Fet voltages

[gar13]

I am trying to bias the FET before completing the finally tweaking of the plate voltage in the preamp for the #102 of finishing up. I have a 25k pot in place of the 12k resistor shown in the schematic. I used a 2N4416 FET in the circuit. Problem is, I have the same voltage on either side of the the drain resistor. Both voltages track up and down with adjustment of the source pot.

Does this mean the FET is toast?

[martin manning]

If you have the same voltage on either side of the drain resistor, that only says the drain resistor is shorted.

Are you powering the FET board with the amp's power supply, and what range of voltages are you seeing at the FET? I just posted something in the Dumble Files section on biasing the FET using batteries.

[gar13]

I forgot to mention, I did check the drain resister, it measures a full 10k. Yes powering with the amp's PS. I get a range of voltages up to 16v, they track down toward 0, but I haven't used the full range of the pot (wasn't sure if shorting the drain to ground would harm it).

[markr14850]

If you're not dropping any voltage across the drain resistor, there must not be any current flowing through it. Check your FET pinouts - you might have your drain resistor connected to the gate.

[gar13]

FET board looks just like in this post:

https://tubeamparchive.com/viewtopic.php?t=3205

and I double checked the spec sheet. I think I have an MPF102 to try from rat shack.

[gar13]

I'm getting the same results with the MPF102 as well. Please have a look at the board and see if I have wired incorrectly, I am unable to find an error.

[markr14850]

Dunno. Maybe check the resistance from fet source lead to chassis ground.

[gar13]

Resistance from source tracks perfectly with rotation of the temporary pot I have installed.

[martin manning]

Tru enough that you could see the same voltage on either side of Rd if there were no current flowing through it, but I don't know how you could maintain that condition and get that voltage to change by varying Rs. What is the source of the pcb?

[markr14850]

Any chance there's an accidental solder bridge on the back side of the board?

[gar13]

It's one of Funk's boards. Got these parts years ago, just getting around to building it now. Here' a pic of the back side of the board.

[martin manning]

Your part placement looks good to me. I think a more likely value for the 8k2 you have there is around 22k, and Rs will probably want to be around 2k. I am a fan of setting up the bias outside the amp with a couple of 9V batteries.

[gar13]

Finally did what you suggested. I pulled the board out and biased with a 9v. I settled on 22k and a 2.2k giving me 22vsupply and 12v on the drain.

Now, how do I adjust the trimmer on the FET board? Right now I have it about 3/4 of the way up.

Thanks for the help,
Dave

[CHIP]

I set mine ,(FET input), so the volume level of my Strat is about the same as the normal input when using a humbuck type guitar. 3/4 is about where mine ends up being.