Alternate FET circuit

[markr14850]

All,

I've been playing with FET circuits lately, and made a few tweaks to the "standard" ODS FET input. The result is something that I really, really like with my strat.

The main changes are:
- Use a 2n5457 instead of the NTE452. It's available, cheap, and has low noise. The ones I have used have a Vgs(off) of ~1.5V and Idss of ~3mA. You should measure the Vgs(off) and ensure that it's at least 1.4V, or you will clip the input with a strong hit from humbuckers. Note that the pinout of this FET is different than that of the 452 - look at the datasheet.
- Drop the gate resistor from 1M to 10k.
- Remove the gate resistor bypass cap.
- Drop the source resistor to less than 1k. In my case, best bias was at 670R. I'd suggest using a scope to set the bias here.
- Raise the operating voltage to ~22V by changing the lower resistor of the divider to ~27k.
- Change the output level pot to 250K. I run this at about 3/4 for a strong boost over the normal input.
- Remove the source resistor bypass cap. It's not needed. With these values, this circuit will produce clean gain up to the voltage limit of the FET. Also, removing it removes phase shift, which makes it easy to set the bias by setting my scope in x-y mode and watching for departure from linearity.
- Optionally, add a 150pF cap on the input to simulate more Miller capacitance. This flattens out the response nicely.

These changes are all easy to do on an existing FET board.

I've attached a schematic.

Thanks to all that have shared their findings in the past,
--Mark

[Mark]

Has anyone tried using a mosfet instead of a regular FET?

[markr14850]

For MOSFETs, you could adapt ideas from these sources:
http://www.muzique.com/schem/mosfet.htm
http://www.geofex.com/Article_Folders/m ... sboost.htm

You'd need to locate devices that have both low capacitance and low noise at the estimated input levels.

[BobW]

Has anyone tried using a mosfet instead of a regular FET?

Mark, A typical MOSFET requires a higher gate voltage for turn on (Vgs). Since both FET types have a high impedance input you wouldn't gain any advantage over a standard fet, other than higher Vds and Ids ratings.

[Mark]

Thanks for the replies.

Mosfets do seem to be preferred over FET's these days, so I assume there must be something to it.

[ElectronAvalanche]

Hi Mark,

thanks for your alternative FET circuit.

Since I did have all parts here (did not have a 2N5457 so I used a 2N5485) and some time on my hands, I build the circuit and it worked right away.

I get 18.7V operating voltage using a 50k trimmer set to almost 50k in place of the 27k.

The output is not too high, even with the 250k output (I use a 220k trimmer) set to 220k to ground I get only a slight boost compared to the Normal Input.

I need to measure the voltage at the drain and as of yet only set the bias by hear (loudest signal). I used a 1k2 trimmer in place of the suggested 670R bias resistor.

Did you ever try to use a bypass resistor parallel to the 670R?

Thanks again,

Electron!

[Structo]

Wow a 2009 thread.

It really depends on your B+ supply voltage (from V1) to the 150K on where your voltage and bias will be.

You want approx 10v at the Drain and 20-22v at the junction of the 10K and 150K.

I used a larger input cap of 470pf and a larger Source bypass cap to thicken the tone some.

[markr14850]

Hi Electron,

The output is not too high, even with the 250k output (I use a 220k trimmer) set to 220k to ground I get only a slight boost compared to the Normal Input.

Hmmm. Going by the specs, a 2N5485 on average has more gain than a 2N5457. Of course, you have to adjust the bias tightly to realize all of it.

Do you have more FETs? If so, try another one. I find that if I expose a FET to over-voltage, it maybe not directly fail, but instead just lose a lot of gain. This happens to me often when setting up the dropping string off the B+.

Did you ever try to use a bypass resistor parallel to the 670R?

