oscillations in Dr. Boogie...

[Davide]

oh i've also used some "temporary" solutions ..

2.2M pots with 2.2M R in parallel instead of the three 1M pots and 4 resistors to obtain 329K instead of the 330K R ...

[Bucksears]

Hmmmm.........I'm going to go back and check my wiring.
If it looks fine, I'm going to try some MPF102's just out of curiousity.

[Bucksears]

Wow. I dropped the gain pot down to 500k and replaced Q1, 2 & 3 with MPF102s.
Dramatic difference. The gain actually has some range where it didn't before, although it is lower now.
The only noise now is a somewhat loud 60-cycle hum (probably due to long wiring and it's not in a case); no squealing, no farty misbiased distortion.
I'm wondering now if I had a bad transistor or two; odd that it's not squealing at all and yet the wiring hasn't changed any.

Anyhoo, it sounds pretty incredible in terms of no muddiness and just flat-out distortion, but I will probably go back to a 1M pot and a J201 in Q3 to see if we can get some metal back into it.

[Connoisseur of Distortion]

with all the varied results, i think the doctor deserves some sort of warning sticker. it's just so damn unpredictable!

[Bob N]

Yeah, I didn't have any issues with it at all either. It worked just fine for me, but I didn't box it up tight... It's rack mounted with the JCM800 and the AC30 emu

[Bucksears]

BobN,
Did you use my layout as well?
(just trying to decide if it's a layout issue or a wire-this-up-really-freaking-carefully issue)

I agree, though, with Conoisseur: it's not a simple, 'it should work right off the bat' kinda build. But, if it sounds great, it's worth it once it's working right.

[Bob N]

I sure did. I didn't find anything wrong with the layout.... I'm not sure what I did differently outside of possibly component selection (1% metal film resistors, Orange drops ( b*tch and a half to fit in), Mica, etc...). Well, that and a rack mount chassis with a shield over each board... Outside of that, I stuck hardcore to the design...

Let me get the board back from one of the guitarists I work with and put it through some tests. I'm having a helluva time getting it back since he got to play with it. One thing I did notice though is that the Mid Pot doesn't have that much of a swing, so I need to take a closer look at that. There's still some tweaking to be done, but heath as well as myself are very happy with the design...

edited: Oh yeah, I had to be REALLY careful about the biasing though. I noticed that on my Telecaster it would go into distortion very quickly and when I brought it over to another rig, it was completely clean. I Re-Biased the JFets and everything worked out ok... Once you get it biased in, dab some nail polish on the trimmers so it doesn't "Re-Bias" itself through handling. Those trimmers are REALLY touchy.

After going through my BOM at Mouser, the only other thing I can find is the Fairchild J201 JFets for components... Let me get this thing back and run it through the paces. I should see Heath this weekend...

[Bucksears]

Thanks BobN.
Yeah, I noticed that the trimmers were REALLY precise and it took a while to get them exact; the swing seemed rather extreme as well, going from under 1v to over 5v in about 1/8 of a turn. It's extra hard for me because they are VERTICAL trimmers instead of the flat 'tabletop' trimmers; I'm using needlenose pliers to make the adjustments on the inner trims and a tiny screwdriver on the outer ones.
Glad to know that others have had no problem using my layout, though. I may eventually cave in and go with high-quality parts.

[Bob N]

I don't know... maybe the parts ARE the difference along with short leads and such... I'm pretty anal about stuff like that because I know that the guys use this stuff both live and recorded. I'm definately no expert on the subject as I am more of a "can build what someone else designs" kind of guy, but I have had very good luck with what I have built... maybe just dumb luck... have no designing talent what-so-ever, which is my biggest downfall, but I can get it to work...

Connoisseur: What is the full frequency of KUFO? I have an FM Transmitter here at the house and I'll try to replicate your findings with mine when I get the board back... It should be more than powerful enough to replicate the problem in close quarters....

[Davide]

I dont think it's a component quality issue.. I've used five 0.022 uF ceramic caps and three-four resistors taken from a 25 years old radio.. without any problem. I'm also using it without perfectly biased fets, i've adjusted trims by ear ..
Maybe the fets ?

