Author Topic: Electric Mistress Clone - wont bias  (Read 239 times)

rundgrenrules

Electric Mistress Clone - wont bias
« on: January 28, 2021, 03:49:35 PM »
Hey everyone!

I built the Electric Lover project from Guitar PCB Mania.  Its a modernized Electric Mistress clone.  It is passing signal, but I cannot get it to bias up correctly.  There is only a small range of the BIAS trimpot near max-clockwise position that allows signal to pass, regardless of the settings for the CLOCK, T1 (max feedback), and VOLUME trimpots.  Here are the directions for the setup:

Set Clock and Bias in the middle, at 12 o’clock.
Set T1 and the Rate knob fully counter-clockwise
Set Range, Volume, and Feedback in the middle, 12 o’clock.
Filter Switch pointing to the right position.
Plugin your guitar and amp to the board, and the DC power supply, this
project works great at 12V, but also could be feed with a standard 9v
power supply. Never plug it on 15v, this would be risky for the MN3007.
Now the signal should be going through the effect, although you may not
hear any flanging yet. If you can’t hear any signal through the effect, double-check all your solder points, components on connections to ensure
everything is okay.
In order to get the flanging effect to show up, adjust the Bias trimpot. This
trimmer is in charge of feeding the BBD MN3007. You don’t need to be
extremely precise with this one, just till you hear the flanging effect and the
BBD working.
Adjust the Clock Trim pot. As we said before, this trimmer is pretty sensitive.
Set it up to the point where you achieve a wide sweep with minimum noise.
If the flange starts to whine or “chirp” at the extremes of its sweep you will
need to do a little more adjustment.
Go back and tweak the Bias trim pot further clockwise until you get smooth
up and down flange without distortion.
With T1, we can control the maximum of Feedback desired, as this trimmer
acts as a limiter to the maximum allowed. Set Feedback at max, fully
clockwise, then start adjusting T1 as you get the maximum desired
feedback.
Once we have almost everything set-up, use your ears to make the final
adjustments on the trimmers while testing the limits of Rate, Range, and
Feedback.


And here is the link to the page with the build doc, schematic, etc.

https://pcbguitarmania.com/product/electric-lover-flanger/

I am honestly not sure what the deal is.  I've tweaked and tweaked the trimpots in every order, tweaking this one then that one then this one and then back and forth and all directions.  The bias seems to be the one throwing a wrench in getting a good clear sound.  There is such a tiny range of adjustment and a lot of that trimpot range completely cuts out the signal and another chunk of it is significantly crunchy in an overdriven way and then there is a small range where there isnt crunch and there IS flanging, but the sound is overall weak and tinny, and then I cant turn it anymore clockwise from there.

Here are some pictures of the build:




And here are the measurements I took with all trimpots at middle position except for the bias which I turned far clockwise:



Any suggestions would be greatly appreciated!

DrAlx

Re: Electric Mistress Clone - wont bias
« Reply #1 on: January 28, 2021, 06:13:36 PM »
Looking at their schematic:

https://pcbguitarmania.com/wp-content/uploads/2018/05/Electric-Lover-Schematic.pdf

I would say R10 is completely unnecessary and is what is stopping you from setting the bias any higher.
I would short out R10 on the back of the board (stick a wire between its pins).
There is only a small range of bias voltages that will work.  That is the same for any BBD.  Too low or too high bias and the BBD will not pass any signal at all.  That's why I like multi-turn (precision) trim pots.  One way to set bias it is to find the 2 bias voltages (high and low) where the BBD starts to distort or stop the signal going through, and then set the bias voltage half way between those levels.


R30 of 15k is a design error in my opinion, which was made in the original Madbean Current Lover from which this is copied.
If you look at the original 9V EM circuit (http://www.metzgerralf.de/elekt/stomp/mistress/images/1981-electric-mistress-v6-schematic.gif) you will see they used 39k connected to the range pot, not 15k.
The reason Madbean lowered R30 was to try and give the pedal a bigger sweep than the original.  Lowering it to 15k can work OK so long as the pedal supply voltage is 15V.   The problem is that at 9V supply, that attempt at a bigger sweep can end up setting too low a control voltage at pin 2 of the LM311 (part of the VCO) and the VCO can then freeze because it can only go so fast.  The end result is a nasty drop out at the top of the BBD sweep.  This has been reported here before for the current lover at 9V.  If you get a dropout in your sweep, you should replace R30 with 39k so it is like the original 9V EM.

The 22pF clock cap is not an ideal choice and arises from a too simplistic understanding of how this VCO works.
The original 9V EM used a 47pF,and the thinking was that to double the clock speed (as needed by 3x07 chip) the cap needed to be made about half as small (i.e. 22pF).  That can work, but the problem with that approach is that you get a non-linearity in the top end of the sweep due to diode reverse current charging up the cap when the discharge diode switches off.  All analysed and explained on the EM3207 thread.   So I would personally recommend using 47pF like the original too and just twiddle the clock trimmer to give the VCO double the speed of the original 9V EM..
« Last Edit: January 29, 2021, 03:42:37 AM by DrAlx »

rundgrenrules

Re: Electric Mistress Clone - wont bias
« Reply #2 on: January 29, 2021, 03:44:37 PM »
Thank you!  I did all of the things you suggested.  Now the bias range for the trimpot seems correct - right in the center of the sweep.  I've got a much better signal happening now, not crunchy and higher fidelity.  I suppose the nature of the flange isn't quite as I expected.  Cuts a good amount of bass from the signal, not a noticeable flange effect unless the Feedback is set high, and the Range knob doesnt seem to do a ton.  Also, the dreaded LFO ticking!

I have a Mooer ElecLady coming in that I want to compare it to.  Once I do, I may try to modify it if I determine that its far enough off the mark.  As it stands Im not quite sure this clone gets the Mistress sound as I have never owned one!

DrAlx

Re: Electric Mistress Clone - wont bias
« Reply #3 on: January 29, 2021, 05:58:39 PM »
The Mooer ElecLady is pretty much a 100% copy of Thomeque's EM3207 on this site.  Very similar to the circuit you have just built apart from a few part value changes.
I am surprised it ticks. I have built this sort of circuit many times, even on vero, and never had ticking from the LFO.
« Last Edit: January 29, 2021, 06:03:04 PM by DrAlx »