Electrictab´s JCM800 Emu - Problems again

[snyder80]

Hi Folks!
Its my first posting in this cool forum and it could already help me out with building a Prof. Tweed and so on.
But now, well im realy lost in problem
I already used the searching function but nothing that suits my problem is discussed in one of the topics, so maybe YOU can help me out:

My problem is about the last two of six J201 Fets which should emulate the poweramp-section or something like this...
First i build my JCM800 emu (Electritabs Schematic) on perfboard (only inspired by the Xavier layout cause it seems to be buggy in some spots and is more a mixture of erik van Hansen and Electritabs layout http://diy.erikhansen.net/jcm800emu.htm).
Biasing by the way was no problem, every biasable Q is at 4,5 V DC at Drain, All Source is bout  0.9 V DC.
No substitutions and billion time triple checked the wiring. Elkos are used and are well fit in (-) to ground, (+) to signal.

After assembling the pedal i checked it and recognised it had about 4,5V DC (unwanted) on the output as well as AC Voltage (it should only be AC, right?!).
So a chap helped me and gave me the hint to add that 15n Cap between the VR5 and the last 2n2 cap as its shown in Erik van Hansen schematic. It realy blocks the DC, so just smooth AC on the oputput.

But the sound was some sort of phased out (not misbiased noisgate-style, just not that full and open sound )and had oscillations when Gain and MVolume are full up (even if i turn the Post-Volume near zero!!!). The DPDT input-ground-wiring didnt help me either, though it should help with highgain circuits .

I tried the box without connecting the 2n2 /15n cap to the VR5 trimmer and it sounds very full and like it realy should be, also with a extremly versatile EQ section and good bass and so on. Its just quite silent without the "Poweramp"-section.

I dnt want to blame anyone, but is the layout from Electritab and Bucksear( did the PCB layout) and van Hansen still buggy/ not working, i read across this forum Electritabs schematic should be varified?! But It must be realy a problem with only the last two FETS-section.

Thx for your comments,

Snyder80

[snyder80]

Well in the End i found a quite similar posting but there was no helping information how to tweak the problem...

look at http://www.diystompboxes.com/smfforum/index.php?topic=30120.0 , but he´s got biasing problems that i have not!!!

And it seems like the last overall volume also dials in/out the amount of voltage coming from the VR5 and Drain of Q6 to the output. I think the last 100k Volume pot should just be a overall and tone-and gain- independent volume...

And ther was another post where someone mentioned the strange 2n2 cap to output wiring and vanhansen told that all the DC voltage should be kept out of the signal because of the 2n2/100k/100n section in the "end"... Well mine didnt (see above posting)

And at least one thing i found out:
When i use the original wiring  but with the blocking 15n cap before the output, i have different drain DC voltage if biasing the drains when input and output is connected to guitar and soundcard/pod XT for cabsim... Since i bias pedals when connected to guitar/output i have no changing values...

[snyder80]

well, well, 

After sitting in front of this "#=(§?-in a box" i checked everything for the billion and first time. Well i had nothing wrong i just remade the whole part of the last Q5 and Q6.
And suddenly the AC/DC problem got solved even without the additional 15n cap... and sound good.

But one Prob remains: Q1,2,3,and 6 are all easy biased D=4,5 V, S= ~ 0,24- 0,39 V and G=0. Q4 is D= 8,9 V, S= ~4,5 V, G= 4,54 V (=Q3 Drain).
Only Q5 is D=4,5 V, G= ~2,7 V (!!) and G=1,12 V...
I tried the hint from Electrictab (http://www.diystompboxes.com/smfforum/index.php?topic=20380.0) but it made D= min. 4,9 V, D= ~ 0,7 V and G= ~ 0,3...

And it matters wether i bias the trimmers whith Gain and Mastervolume on 12 o clock or full on ten.. AND if i play with the overallvolume and try make it louder in aclean setting, it lost the signal and nothing is on the output. So i can only get bout 1/3 of the overall volume...
So any last tweaks supposed, as i am debugging this thing for nearly a 6 DAYS !!!!

Greets,

Snyder80

[Bucksears]

I've had no problems with biasing, the layout or the unit not functioning correctly. The only problem I had (and I took mine apart this weekend) with the JCM800 is that it was too #%&@! bright. I just couldn't deal with it.
Sorry I couldn't be of further help, but the problem is still there somewhere.
- Buck

[snyder80]

Hi Bucksears!

Thx for joinig my problems!!! Its good to know that there is someone with great knowledge

So i have no probs with the brightness and read across that u can add that 15n like van hansen did in his schematic to add more lows... also here http://tone-lizard.com/Ultimate_JCM800.htm are tons of soundtweaking tips... i came across that page while looking after the jcm800 tube schematic to "veryfie" the Electrictabs schematic...

At least i have edited my last post, so maybe any tips bout the Overallvolume problem?!?

[snyder80]

Well though it seems like talking to myself, i have some news:

It is the 10k coming from the Q4´Gate to ground which behaves very strange. I measuerd it in the circuit and first got only 3,3K . After llifting the ground connection, it shows the desired 10k value again. Now im redoing the Ground-connection surrounding the circuit. without having done that completly, that 10k does only give me a ~4,9k value...
Hell, there must be something cross-wired or what could make a resistor behave as if it is paralleld?! 
Also without the Q4 put in, there is this lack of 1/2 the resistors value

[snyder80]

So in the end i forgot to mention i bought the parts of microsoft, so it was shure they would have "bugs".

Its realy mysterious, cause everytime i posted my advances and the following problems, i did look at the box and it worked!!!

So im shure there is a demon in my JCM666 and i know why only the demon-like rockers use it that much....

@Bucksears: After i boxed it up, there realy is no problem anymore and it has that much bass, i cant hardly use it on full. And the mids do realy scoop out.. Middle poti is now a 47k and the highs are 220k poti. Ive got a 33k resistor after Q4 for the tonestack and it dials in, well.And USE that 15n cap between the 2n2 and the output. by the way, Q5 (Drain Resistor changed to 10k)  doesnt still bias well (is D=5,6, S=0.7 G=0), but i have no blatty or misbiased tone.

RAWWWK on...