JCM800 Emu - Bias/Pop

[mydementia]

I finally got the pots to finish this project and put all the off-board wiring together this morning.  Once I had it all wired up, I put power to it and set the all the drains...except for Q5 - it would only dither between 9.1V and 8.4V.  I used 'open' 100k trimmers from futurlec (they're okay - the sealed ones are nicer to work with...but 3x more $) and when I checked the resistance the Q5 trimmer maxed at 84k so I yanked it and put in my last sealed trimmer...no joy - same problem.  Since it was all together, I thought I'd try it out.  I plugged it in to my rig and noodled for a while - first impressions are that it's a pretty darn good emu.  A little shrill (compared to my Dr. Boogey emu) but rolling back the guitar tone pot helped a lot.  I also notice popping when I switch the circuit on and off.

So...
1) Any thoughts on biasing Q5?  I've chased the wiring several times over and just can't figure it out.  There are very few parts in that area of the circuit...not sure where up or downstream I'm having a problem.
2) What am I supposed to do about the popping?  I just checked and it looks like the 1M input resistor is going to ground like it's supposed to and the polarity of the 100uF cap is correct...

I used this schematic:
http://www.erikhansen.net/diy/schematics/misc/jcm800.png
...and this layout:
http://www.erikhansen.net/diy/schematics/misc/JCM800emulatorParts.jpg
...and I built it on perfboard with zero subs.

Yes, I used the search button...but none of the threads seemed to offer a solution to the problem I haven't already tried (forwarding to Thunderchief bias problem threads just seemed to be more confusing than helpful imho). 

Thanks for looking.
Mike

[Bucksears]

The best thing I can recommend would be to try another JFET in Q5. I didn't really have any problems with the JCM800 emu as far as biasing or working right goes.
Yes, it's a quite a bit brighter (and CAN be a little more shrill) than the Dr. Boogey, but it sounds really good. I originally had an 820ohm resistor in front of the tone stack (should have been 33k) and got a killer VH1 tone, but the eq didn't really work.
I screwed up the board on mine and need to build another one, but I'm having too much fun with my Guv'nor and BSIAB II at the moment.
- Buck

[mydementia]

Hi Buck.
Yeah, I tried a few different J201's and got the same result...strange...
Does yours pop when you engage the circuit? 

I'm not all that thrilled with my BSIABII - mine has a nice tone when I push it with something bright...I haven't messed with it in a while...everyone loves the BSIABII so much, maybe I should take a break from emu's and give it another shot...

I need to box up my guv'nor.  Based on my initial noodling, this one sounds pretty darn good too...  I was hoping to get the JCM and the Guv boxed up today for band practice...but that's just not happening...stupid biasing...

Thanks for your response.
Mike

[vanhansen]

Try a 2N5457 or an MPF102.  They are lower gain but might bias for you.

For the pop, put a 1M pulldown resistor at the output.  That should help.  The 1M at the input should already be helping eliminate the pop.

[mydementia]

Tried both different JFET flavors (2N5457 and MPF102) - same lack of bias..
I'm probably going to box this up with Q5 biased as far down as I can get it (max trimmer biases to 8.7V with my 9.0V battery).  Would there be any ill impacts of adding a larger resistance in place of the 100k trimmer?  I could futz around with resistor values on my breadboard until I find 1/2 supply voltage...just thinking out loud here...  I really do think there are some useful tones the way it is working right now...  From the schemo, it looks like Q5 flavors the Master Volume...the Q5 drain also goes to the Q6 gate that goes through a 100k resistor to ground...since this goes to ground, does it matter what voltage hits it?  Also, any idea why the Q6 trimmer power rail goes to the 2.2nF cap to the output?  Seems strange to me to put DC into the output signal...

Also, engaging the effect doesn't pop when I use my Ruby test amp...hmmm...might just get this in a box and test drive...

Regarding the output pulldown resistor - do I just throw a 1M resistor across the output jack lugs (tip and sleeve)? 

[vanhansen]

Yes, just throw a resistor across the output jack lugs, or solder it on the board from the output pad to ground (on the solder side of the board).  That should do it.

You could try a larger resistance trimmer on Q5 if you have one, and then put in a resistor matching the value or leave the trimmer in place.  Also, double check the values of everything around that JFET, including the resistors/caps from source to ground and the 1M resistor that connects the gate to source after the 22n cap.

Q5 and Q6 are actually the emulated phase invertor stage of the 800 amp.  If you look at the schematic for the 2203/2204 JCM800's, you'll see that Electric's schematic is exact.  Coming off the drain of Q6 is fine.  The 2n2/100k to ground should be taking care of it. 

[mydementia]

Am I putting a resistor or a cap across the output?  (Buck says resistor/Erik says Cap...).  I'll try whichever one you guys recommend tonight when I get home.  I'll also re-re-re-re-re-re-verify component values around Q5/Q6...my local surplus shop isn't the best at putting resistors in the right bins...so I usually measure every cap and resistor before it goes into a circuit - I might have gotten lazy on this build...we'll see.
Mike

[vanhansen]

Sorry, I meant to type "resistor".  I modified my post.  Take notice at the time of the original posting. 

[mydementia]

It's official - I'm a *&^*&^!  I can't blame this one on my local surplus store!  I had a 470k resistor coming off the Q5 source instead of a 470ohm  - dumbass!  I swapped in the correct value and now it works great - no popping. 

Two observations:
1) This thing has a TON boost - my 'level' pot is barely above zero.  Is this behavior consistent with everyone elses builds?
2) It's VERY bright...can be shrill (as Buck mentioned) - should sit just fine in the mix with a bass a-la 80's big hair!

Thanks for your help on this build...turns out it was completely me this time!
Mike

[vanhansen]

Glad you got it working.     Don't feel bad about the resistor oops.  I've done it before.  It happens.

The boost you're experiencing is normal.  What you need to do is fiddle with the gain/master volume controls together.  They behave much like the actual amp which means they interact with each other.

The brightness is another characteristic of the actual amp.  That can be tamed with some simple mods.  I have a few listed on my site.  Have fun!