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Re: FET help, please

5/28/1999 8:42 PM
John C.	Re: FET help, please >As noted in the schematic, if you clip the 22K resistor going to ground in the tone control circuit, you get a totally different sound from the standard big muff tone control. I made the board with room for the 22K resistor, wich is now on. And I haven't stoped playing with this nice effect for two days now. I like it. A tried some different combinations for the diode part, but I left it as you reccomend. Next, I'll take off that resistor to see the difference. Right now I'm "cooking" in the oven a beautifull black box to install the effect >You want JFETs that are smoking so you get some distortion from the MiniBooster circuit. Another mod is to put a 100K trimmer before the 1Meg resistor of the MiniBooster. The one that's connected to ground in parallel to the diode arrangement and the 220pF cap, you mean? Wire the trimmer as a volume control (signal to outer lug), the other outer lug to ground and the middle lug to the MiniBooster. I think it has enough distortion as it is now. I set the Clean Boost trimmer to zero. With the 500K pot adjusted to max, I have more than enough distortion, and set to the minimum, a fair clean boost. The 10K trimmer, all way up. I like it that way. Would it be possible to make it a little bit smoother? The "ringing" distortion. I can't explain myself much better, I'm sorry; I mean that it is a bit (just a little bit to my taste...) "acid", "octaving" or so. Because of this, I like to set the tone, and my guitar, near to full low, to avoid those "ringing highs" which are more metalic, than bluesy,<-- that is what I prefer. JohnC

5/28/1999 8:42 PM

John C.

Re: FET help, please

>As noted in the schematic, if you clip the 22K resistor going to ground in the
tone control circuit, you get a totally different sound from the standard big
muff tone control.

I made the board with room for the 22K resistor, wich is now on.
And I haven't stoped playing with this nice effect for two days now. I like it.
A tried some different combinations for the diode part, but I left it as you reccomend.
Next, I'll take off that resistor to see the difference.
Right now I'm "cooking" in the oven a beautifull black box to install the effect

>You want JFETs that are smoking so you get some distortion from the
MiniBooster circuit. Another mod is to put a 100K trimmer before the 1Meg
resistor of the MiniBooster.

The one that's connected to ground in parallel to the diode arrangement and the 220pF cap, you mean?

Wire the trimmer as a volume control (signal to
outer lug), the other outer lug to ground and the middle lug to the
MiniBooster.

I think it has enough distortion as it is now.
I set the Clean Boost trimmer to zero. With the 500K pot adjusted to max, I have more than enough distortion, and set to the minimum, a fair clean boost.
The 10K trimmer, all way up.
I like it that way.

Would it be possible to make it a little bit smoother? The "ringing" distortion. I can't explain myself much better, I'm sorry; I mean that it is a bit (just a little bit to my taste...) "acid", "octaving" or so.
Because of this, I like to set the tone, and my guitar, near to full low, to avoid those "ringing highs" which are more metalic, than bluesy,<-- that is what I prefer.

JohnC

5/30/1999 2:56 PM
aron	>Next, I'll take off that resistor to see the difference. Yes, if you take off the resistor, you don't get that scoped mids sound that most people are hearing. >The one that's connected to ground in parallel to the diode arrangement and the 220pF cap, you mean? Yes, before the 1MEG resistor. This lets you get LESS distortion than before by letting you trim the signal going into the MiniBooster down and precisely control the amount of drive. Think of it as an internal drive control for the MiniBooster. >Would it be possible to make it a little bit smoother? The "ringing" distortion. I can't explain myself much better, I'm sorry; I mean that it is a bit (just a little bit to my taste...) "acid", "octaving" or so. Try and clip that 22K resistor. Also, experiment with different value caps across the diodes. Did you use the MOSFET diodes? Try those if you can. BOTH 220pf caps can be larger to cut more highs and smooth out the sound. Let me know how it goes! Aron

5/30/1999 2:56 PM

aron

>Next, I'll take off that resistor to see the difference.

Yes, if you take off the resistor, you don't get that scoped mids sound that most people are hearing.

>The one that's connected to ground in parallel to the diode arrangement and the 220pF cap, you mean?

Yes, before the 1MEG resistor. This lets you get LESS distortion than before by letting you trim the signal going into the MiniBooster down and precisely control the amount of drive. Think of it as an internal drive control for the MiniBooster.

