Fet trouble

[passfan]

The FET I'm fiddling with has at least 1/3rd less gain than the normal input and is dark and muddy sounding. I'm sure this is far from normal. I measured around 13 to 14 volts at the 150k and 9.2 volts on the drain but at full trim its was behind the normal input. I have a feeling it's not working to full capacity. I also get a lot of hiss if I bring up the preamp or master volume. Any ideas? I'm going to pull it in the am and check it and rebuild it if needed

[ToneMerc]

The FET I'm fiddling with has at least 1/3rd less gain than the normal input and is dark and muddy sounding. I'm sure this is far from normal. I measured around 13 to 14 volts at the 150k and 9.2 volts on the drain but at full trim its was behind the normal input. I have a feeling it's not working to full capacity. I also get a lot of hiss if I bring up the preamp or master volume. Any ideas? I'm going to pull it in the am and check it and rebuild it if needed

No, it's not normal. What FET part number are you using and what resistor values did you use for the divider, source and drain?

TM

[passfan]

Nte 452 with8.2k , 8.2k and 10k right off the 1988 skyliner schematic

[martin manning]

Passfan, you might want to give this a look: https://tubeamparchive.com/viewtopic.ph ... highlight=
Your bias point is a bit off-center and your supply voltage is low. Most find that the lower leg of the 150k/8k divider should be 22-25k to get 18-20V at the top of the drain resistor.

[passfan]

I tweaked around with that for half a day yesterday Martin. I wound up super compressed without any change in volume or tone. I fear I'm dumping something to ground with a miswire somewhere. I didn't use a prefabbed card ; I just drilled some holes and soldered some leads on the back side. If my device was blown it wouldn't pass anything so it should be good , correct ? I am pulling it out today to go through it.

[martin manning]

I suppose if you can bias it and it passes signal that is something, but it still could be failed or defective. Did you attach a heat sink to the leads when soldering?

[passfan]

Component error. 3.3 k to ground instead of 3.3 meg . Draining off most of my input signal before it even hit the gate. Device should be as good as new. I must have originally misread it off the layout. Think it will make a difference

[martin manning]

Ah, that would explain some signal loss, wouldn't it. I still recommend doing the battery bias drill to find an Rs, and then tweak the 150k divider.

[passfan]

Yes as well as the the muddiness with that low pass input looking at 3.3k instead of 3.3meg. I am going to check it out with the b+ and see how she looks. Yesterday when I fiddled with those values it came up almost right on the button but it compressed something awful. I feel my experience today will be much improved , thanks

[passfan]

Came right up with good numbers and easily twice as loud as the normal input at full trim and crystal clear. This thing is kicking butt now.

[Colossal]

Nice!

[passfan]

Wish I hadn't built her to sell now. Oh well, off to pack her up.

[Colossal]

Someone will be stoked when they get it.

[vibratoking]

Glad you got it going. I like the FET with a Strat.

[sonicmojo]

I find FETs in general to be all over the map and need biasing. For the pedals I'm building that have a Dumblesque FET boost, I made a little protoboard and sorted a bunch of them first. Most were in the range I wanted(source and drain), but every now and again....not. If you are way off, the first symptom is usually that fizzy gated sound.

Another option if you don't want to sort first is to use a socket and just swap a few. You can always dab a little silicone on it to lock it down once you one you like.

Yet another option is to replace the drain resistor with a trimpot for biasing. It would be easier to tune but not true to the Dumble layout.