thunderchief problem

Greg Moss · October 03, 2004, 01:45:07 AM

I finished perfing a thunderchief build (without tonestack) but I'm getting some un-smooth blatty kind of distorion. All of the drain voltages are between 4.3V and 4.5V. Looking at Runoff Groove's pin voltages I'm noticing that Q3 is getting some weird measurements:
Q3
-------
G:2.2V
S:3.8V
D:4.5V

I feel like I've checked through the 3rd gain stage for shorts and open solder connections pretty well. Does this look like any wiring problrm specifically? I pulled the transistor and checked the voltage for the gate to make sure no DC was getting through the cap from the output of Q2, but that looked ok.

I'm sure it's something obvious, but I'm stuck! Anyone have any suggestions>?

Greg

jimbob · October 03, 2004, 02:33:16 AM

I had the same thing going on and found it takes some carefulll trweeking of the trimmers one at a time--and look for the sweet spots--be carefull not to pass over them to quick either..Give that a try.

Greg Moss · October 03, 2004, 02:35:25 AM

Oops - I see that this problem has cropped up before .

Hmmm. I tried a few fairchild J201's and a couple of the J201's I got from Smallbear, but same weird high readings on the source and gate pins. Is it just an issue of raising a value of the 4k7 resistor between the 1M and resistor and the 100n cap?

Did anybody find a fix for this that worked for them?

G

Greg Moss · October 03, 2004, 02:42:44 AM

Quote from: jimbobI had the same thing going on and found it takes some carefulll trweeking of the trimmers one at a time--and look for the sweet spots--be carefull not to pass over them to quick either..Give that a try.

Thaaks for the quick reply!

I've tried both biasing all the trims by meter and by ear to see what seems to work,. It almost sounds great, there's just a little squarted blatty sound introduced around Q3. Plus from what I've read elsewhere on the forum, the high gate and source voltages are a problem.

Somebody suggested varying the value of the resistor attatched directly to the source of Q3 (470 ohm) and someone else recommended increasing the resistor following that (4k7). Has anybody found that one of these tweaks worked out for them?

Jimbob - did you have the Q3 problem at all? if so how did you get around that?

-G

jimbob · October 03, 2004, 02:51:35 AM

I had problems with them all as I wzs unsure where to start in terms of adjusting the trimmers. I think i started at trim 4 and listened for the volume to get louder than i went to the 1st one and listen as good as i could adjusting it and then to the next. I think its probably right but just needs adjusting due to the decaying/blatty sounds im sure its making. Also re- chk where the 3 lugs of each trimmer is going. The 1st 2 go one way and the last 2 go another if i recall.

Greg Moss · October 03, 2004, 04:06:35 PM

I've read these threads through several times:

http://diystompboxes.com/sboxforum/viewtopic.php?t=22939&highlight=thunderchief
and
http://www.diystompboxes.com/sboxforum/viewtopic.php?t=20380&highlight=bias+thunderchief

OK, I tried replacing the 4k7 between the 1M/470ohm junction and the 4k7/100nf junction with avarible resistor and tried tweaking that, and I tried replacing the 470ohm resistor with a high value (not 10k, as had been recommended, abut 4k7) but to no avail. :

I could get the source and gate voltages down to about a volt with the pot replacing the 4k7R set for no resistance, and increacing the value of the 470ohm R helped, but then I was unablle to get the Drain voltage below 6V.

I have set the source voltage to 4.5 V (measuring between drain and ground) on each Q, band every jfet besides Q3 is behaving. I've tried swapping out abiout 10 j201's, and they all wind up with the same voltages within 10mV:

G:2.2V
S:3.4V
D:4.5V (witch I set with the 100k trimmer).

Everything else sounds really good, - I'm just picking up biasy- sounding problems off of Q3. I know others had this problem, were any of you able to resolve it?[

RDV · October 03, 2004, 05:14:21 PM

I've given up on mine for the time being. I was hoping someone would work it out.

RDV

RDV · October 03, 2004, 08:24:53 PM

I revisited my Thundercheif which has been making me crazy. Muffled, blatty, hard to bias Q3, you name it, it's happened. I put the whole thing in a plastic bag and threw it in a drawer, waiting for the inspiration to fix it. Well, this thread provided some impetus for getting it back out and trying again. When I looked at the schem I used, I realized I had used the 1st version(1.0, there's now a 1.1), so I pulled the parts that were different, but no change, still quiet and muffled, still hard to get a consistant voltage reading on Q3.

