thunderchief voltages

[jimmy]

hi everyone

i built a thunderchief the other night with 2N5457s i think it was and it works, but it sounds a little odd. the single notes have some weird kind of tremelo on them.

so i checked the voltages and everything is ok up to q3 where the source pin has 4.3volts, and ISNT shorted to the drain.

any ideas?

cheers
Jim

[erio fraga]

This problem has been discussed here a lot. I really suggest you to perform a search using the “search ‘facility at the top of this page. Specially, read the posts under the topic name “New at runoffgroove.com: Thunderchief”.
In my TC, I couldn't fix the problem you are facing, I used the BF245 and MPF102. I'm trying to get some J201 to see what happens.

Erio

[Lonestarjohnny]

Jimmy, Make sure your pinout is correct on your 2n5457's,sounds like you may have the gate and sourse swapped around,
If not try another transistor in that positon, and also make sure you got the right value resistor's tied to the tranny, it's very easy to confuse a 4k7 with a 470k or simular resistor value when you hurry, so take your time and go back trough the complete build, I find you learn more by doing this, you'll understand more about the circuit if you eyeball the thing, an audio probe is cool but some time's makes things to easy for you when your try'in to learn circuits.
JD

[ODwan]

Hi, Jimmy!
I have just the same problem with my TC-build using BF245A's. I also checked the pinout:ok! Checked the resistors surrounding Q3:ok!
Checked all capacitors:ok! I just can't figure out what's happening there.
Right now I'm waiting on a bunch of J201 to try if it's any different with them. I'll post my findings here.
BTW I also build the Meteor with BF245A's and all voltages are just what they should be.

[Gary]

For all those substituting jfets for the J201, the problem lies in the differences of the jfets' characteristics.

See the 4k7 that connects to the 4k7/100n/100k on one end and the 470/1M on the other end?  Make that a trimmer or modify the value of that 4k7 resistor.  4.3V on the source of Q3 is way too high.  You should see no more than 0.5V.  If you reduce the 4k7 I mention, you will be able to adapt your substitute fets to the circuit.

*VORSICHT*
Substituting fets will alter the sound.  You may not like the results.  This circuit was tuned to J201 fets.

Best regards,
Gary

[jimmy]

huh, i was not aware of that.

thanks alot for the help, ill see what i can do in the way of j201s, or modifying resistors.

cheers
Jim