'umble Build Report - and a lil help please..

[vdm]

hey everyone,

i've thrown an umble together over this weekend, and after getting the trimpots dialed in (by ear and audio probe - not meter :? ), it sounds pretty sweet. not exactly what i expected, but it's a really nice tone.

im not getting as much gain out of it as the sound clips seemed to have, but that is no doubt because im using 3 2n5457's and a mpf102 not j201's. i was going to try to get hold of some, but i worked out that it would end up being $6US to get them, and i dont have the money at the moment to warrant that - or to do a bulk order from small bear.

anywho - i've got a bit of noise problem with it as well, and it's not the standard too much gain problem either, because as i turn the gain from max to min, the noise stays the same the whole way. at first the noise was louder than the signal - then i changed the gain pot cuz it was giving lots of noise when i touched the shaft or casing - changing it helped but didnt fix it.

what makes me think it's the treble pot is that when i touch the shaft it makes the noise much louder. as i turn the treble down, the noise gets lower, but i start to get heaps of radio interference. so im guessing there's something wrong there.

i'll have to get another 250k pot to try, but i could only get a 16mm pot and it was audio taper not linear - cuz that was all they had.

anyway - if anyone has suggestions about what could be causing the noise or how i could get some j201's in australia - it'd be really appreciative.

thank you,

trent

[petemoore]

Do the Pin V readings look good? [did you measure transistor voltages?]
 The Umble is a big one.

[vdm]

i dont have a mm at home, so i just tuned the trims by ear - but i can check the voltages on wednesday at school hopefully. but the jfets only seem to turn on for a very small section of the rotation for the trimpot, so i just got them in the middle of that, where the sound is loudest - i assumed that's the right place to have it....

[petemoore]

Probly is, or could be. Always fun to check and see where  they are voltage wise with a DMM, I find if I'm getting 4.5v at drain, and the thing seems to be working right, all the other voltages fall in place for Jfet biasing...other wise there a wiring error.
 Jfet in backwards or other biasing issues make it jump right past the 4.5v at drain, the trimpot seems to be very fine adjustment, and hard to get the drain near 1/2v.
 With the 100k trimpot, setting for 4.5v is about as easy as tuning a radio station.