Then I decided to do the twisto on the board and lo and behold, there was the correct sound! I then proceeded to reheat all the solder joints to see if that would do it. Nope, still sucked. Twisted again and couldn't make the sound come back like it had before.

Then I accidently touched the pin 1 of the volume pot to the output jack tip. There was the sound again! So I now removed the wire from pin 2 of the output pot to the output jack, cleaned the contacts good and resoldered a new wire in and VIOLA! I suppose the wire was badly soldered, or attached to a dirty used jack or something. I did the same for the ground on the jack also. There was some definite capitance and the associated unwanted filtering going on.

Now I can actually adjust the 3rd 100k trimpot while watching the voltage(I had not been able to do so before), thus easily biasing Q3. I can't wait to try it on the big amp, cause the Ruby lies like a dog about the true nature of pedals.

Obviously this circuit is very sensitive to the circuit being very correct in respect to how the DC flows throughout. Any disruption of ground is likely to throw off the bias and give a muffled, blatty sound.

RDV

RDV · October 03, 2004, 08:27:50 PM

Quote from: Greg MossI have set the source voltage to 4.5 V (measuring between drain and ground) on each Q, band every jfet besides Q3 is behaving. I've tried swapping out abiout 10 j201's, and they all wind up with the same voltages within 10mV:

G:2.2V
S:3.4V
D:4.5V (witch I set with the 100k trimmer).

Everything else sounds really good, - I'm just picking up biasy- sounding problems off of Q3. I know others had this problem, were any of you able to resolve it?[

Make sure you have Q3 & Q4 turned the opposite way of Q1 & Q2.

RDV

tcobretti · October 03, 2004, 09:36:55 PM

I just built an eighteen and had the same kind of problems that RDV had - bad solder joints that were invisible. It took patience, a continuity tester and an audio probe to figure what I'd screwed up. I didn't have any alligator clips, so I rigged my audio probe with the cap soldered to a jack and the ground connected to a 1/4" plug so I can plug it into the output jack for ground and then use the probe. That probe is a pretty cool device. I want to spend some time poking at some of my pedals with it to see what's happening at different points in the circuit.

RDV · October 03, 2004, 09:41:16 PM

Mine still seems a little muffled(though not nearly as bad), but I'm not changing anything till I try through the Marshall.

RDV

Greg Moss · October 04, 2004, 04:00:31 AM

Well, I have most of the project perfed, and then the output and drain bias stuff on a breadboard so there are definately possibilities foir little ground inconsistancies. I'm pretty contientious of the bad solder joint phenomenon, and I've reheated everything around that gain stage, but to no avail.

I should probably give my self a week off from it and come back with renewed patience. I'm building it for a friend who was (only slightly) disappointed with the Blackfire I made for him (this is fine, because I totally dig the BF) and I want to finish the TC before I hit the road in Nov.

As a last resort I may just omit the power amp part of the circut - audio probing after Q2 sounds really great too - a little less thick, but still retaining that great jfet/valve warmth and sparkle.

Thanks much for the encouragement - if I solve this problem, I'll make a point of documenting it for my frustrated peers of the future!

Samuel · October 04, 2004, 08:49:59 AM

You need to make Joel a pedal that when you step on the switch it pops up a little paper flag that says "Dude, just buy a new JCM900 and get off my jock, alright?"

Greg Moss · October 04, 2004, 01:07:21 PM

I not going to recommend that joel get anything with a voltage rail higher than 9V until he stops blowing things up.

Gary · October 06, 2004, 12:17:34 PM

If you have issues with getting the proper sound, even after all the connections are verified good, check out this post.

http://diystompboxes.com/sboxforum/viewtopic.php?t=25753

Best regards,

Greg Moss · October 06, 2004, 02:05:51 PM

I will most certainly give this a shot - how about sticking in a 10k pot innstead of the 4k7 and varying that until I get the desired voltages?

There was a suggestion on a separate thread to increase the value of the 4770ohm resistor to 10k and go on to the normal drain biasing from there. It seems to get the pin voltages closer to what I'm looking for, but I actually have that resistor up to 12k and itthe gate and drain V's are still too high, and there's still a bit of square wavey blattyness going on.

Is this a reasonable remedy to the Q3 problem? I don't know if it makes any difference, but the bad bias sound that I'm experiencing is related to the peak of the signal, and not a lack of sustain, or distorted "gating" sound during quiet signals.....

Thanks for the reply!

Greg

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