fetzer valve pedal help

[night0wl]

Hi peeps,

While I was on a roll from making the overdrive pedal I thought I'd have a go at the Fetzer Valve - FET Booster schematic

I used a MPF102 FET because I can't get J201's or 2N5457's here and the "boost" works fine

. I have a slight prob with the project...The tone is poor in bypass mode (ie; not "bright" sounding), when the effect is bypassed and I turn my guitar volume knob it sounds scratchy. When the effect is on everything is boosted and no scratchy pot. Then when I press the switch to turn the effect off it pops (doesn't pop when switching the effect on)

Here is how I wired it all up (please excuse the ordinary diagram )


Any ideas??

Thanks
Shane

[UnklMickey]

night0wl said:

Hi peeps,...

...Any ideas??

that looks like the switch is configured to do a complete bypass. so i'd start there and look for a wiring error or a bad switch or the wrong switch.

[night0wl]

I used the same wiring to put together the overdrive pedal I made and it seemed to work fine. Although thinking about it now, the overdrive did suffer a little in the tone area ie; very "midi" and not alot of high frequencies.

The switch wiring is from the "LED trick" file I have, copied every last detail.

The switch is new and I have been over the wiring a few times. It isn't in a box yet so would that make a difference?

[JohnH]

Hi Nightowl - at last someone nearby. I live near Wollongong. Ive also been playing with JFETS recently. Isn't it a pain trying to find what you need here? Ive bought some 2N5457's from % Smith, but they are going out of stock. MPF102s work in these circuits, but not as well, you end up with less available output swing, and also gain, since they need more bias voltage gate to source. I bias mine to 6V at the drain instead of 4.5V as on runnoffgroove. Anyway, they are working for you and that is great - you are on a roll

Now your problem - it seems there is something odd happening that is not obvious from your fine diagram. One thing that always causes a pot to sound scratchy, when it doesnt usually, is if there is some dc running through it. In most circuits, including this one, the pot is protected from dc by the output cap. The guitar volume pot should be OK, since the input to the booster input is biased to zero.

However I always put a small cap between booster and guitar, and I use 39nF which is plenty. That may fix the scratchy guitarpot, but there may be an underlying issue with the way the battery is connected via the jack. Are you using a stereo jack socket as a way to engage the power? By chance, I posted a design on the Schematics page yesterday (LP maximiser) with a JFET buffer, and the wiring to a jack socket. If you take a look at that, it might lead to something.

John

EDIT - an outstanding discovery: Im not allowed to write the name of the most well known electronics supplier in Australia, which is D-I-C-K Smith.

Lets try it again:

Dick Smith, Dick Smith

See?

[night0wl]

JohnH, I am using a stereo input jack with the tip going to the DPDT, the sleeve to ground and the last lug to the negative battery snap. Could the ground wiring have anything to do with my problem? Maybe move the wire connecting the stereo input jack (-)/100k pot over to the circuit ground/100k pot?

I will try the cap before the booster and see if it fixes the scratchy pot.

What else could cause the dull tone I am getting?

Thanks,
Shane

[UnklMickey]

another thought came to mind, as a possible cause of these symptoms.

if the connections at the input jack are resistive, that could cause some current to be flowing through the volume pot of the guitar. i would think that the same thing would occur when the preamp was "on". (actually it's always on, just bypassed when it's "off").

do you still have these symptoms, when the switch is in bypass, and the battery is disconnected?




BTW: i have always thought that, this method of power switching through a stereo jack was prone to problems.



i see that you have posted again, while i'm writing this.

your diagram seems to indicate that the power (-) is connected to the sleeve of the jack and the circuit and output grounds are connected to the "ring" connection. but maybe i don't understand the drawing conventions you have used.

[JohnH]

As you describe it, the jack wiring sounds right. Getting the tip and 2nd output or sleeve connections mixed would be the sort of thing that could explain the problem however. You could try a test where you just hardwire the battery to the circuit, not via the jack, and treat the jack as a normal mono jack, without using it to control the power

Dull tones from the guitar can be caused by lowish resistances between ground and hot, eg, if your booster volume control was also in the circuit in bypass mode, that could explain it (not apparent on the diagram however). Or some solder gunk causing a partial short between lugs somewhere.

On your OD circuit, it has a fairly low input impedance, and so will tend to damp out high frequencies. The JFETS however have a very high input impedanc, so that should not occur with this booster active

John

[night0wl]

I just added the cap between input and circuit and the popping and scratchy pots has stopped

Aswell as a dull tone, the bypass signal seems a little bit weaker than if I run my guitar directly into the amp. Would a different cap before the output help?

[JohnH]

The bypass as drawn should be a pure electrical connection through the switch, with no affect on the tone. Hence there is an issue in the construction relative to the diagram at the root of this. BTW that extra cap can be between switch and circuit input, it should not be needed in the bypass mode, but may be helping at this time, because it is treating the symptom, but not the cause, of the main problem.

John

[night0wl]

Sorry John I meant I put the cap between the switch and circuit

I disonnected the battery and the signal is fine. When I connect the battery the signal loses volume and tone again. I will try hardwiring the battery neg to the circuit like you said and see how it goes.

[night0wl]

ok same deal... poor tone with battery, normal without.

BTW, when checking trim voltage, what setting on a multimetre do you use? I had it on DCV 20 and the highest the trim pot would go is 4.3. Then on DCV 200 I get 10.3 reading. Which is the reading to use?

[UnklMickey]

night0wl said:

... I had it on DCV 20 and the highest the trim pot would go is 4.3. Then on DCV 200 I get 10.3 reading. Which is the reading to use?

something isn't right there.

if you got a reading of 10.3 on DCV 200, then on DCV 20, you should have read something like 10.24 or 10.37.

either there is something wrong with the meter, or it's loading the circuit HARD.