Do you mean bypass capacitor? If so, I omit it, as I had plenty of gain, and removing the cap gives some negative feedback to linearize the response. My own goal for that stage was just a few X of gain, very clean, flat, and low noise. In particular, I wanted to never clip the stage's output when hitting the input hard with humbuckers.

If you want to, you certainly could bypass for more gain - either fully or partially, just like a triode. Just be clear on what you want from the stage.


--Mark

[ElectronAvalanche]

Hi Mark,

thanks for your answer (and of course thanks to Structo as well).

I could not get the bias voltage higher than 2.3 V using a 1k2 trimmer in place of the bias resistor. Still the boost was lower than the Normal channel.

Unfortunately I only have non exchangeable FETs here at this time, so I tried to install a 10k trimmer in place of the 1k2 and got around 10Vs (voltage supply is 22Vs) but there was not much more gain and oddly enough now I get a loud pop when switching pickups on my tele playing through the FET channel (no pops in the Normal channel).

So I guess the FET is defect or the coupling cap 4u7 (50V) somehow got defective. Taking out the FET gets rid of the pop (and the signal of course), with the 220k output/level trimmer all down, no more pops as well.

I am waiting for some FETs but wonder if anybody could explain the pops?

I wonder why you changed the gate resistor from 1M (with 100pF cap in parallel) to 10k. How did the sound change by doing that? Is the sound comparable to the Dumble FET circuit?

Thanks again for your help!

Electron

[markr14850]

Hi Electron,

I could not get the bias voltage higher than 2.3 V using a 1k2 trimmer in place of the bias resistor. Still the boost was lower than the Normal channel.

It's hard to say for sure, but that could be too much. Like Structo said, we want about 10v (1/2 of the supply voltage) at the drain. Adjust the source trimmer to target that 10v at the drain.

The higher you bias the source (both in resistance and voltage), the less current will flow through the FET - this will lead to the drain having higher than 1/2 the supply voltage.

Remember, the conversion from triode to JFET is:
plate : drain
grid : gate
cathode : source

I am waiting for some FETs but wonder if anybody could explain the pops?

I'm guessing here, but the FET might be biased fairly cold, putting you in a very non-linear region of the device's response.

It might help if you draw up exactly how you built the circuit, and show voltages everywhere.

I wonder why you changed the gate resistor from 1M (with 100pF cap in parallel) to 10k. How did the sound change by doing that? Is the sound comparable to the Dumble FET circuit?

I found the original circuit to be a bit thin and noisy. The 1M made a bunch of resistor noise, and the 100pf cap let in a lot of "jangly" high frequency string transient "presence" that I disliked. So, no, it doesn't sound the same.

It's such a simple circuit, I suggest you throw one together on a breadboard and try different variations.

Just remember, don't ever let it see more than 25 volts, or the FET will start to fail in unpredictable ways, which will drive you insane.

Kind Regards,
--Mark

[thyx]

Just remember, don't ever let it see more than 25 volts, or the FET will start to fail in unpredictable ways, which will drive you insane.

Kind Regards,
--Mark

Now THAT is a tip worth remembering. More than once the little men in white suits have come looking for me, whilst I stewed in my basement shop.

[ElectronAvalanche]

Hi Mark,

thanks again for your suggestions. I ordered some new 2N5457 and it was indeed a faulty transistor that lead to the low output.

I went back to the Dumble circuit using 4.7uF as source resistor bypass, 1M paralleld by 470pF and a 100k output trimmer and really like the sound now.

When I first built my amps years ago I thought that the FET was not worth it, but I must say that the FET really brings some nice additions and can give a great OD sound.

Absolutely recommended.

I will post some pics of the FET board.

Thanks again,

Electron

[fp2000]

They should make this a sticky, for those who like to fiddle with the FET input. I would like to try it myself

[heisthl]

Another alternative, and as a bonus the board is smaller.

http://www.generalguitargadgets.com/com ... gory_id=14

http://www.generalguitargadgets.com/pdf ... a841b3e85a