Azabache bias and split question

[screamersusa]

I just built the ROG azabache. Very nice.
Before it goes in the box, whats the best way to finish biasing it?
I'm guessing use gain to keep it from clipping with a 1k tone,
adjust the bias for each j201 to just peaking? Is that about right.


2nd question. It's not playing nice in front of my carvin pre board.
Is there any thing to consider when configuring the first stage like the AMZ
jfet splitter (http://www.muzique.com/lab/splitter.htm).
I'm thinking of pulling from the Azabache, pre 2n5457 and simply feeding over to
the carvin boards first 2n5457 omitting the decoupling cap on the carvin.
One input to both preamps, switching the outputs later as the carvin is set up with the boogie eq.

[midwayfair]

Before it goes in the box, whats the best way to finish biasing it?

Not to sound snarky, but if you want it to sound like it's supposed to sound, use the biasing they give on their schematic. The biasing affects not just the output (and thus how far they overdrive the following diodes or transistors) but also the character of the distortion. For more info, check out the FETzer valve article on ROG. It discusses how the biasing affects the ratio of even vs. odd order harmonics.

[screamersusa]

Midwayfair:  Its set as per the schematic, q1 at 6v, the rest at 5v using a 9v supply.
Since j201s vary alot I wanted to see if it needs adjusting, I also run everything else at 12v.
Mine does not clip very much at all unless gain is almost all the way up. Its quite clean.

[midwayfair]

Midwayfair:  Its set as per the schematic, q1 at 6v, the rest at 5v using a 9v supply.
Since j201s vary alot I wanted to see if it needs adjusting, I also run everything else at 12v.
Mine does not clip very much at all unless gain is almost all the way up. Its quite clean.

If you have the gain at max in "tweed" mode, is it enough distortion (as in, it could be a pot taper problem - could switch to a B taper)?

Where are your J201s from? There have been a lot of fakes lately -- the ones from Tayda, for instance, were almost universally lower gain than the Fairchild ones.

I suppose you could adjust the 100K resistors between each stage to get a little more out of it without rebiasing.

I should have added in my first post, though, that the "right" bias in any circuit you build is the one that sounds best to you.

[screamersusa]

They are fairchild originals. I used my last few in it.
Compared to the demos Ive seen, mine is running lower gain unless
the demos are through edge of breakup amps already as many demos are.

[midwayfair]

They are fairchild originals. I used my last few in it.
Compared to the demos Ive seen, mine is running lower gain unless
the demos are through edge of breakup amps already as many demos are.

Here's my demo through a squeaky clean amp to compare ... http://www.youtube.com/watch?v=LsQWtJyYBIM ~2:50 for higher gain sounds. I can't remember if I cranked the gain to 5:00 at any point.

Have you audio probed a bit to see if maybe you're losing a lot of signal somewhere?

[screamersusa]

oh, ok. Your demo is the one that made me build it....lol.
The other demo with the scratchy black box is a lot gainer (too much).
I did have to sub the last 2 220nf caps for 150nf as RS was out of 220nf.
I have 300nf, should I use those? First 4 caps are 220nf.
I probably need to run it through my Crate Gxt100 (unmodded as yet) to
test it. Unfortunately I'm limited to headphones and solid state most of the time which
is why I'm probably annoying the hell out of everybody asking silly questions.
My digital modelers drive me Nuts as I can hear them trying to sample, apply the
algorithm, and reproduce the sound like falling down a staircase.
I plan on using the azabache in parallel with my boogie-ish front end switching between the two.
Driving either the amp or the GNX3k in direct mode.

[B Tremblay]

Are you powering it with 12v?  Keep in mind that it was developed for use with a 9v supply.

What are the pin voltages of the FETs?

How does it not "play nice" with your Carvin pre board?

[screamersusa]

I'm testing it with a 9v. Drain voltages are as listed
in schematic/layout. Q1 2n5457 6v, Q2,3,4, J201 at 5v.
I had a bc549 in there and wound up pulling my only 2n5088 out of
the cot for it,which made a big difference, (before I asked for help by the way).

The Azabache sounds quite nice but quite clean.
The carvin sounds warm and full, the switchable Boogie eq is a little weak. (need lo z driver).
Running the Azabache into the carvin without the eq results in the Azabache going very thin
and searing like there is a severe impedance issue. I figured I'd run them parallel in a worst case.

[B Tremblay]

What are the voltages of the FET source pins and all three pins of the BJT?

[screamersusa]

Its put aside at the moment, I only get 20 min or so to work on this stuff before
she gets p/o ed.     Just figured out my eq issue last night.

Will get to it tonight.

[screamersusa]

FIXED!!!!!!!!!!!!!!

Source voltages were q1 .7, q2-q4 all .3

It was the gain pot lugs 1 and 2 wired backwards.
I had to make geographical adjustments due to trimpots and wired
the gain pot back azzwards.
Good gain now at 9v and it seems to like 12v as well.
Thanks for the assist

[midwayfair]

Yay! How do you like it so far?

[B Tremblay]

Very glad to hear that you determined the fault and fixed it!

[screamersusa]

So far I like it and its probably a keeper. It's in an old tube MP casing.
Gotta play with it for awhile to get a good feel for it.
I pulled the output of Q1 through a second decoupling cap to the boogie mark2 eq
which feeds a transistor makeup gain stage. No effect on the Azabache at all.
I can switch from the "Fender" tones to a bell clean Boogie tone.  I'm using
it additionally as my Mark2c/savage research attempt (The Bengl).