[night0wl]

ok I did another reading with the battery wired to the circuit and I got....
DCV200 = 00.7
DCV20 = 0.32
The trim was on full. Maybe my cheap MM is crap??

[night0wl]

unklmickey said:

another thought came to mind, as a possible cause of these symptoms.

if the connections at the input jack are resistive, that could cause some current to be flowing through the volume pot of the guitar. i would think that the same thing would occur when the preamp was "on". (actually it's always on, just bypassed when it's "off").

do you still have these symptoms, when the switch is in bypass, and the battery is disconnected?

Sorry unk, I didn't see this post until now. The signal is fine without the battery connected but loses tone with the battery connected. In what way could an input jack connection be resistive?

[UnklMickey]

maybe,

on some meters, there are additional jacks for the probes, those are to be used when you are measuring miliamperes (current). if you inadvertently plugged your probes into those it wouldn't work right. also, double-check, just to be sure you're on DCV not ACV.



the latest report of: poor tone with battery, okay without. is the battery still connected to the jack. or did you disconnect the wire to the jack and connect it straight to the ground of the board?

[night0wl]

I disconnected the battery from the jack and put it on the board with the same results.

I used the MM exactly the same way as earlier but it still gave the same readings.

[UnklMickey]

night0wl said:

...Sorry unk, I didn't see this post until now. The signal is fine without the battery connected but loses tone with the battery connected. In what way could an input jack connection be resistive?

no worries.

if the contact between the "ring" connection (the one that's supposed to connect to the next contact, just behind the tip of a stereo plug), is loose, oxidized, or has solder flux or grease on it, it won't make a good connection to the sleeve of the mono plug. you're dependent on that, to make your ground connection properly with this circuit.

[UnklMickey]

night0wl said:

I disconnected the battery from the jack and put it on the board with the same results....

and you disconnected the battery wire from the jack?

night0wl said:

...I used the MM exactly the same way as earlier but it still gave the same readings.

well at least it's consistent.

[night0wl]

Yeah I disconnected the battery wire from the jack and connected it to the board ground. When the battery is not connected to the battery snap everything bypasses fine. When the battery is reconnected I lose tone again.

So now I have..... battery (-) to board ground, Battery (+) to board, Board ground+pot ground+input ground+output ground joined together.

[JohnH]

Two weirdnesses here, being the tone loss and the odd meter readings. Can you get a consistent 9v (plus a bit) measuring just a fully disconnected battery, on the 20V and 200 dc settings? And with the battery in teh circuit, do you get almost the same readings?

When you switch the boost on, are you getting a reasonable increase in volume relative to a direct guitar volume level? Not related to your problem, but with those FETS I would increase the 1k source resistor up to somewhere around 10k, and rebias. Runnoffgroove base all their designs on the J201 which needs much less bias.

This is a fast moving thread - sorry if im a few posts behind

John

[night0wl]

when I test just the battery by itself i get..
DCV200 = 23.8
DCV20 = 10.37

Maybe I should chuck my MM lol

I am getting a great boost! It sounds great too. No loss of tone at all, infact it sounds better than the bypass signal.

The bypass still has a little loss of tone but not like b4.

[UnklMickey]

night0wl said:

Yeah I disconnected the battery wire from the jack and connected it to the board ground. When the battery is not connected to the battery snap everything bypasses fine. When the battery is reconnected I lose tone again.

So now I have..... battery (-) to board ground, Battery (+) to board, Board ground+pot ground+input ground+output ground joined together.

totally WIERD!

in the bypass mode, there shouldn't be any path for signal or current via the board, unless there's current going through the output pot to ground via the brown wires.

but the 22 nF cap shouldn't allow that.

i'll be scratchin' my head on this one for a while.

too bad that meter is acting strange. do you get wrong resistance measurements too?

[JohnH]

What Id say is:

Chuck the meter. 23.8V and 10.37 are just two of the many voltages which are not available from a 9V battery New ones tend to provide about 9.5V

Given that the circuit works well when active, it must be a really simple problem related to the switch and jack areas. Im guessing some solder residue or partial short somewhere.

John

[night0wl]

When I check resistors with the MM I get a good result.

OK.....don't strangle me...I'm a beginner remember??

It seems I may have mistaken the bypass for the boost because the louder signal (which I thought was the boost) is not controled by the volume pot. But when I switch to the softer signal the volume pot works! No wonder the boost sounded so good!! lol So I guess it is the boost signal that is the problem now, not the bypass?

I tried changing the 1k5 resisitor for a 10k but nothing happened. The unit stills runs the same without a battery connected to.

[UnklMickey]

night0wl said:

...OK.....don't strangle me...I'm a beginner remember??...

we don't (normally) do that!

[JohnH]

So just to confirm that, your louder signal should be identical to the pure guitar directly plugged to the amp without this circuit at all?

You are already one ahead getting your OD circuit to work. Nothing Ive ever built worked first time.

John

[night0wl]

yeah the louder signal is the same as the direct guitar-to-amp signal. The softer signal is meant to be the boost so is there anything I can change to make it boost instead of reduce?

Shane

[JohnH]

What I can do is mock up that circuit on a breadboard - It wont take long - Ive got some of that JFET - then Ill let you know!

J

[night0wl]

Great!! Thanks John it's much appreciated! I must get a breadboard setup so I don't waste any more parts!

Shane
P.S. We really need a chat room for this! lol

[UnklMickey]

night0wl said:

...I must get a breadboard setup so I don't waste any more parts!...

this is similar to what we used in school.



looks like a lot of value for $70

www.circuitspecialists.com/prod.itml/icOid/6027