volume pots, then all volume pots must be 500k
d) so the Fuzz pedal I'm designing, which goes between a guitar and an amplifier, must also have a 500k volume pot, so we don't offend the Gods and they send a second Deluge or at least a Tsunami.
e) not forgetting that Sally mistyped a purchase order and now we have 10000 500k pots in the warehouse, instead of the 1000 we actually needed
:lmao:

3) that horrible Fuzz, straight from the beginning, (notice it didn't exactly become widely used and remembered as, say, Fuzz Face or VOX Tone Bender), must at least have been bearable because Germanium transistors lacked gain (so they didn't fuzz that much) and had poor highs (which somewhat smoothed buzziness) while the silicon version must sound like having your teeth pulled with a gas pipe wrench.

the 100 ohms resistor is needed so with Fuzz set to minimum just a very little signal reaches Q2 .

4) not forgetting that it's probably wrong.
Very much doubt it's the actual circuit found inside an original PepBox, which must have been at least barely usable.

If you want to build a similar but far more usable pedal, try this , basically the same but functional:



I think the resistor values were misread, the copier had not much clue, as shown by the "350k" and "33k" pots, not such thing ever, he must have read them in circuit and most certainly were 500k and 50k , standard values.

With Germanium transistors reasonable collector resistors might have been 47k, not 470k , and with silicon transistors 4k7

Roly

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Re: HELP! One knob Silicon Pep Box?
« Reply #6 on: March 27, 2015, 02:51:36 PM »
{I'm just too trusting, right?}

Quote from: J M Fahey
From the mind of the "designer":

Quote
b) guitars have 500k volume pots
c) if amps and guitars have 500k volume pots, then all volume pots must be 500k

Now there is a real gem of design drivel.  ::)   The "designer" seems so oblivious to the fact that this is total rubbish that they are willing to post it on the net.


Quote from: J M Fahey
Then, why didn't they use the proper 50k pots?

My own thoughts exactly.


Quote from: J M Fahey
4) not forgetting that it's probably wrong.
Very much doubt it's the actual circuit found inside an original PepBox

And you're right.  Looking at pix of the actual PepBox circuit board I notice that the output 56k is in the original, however there are also some resistors with values not on the circuit above, 470k, 1k5, 820k, and what looks like 18k, so the circuit above is at least a pretty poor match to the original.  When four out of seven resistors are different values and both transistors have been changed from germanium (ACY11's) to silicone (BC107's) you have to wonder if this highly modified circuit deserves to be called a "PepBox" at all.  I certainly don't think so, even "derived from" would be a stretch.  Avoid.


When you are looking at circuits, particularly on the net, and find something like the "?" control that will blow up the associated transistor if turned too far, loud alarm bells should be ringing.  This circuit follows the "you never find just one roach in a kitchen" rule, even in a simple circuit like this one, and you should be very alert for other electronic bloopers.
If you say theory and practice don't agree you haven't applied enough theory.

Ben79

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Re: HELP! One knob Silicon Pep Box?
« Reply #7 on: March 29, 2015, 11:25:42 AM »
I think WEM made 2 versions - a Ge and an Si. That would account for the differences in the circuits, wouldn't it?

I've spoken to Ian at Ghost Effects who has traced an original and he confirms the schematic is correct except for the 56k which can be omitted and the electros which should be 12uf.  He specifies a 500k bias trimmer set around 120k.

I breadboarded one a while ago and it sounded pretty good.

Roly

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Re: HELP! One knob Silicon Pep Box?
« Reply #8 on: March 29, 2015, 01:21:55 PM »
Silicon version internal;

Shame on WEM for not putting in one more resistor to protect the second transistor from twiddling guitarists.  If the required value is 120k then use a fixed resistor rather than a (circuit-destruct) trim pot.  Murphy's Law dictates that if it is possible to crank it around and smoke the transistor, someone, sometime, will do so (most likely you in a moment of forgetfulness).  If you must use a trimpot then at least put a 47k in series with it to cheat Murphy.

This is the germanium version;


The "effect" control has a different action here to the later silicon version in that it changes the bias on the second transistor.  Pity they didn't call it something else because it's significantly different.
If you say theory and practice don't agree you haven't applied enough theory.

Ben79

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Re: HELP! One knob Silicon Pep Box?
« Reply #9 on: March 29, 2015, 03:02:04 PM »
Can you say a bit more Roly about how the 'effect' pot is doing something different in the Ge version because it looks the same as in the Si to me. 

« Last Edit: March 29, 2015, 03:03:08 PM by Ben79 »

Roly

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Re: HELP! One knob Silicon Pep Box?
« Reply #10 on: March 30, 2015, 04:12:53 AM »
In the Si version there are three resistors in the bias divider, "?" pot, the effect pot, and 100r.  The effects pot changes where the AC signal is injected into the DC bias chain but it doesn't change the DC conditions on the Base of the second transistor.

In the Ge version the 820k ("?") goes to the effect pot wiper instead of the top of the pot, so that when the effect pot is changed it also changes the total value of the bias divider chain, and thus the voltage at the Base of the second transistor.

If you are having trouble visualising it try considering the two limiting cases, effects pot at max, and at min.

At max the divider chain consists of 820k, (Base) 10k effect pot, 100r.

R = 820k + 10k + 100r = 830.1k

I = E/R
9/0.8301 = 10.8421uA  (volts, megohms, microamps)

VB = I * R
10.8421 * 10.1 = 109.5052mV

At min it consists only of 820k and 100r, with the Base connected via the 10k pot (which will have very little effect because of the low Base current).

R = 820k + 100r = 820.1k (10k less than above)

9/0.8201 = 10.9743uA

10.9743 * 0.1 = 1.0974mV

At this setting the transistor will be cut off due to insufficient VB and only conduct on signal peaks.

This also means that the driving stage will now be seeing a load of only 100r and its gain will therefore be seriously reduced (true for both versions).

In simple terms the effect pot in the Si version changes the AC level but not the DC level at the Base.  In the Ge version both the AC and DC levels change.

HTH
If you say theory and practice don't agree you haven't applied enough theory.

Ben79

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Re: HELP! One knob Silicon Pep Box?
« Reply #11 on: March 30, 2015, 07:24:00 AM »
Thanks Roly, I see now.

LJN

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Re: HELP! One knob Silicon Pep Box?
« Reply #12 on: April 08, 2015, 09:43:55 PM »
hello there. I wired up a silicon pep box the other night and absolutely hated it. too dark and sounded mis biased. The electrolytic I used were 22 uf.  maybe I should try replacing them with 10 uf?  and try the 120 k resistor on the base of Q2?  Are they supposed to sound this bad?
If it sounds good, USE IT!

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J M Fahey

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Re: HELP! One knob Silicon Pep Box?
« Reply #13 on: April 09, 2015, 01:40:41 AM »
Just looking at the schematic hurt my ears ... does that answer your question?  xP

The schematic has errors , but even the original one can't be good, specially today.

Maybe in the old days it was somewhat acceptable; but with a similar circuit, two single transistor stages , cascaded, the Mosrite schematic I posted is better.
At least it works, try it, you can modify the one you already built.

LJN

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Re: HELP! One knob Silicon Pep Box?
« Reply #14 on: April 19, 2015, 08:22:52 PM »
Thanks. I did a little more work on it. Changed the electrolytics to 10 uf . sounded a little better. right now I'm finishing my maestro super fuzz. I love that one. :dbtu:
If it sounds good, USE IT!

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