JCM800 emu problems...

[NaBo]

I soldered up electrictabs's JCM800 emu today: http://www.diystompboxes.com/sboxforum/viewtopic.php?t=28243, using Bucksears's PCB layout: http://www.diystompboxes.com/sboxforum/viewtopic.php?t=28551

I have problems.  ... ... well not ME... the circuit does.  it's the classic, blatty misbiased tranny sound at output.  this may be part of my problem, but i dont think it's my only one.  I'll let you more experienced guys clue me in.

Mods/substitutions:  MPF102's instead of J201s

Battery V: 9.10V ... V at circuit 9V+: 9.10V ... V at circuit ground: 0V

(I'll number transistors 1 to 6 from left to right since this isnt shown on schem/layout)

Q1, Q2, Q3, Q6: All biased for 4.50V at D (i use 20 turn trimmers ), show 1.5 - 2V at S, 0V at G.  It seems these are behaving normally.

Q4: Battery voltage at drain (9.10V), 6.25V at source, 4.50 at gate (from Q3's trim)

Q5: (my problem is probably here) biased for 4.50V at D, showing 4.44V at S (:shock:) and 1.89V at G (:shock::shock::shock:).


I audioprobed and hey, it sounds great at Q4's source.  But yeah... and so it came to pass that i must have DC in my signal or something, cuz when i turned off my ruby amp, it's LED dimmed but it was still on.  when strumming with the amp off the LED would dim and i could still hear it blatty and ugly through my cab  :?... (yep, those JFET's really are normally "on"!)...  i checked my wiring and traces a whooole buncha times, and used continuity testing to make sure everything is connected thats supposed to be connected and stuff that isnt... isnt...  :roll: ... but i still haven't been able to locate my problem.  I turn to you, oh knowledgeable effects gurus, guide me to JCM 800 glory!  i'll try to record some samples of this emu that everyone's been wanting if you can help me get it working

[Greg Moss]

Are you having problems at the phase inverter stage of the emulation?  If so, this was a very common and frustrating problem with the Thunderchief - the Marshall Super Lead emu.  There have been a few threads on the PI stage problem, and I recommend doing a search on this forum for Thunderchief Bias problems.   Hopefully they can help....

Greg

[NaBo]

Ah yes, it would appear so....  electrictabs mentioned his emu was based on the thunderchief, so it must have inherited the same hard-to-bias-FET problem. Stupid FETs!  :x

Well, I've got 25 J201s on the way from SB, so I should probably just wait and see if some of those will bias right before I start subbing parts for one of the MPF102s to work.

Thanks Greg!

[electrictabs]

on the runoffgroove faq there is some info how to fix the misbiased sound which works great