Tillman preamp related help!!

[mmaatt25]

Hi,,
These troublesome circuits always seem to have me scratching my head.  I've just built another Tillman discrete preamp, (I already have one on my main rig which works fine) from http://www.till.com/articles/GuitarPreamp/ built exactly as shown.

When i tried it there was a volume drop??  So measured the ( and this is where I'm struggling) drain to ground voltage, I set my DDm to DC volts and one probe ground and the other drain of J201, result 2.75v.  I went on to try every J201 I've got and they were all between 2.75v & 2.79v.  I then with my ddm set the same touched one probe on the 9v+ and the other on drain and got 6.3v? I don't understand.

I search here and found lots of discussions about biasing J201's, but it seems these circuits as described will never work to all sorts of biasing resistors 1k gate & 15k drain?

Like I said I have one working in my main rig, but now I'm thinking that maybe more luck than judgement.

Many thanks

Matt

[aron]

>I set my DDm to DC volts and one probe ground and the other drain of J201, result 2.75v.  I went on to try every J201 I've got and they were all between 2.75v & 2.79v.

You should measure from ground to the drain.

>but it seems these circuits as described will never work to all sorts of biasing resistors 1k gate & 15k drain?

What do mean by the above?

Most of the FET circuits here use a trim pot either on the drain or source.

Aron

[Paul Marossy]

Most of the FET circuits here use a trim pot either on the drain or source.

Right. But the Tillman article shows a 6.8K drain resistor and suggests trying different J201s until the voltage at the drain is between 5 & 7 volts. It's a LOT simpler to just use a 10K trimpot, which is what I did the first time I built the Tillman circuit.

[aron]

Good point.

[mmaatt25]

>I set my DDm to DC volts and one probe ground and the other drain of J201, result 2.75v.  I went on to try every J201 I've got and they were all between 2.75v & 2.79v.

You should measure from ground to the drain.

Do you mean between the ground of the J201 and drain?  I was measuring between drain and circuit ground!

>but it seems these circuits as described will never work to all sorts of biasing resistors 1k gate & 15k drain?

What do mean by the above?

I searched the forum and came across this post http://www.diystompboxes.com/smfforum/index.php?topic=69631.0 on the first page there's a couple of references to biasing J201's, one suggests that due to their variability very few will work in a specific circuit and another suggested using 1k gate and 15k drain.

Most of the FET circuits here use a trim pot either on the drain or source.

I'm using a 10k trimmer on the drain, I'm getting a signal but with a big volume drop?

Thanks

Matt

[aron]

>another suggested using 1k gate and 15k drain.

I'm pretty sure it's 1K source and 15K drain.

>I was measuring between drain and circuit ground!

That's correct.

>I'm using a 10k trimmer on the drain, I'm getting a signal but with a big volume drop?

I usually use a 25K-50K trim pot. 10K might be too small.

[Paul Marossy]

I usually use a 25K-50K trim pot. 10K might be too small.

I guess it depends on your batch of J201s. I have used a 10K trimpot on all three Tillman preamps I have built and they all worked fine.

[mmaatt25]

OK thanks for all replies.

I've got it now so I'm measuring 7.9V at the drain, that's with 6K8 R3 & 2K2 R2. I went 12K with R3 and got down to 7v, what effect will this have on the output impedance?  What I can't work out is how to measure current at R2, Tillman suggests R2's value to bias the FET around 0.5mA, but how do I measure this with my dmm?

Many thanks

Matt

[Paul Marossy]

What I can't work out is how to measure current at R2, Tillman suggests R2's value to bias the FET around 0.5mA, but how do I measure this with my dmm?

For measuring current, you have to put the DMM in series with what you want to measure.