[Connoisseur of Distortion]

the frequency is 101.1

best (or worst) of luck with that... :?

i have been thinking... what are buffered pickups? active? i have seen mention of buffered pickups as well as buffered pedals, and maybe this is the reason some people are getting it right on the first try... just a thought.

[Bob N]

Wow, that's Q101 here... HUGE station... Home of Mancow. I would definately have got something on that station if it was somehow "Tuned" to that frequency, ESPECIALLY when down in Gary, Indiana which is where Heath is at... I gave him a call at Lunch and he still reports no problems at all outside of the Midrange Pot that we already know about.

As far as the pickups are concerned, my Telecaster is completely factory with the single coils. No mods to this one at all and built in 2001 or there about... Heath's is a Schecter that is also completely stock and built in the same basic timeframe as mine... Humbuckers on that one...

Guitar cable or input leads to the PCB possibly??? With as much gain as what is in this circuit, it wouldn't take much for something like that to be heard... Outside of that, I'm at a loss. It's obviously on the input side though, if it isn't there when the effect is not in the circuit. Or maybe it is there and the gain on this unit just makes it more audible. Well, for the radio signal that is....

[Bucksears]

Psyched. That's all I can say.
Basically, I have MPF102's for Q1 & 2 and J201's for the rest. Q2 was a little trickier to bias, maybe being right next to a J201, I don't know.
Anyhoo, I swapped the 1M pot back in for the gain and this circuit is where I want it. I was getting a LITTLE bit of hiss, but that varied as I adjusted the controls (and touched the contacts), should be fine boxed up with shorter leads. The whine was gone, but while it has SOME noise, it's still not as bad as my DS-1.
This thing kills a Metal Zone. Scoop the mids and dime the gain & bass and it's chugg city. Again, the mids don't really boost as much as they cut.
It's an amazing sounding unit and will be a keeper just simply for those speed riffing and shredding moments.
Again, thanks Electrictabs.

[Bob N]

Bucksears,

Q2 was REALLY tricky on mine as well... I don't think it's a product of the parts, but rather just the circuit itself, but it's not impossible.  Maybe on that JFet it would be best to have a finer tuning trimmer. But I definately agree, this thing REALLY rocks! It makes an awesome "Cut Through" Solo channel, which is what it's being used for off of a JCM1000, which has an awesome rhythm sound.

This is probably one of the most useable projects I have made overall as it can be used live or recorded, in a package that doesn't take up much Real Estate at all...

Definately a HUGE thanks to Electrictabs and You for the design and board, I just REALLY wish I had a better understanding of the design aspects as I just had a request from yet another guitarist for a Plexi... I have the schematics, but don't have a real good grasp on the theory.... I'm going to do some more reading to try to understand this process.

[vanhansen]

Bob,

For the Plexi, check out the Thunderchief circuit from Runoffgroove.  That is an emulation of the 100 Watt Super Lead (a.k.a. Plexi).  Those who have built it love it.

[Bob N]

Cool! I didn't know they were one in the same... Sounds like a winner to me... Still need to understand the theory though...

[vanhansen]

No problem.

As for the theory, I found it relatively simple.  Just replace each of the preamp tubes with a JFET.  I studied the Marshall Super Lead schematic and the Thunderchief schematic to get an idea of what was going on.  That is what led me to do the JCM800 emu at the same time Electric did his.  I have the JCM800 schematic and Electric's JCM800 emu schematic on my site if you want to study them side by side to get a handle on it.

[Bob N]

I will definately check it out! These things have SOOOO many uses, especially in the situation I'm in right now... 2 of our bands that we manage are in Emergenza and are forced to use a backline setup of Krate amps, but the guitarists rely very heavily on the sounds that their amps make (one being specifically what this circuit emu's). It gives them just a head's up on the competition by being able to have their sound without HAVING to have their rig on stage, and this is acceptable by Emergenza standards.

[vanhansen]

That's cool.  Keep in mind though they these emu's don't sound exactly like the amps they emulate but the do get in the ballpark at least.

[Bob N]

I realize it's not perfect, but it sure is a lot closer than the backline amps. At least being in the ballpark you have some of the characteristics that you're looking for, and besides, it's a lot easier to carry than a full blown rig and the tone is pretty close. In addition, the rack mount setup I've configured this into gives a ton of possibilities for effects and setups with the small patch bay on the back.