>Would it be possible to make it a little bit smoother? The "ringing" distortion. I can't explain myself much better, I'm sorry; I mean that it is a bit (just a little bit to my taste...) "acid", "octaving" or so.

Try and clip that 22K resistor. Also, experiment with different value caps across the diodes.

Did you use the MOSFET diodes? Try those if you can.

BOTH 220pf caps can be larger to cut more highs and smooth out the sound.

Let me know how it goes!

Aron

6/4/1999 8:28 PM
John C.	>Let me know how it goes! Great!. It's my pedal now. I left the SBIII as "original", MOSFET's included, except I couln't find a .004uF cap in the tone section, and I used a .0047 cap. Thanks for your help, Aron. JohnC

5/27/1999 9:27 PM

Jack Orman

quote:
"I don't think you can find out if a FET has more output that another except very generally. In my experience, FETs are all over the place."

FETs with a low Vpinchoff usually have high gain. The Yfs (or Gfs) spec is a general indicator of gain as well... Yfs of 1000 is low gain, 3000 is moderate and 12000 is smokin'. However, as Aron indicated, individual FETs can vary from each other as much as 3:1 in gain characteristics.

-Jack

5/28/1999 8:11 PM
John C.	Great, thanks for this accurate info, Jack. I do not have a transistor data book on hand at the momment, but anyway, those 2N5457 Aron told me I think they are powerfull enough... I have been using the same germaniums (diodes) on all my effects, as a most approximate equivalent for the 1N34A, which are almost impossible to find where I live. I never knew their reference#, I'll ask. They are bottle green coloured. I run out of them although I bought all they had in the shop, but I found that the AA series also work, apparently at least as good as the 34A replacements I found. Any experiences with the AA's there? (AA18... AA1xx) (they are called "demodulator diodes" over here, as the 1N34A I think) JohnC

5/28/1999 8:11 PM

John C.

Great, thanks for this accurate info, Jack.

I do not have a transistor data book on hand at the momment, but anyway, those 2N5457 Aron told me I think they are powerfull enough...

I have been using the same germaniums (diodes)
on all my effects, as a most approximate equivalent for the 1N34A, which are almost impossible to find where I live. I never knew their reference#, I'll ask. They are bottle green coloured. I run out of them although I bought all they had in the shop, but I found that the AA series also work, apparently at least as good as the 34A replacements I found.

Any experiences with the AA's there? (AA18... AA1xx) (they are called "demodulator diodes" over here, as the 1N34A I think)

JohnC

6/3/1999 4:54 PM
John C.	>individual FETs can vary from each other as much as 3:1 in gain characteristics. Would it be possible for someone like me with just a basic knowledge of electronics, to measure the gain of a fet? I´m interested in that because I builded a clone of the Vox Treble Booster with a nice germanium transistor but it hasn't enough "power" as the output volume is too low compared with a silicon transistor I installed after that. Would I need some especial equipment to do it? JohnC

6/3/1999 4:54 PM

John C.

>individual FETs can vary
from each other as much as 3:1 in gain characteristics.

Would it be possible for someone like me with just a basic knowledge of electronics, to measure the gain of a fet?
I´m interested in that because I builded a clone of the Vox Treble Booster with a nice germanium transistor but it hasn't enough "power" as the output volume is too low compared with a silicon transistor I installed after that.

Would I need some especial equipment to do it?

JohnC

6/3/1999 9:31 PM
R.G.	It's not all that easy to measure JFET gain without a circuit to test it in. Common digital multimeters have transistor gain testers built in, but not JFET gain testers. The easiest tester is to build a simple gain circuit and listen to several through the circuit. The vox treble booster is not designed for a germanium device, and would probably have very low output with a germanium, unless it's some variation of the VTB I've not seen - which is always possible.

6/3/1999 9:31 PM

R.G.

It's not all that easy to measure JFET gain without a circuit to test it in. Common digital multimeters have transistor gain testers built in, but not JFET gain testers.

The easiest tester is to build a simple gain circuit and listen to several through the circuit.

The vox treble booster is not designed for a germanium device, and would probably have very low output with a germanium, unless it's some variation of the VTB I've not seen - which is always possible.

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