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Electric mistress mods?

4/25/2000 12:41 PM
Electric mistress mods?
Does anyone have a clue how to mod these things?  
For instance IŽd like to make the flanging effect more subtle. My mistress is a reissue and it doesnŽt quite sound like the old ones.  
IŽd be greatful for any help I can get.  
4/26/2000 3:30 AM
Mark Hammer
AT LAST!!! Electric mistress mods
I had mentioned in an earlier posting that I had some mods for the EH Electric Mistress, and several people wrote to me and asked if I could forward the information. I finally got round to digging it up, and when I saw it, I realized that it could be summarized very easily without the original article, and that there was no need to clog up anyone's e-mail with 2 meg of scans when the same info could be conveyed in a few paragraphs. So, here goes.  
These mods are from an article by Thomas Henry that appeared in Device magazine in 1979. Thomas notes that this information only applies to the AC-powered EM, although I suspect the same trimpots are also available on any battery operated versions as well, just in different locations. Same probably goes for reissues.  
The same 5 basic controls, or points of control, can be identified on just about any flanger, although not all commercial models make them available. These basic controls are: the initial delay (1), the range of time delay sweep from that initial delay (2), the rate at which the sweep changes (3), the balance or mix between time delayed and dry signal (4), and the amount of feedback or regeneration of the delayed signal back to the input of the delay portion (5). Ninety percent of commercially available models will make 2, 3, and 5 available, and about 70% will make 1, 2, 3, and 5 available. Only a few will make 4 available as a chassis-mounted control, generally leaving the wet/dry mix at 50/50. The EM makes 2, 3, and 5 available on the panel, but 1 and 4 are lurking underneath, in the form of trimpots on the board. Tom Henry's article simply describes where the controls are and what they can do.  
If you were to magically change the metal case for an invisible one, and looked down at the unit from the standard operating position, you would see the following trimpots:  
- between the RANGE and COLOR controls is the regeneration trim (EH used the term COLOR instead of feedback or regen),  
- just to the left of the RANGE control you would see the bias trimpot (used to set the bias voltage going into the SAD-1024 BBD chip),  
- on the left hand side of the board, you'll see three trimpots; in the upper left corner near the line cord is a trimpot that sets the dry/wet mix,  
- just below the mix trimpot is the clock null trimpot for cancelling the clock signal noise,  
- closest to front of the pedal on the left is the initial delay trimpot.  
The mix and initial delay controls can be productively replaced with panel mounted pots of identical value to yield a 5-control flanger. The regen trim control should be physically left where it is, but it can be used to season the COLOR control to taste. It would normally be set by turning COLOR up full, and setting the trimpot to where it just starts to oscillate without you playing anything, and edging back from that just a little bit. This will give the full range of effect from almost oscillation to zero feedback. Bear in mind that with the ability to tune the initial delay, and mix the delay signal in the background, you can now use the COLOR control to introduce interesting resonances to mix in the background.  
In theory, the mix control should also allow the EM to produce vibrato effects by shifting the wet/dry balance to delay-only, however I'm not sure if this is one of those mix controls that only determines the amount of wet signal, or determines the amount of both wet *and* dry. (If you know, or find out, please post your outcome on AMPAGE.) At the very least, adjusting the mix so that there is more dry than wet permits nice subtle shadings for rhythm work, or for adding just a touch of animation to your tone.  
Other than the inclusion of these trimpots as actively used controls, there are no other mods in the article that would alter the tone. Still, if you've ever worked with 5-control flangers, you know that they offer a broader palette of tones than the 4-knob models.  
Can you screw things up royally by monkeying around with the trimpots? Tonally, yes, but you won't destroy anything, and all trimpots can be set back to their original settings by ear. Bias can be set for the least distorted sound, and clock null can be set for the least clock signal feedthrough (that horrific motorboat sound). If you try to desolder any trimpots, it is good practice to make note of their approximate settings, and to label the trimpots somehow, in case you want to replace them and restore the pedal to "vintage" specs (though I'm not sure how you'd do that after drilling holes in the chassis!).  
Enjoy, and thanks for waiting.
4/26/2000 5:39 AM

Great stuff, thanks a lot!!!!! :)  
4/27/2000 2:50 AM
Synthbum Why mount new pots? Reissue, right?
Tweaking the EH Deluxe Electric Mistress reissue is easily accomplished without mounting and wiring new pots. Plug it in, turn it on, turn it over, and take off the bottom cover. I presume most if not all of the reissues are like mine and use PCB EC1000, REV B - visibly identified along the left side. If so, there are four trim pots arranged in sort of a lopsided 'T' pattern with holes through the PCB to adjust them from the bottom. Couldn't be easier.  
* The top left trimmer (all adjustable with a small phillips or flat head) is near the COLOR knob and governs regeneration. Counter-clockwise turns will induce strong squealing so it's best to set it just short of oscillation. Of course, clockwise turns soften the regeneration effect.  
* The middle trimmer along the top of the 'T' sets the mix. It mixes dry to fully wet with counter-clockwise turns increasing the effect mix. Good for getting some wobble-tone.  
* The trimmer at the right end of the 'T' pattern is the bias (I assume) and affects the overall flange action. Fully clockwise yields a not-half-bad overdrive but it's on full-time so... Counter-clockwise erases the flanging entirely. Set around mid-point or leave as is.  
* The trimmer at the base of the 'T' sets the delay time with counter-clockwise being the shortest. With this set at minimum, really wide sweeps can be had but at the very top of the cycle (shortest delays) with regen way up, it will momentarily click out as it hits zero and can't go higher (in sweep pitch). It's best to increase the delay trimmer about a quarter turn or just enough to eliminate that 'click-out'. That is if very short delay times are needed. I usually leave mine maxed clockwise.  
No clock filtering trimmer around. I may yet measure the trimmer points precisely and drill some 1/4" holes in the bottom so that I don't need to remove it whenever adjustments are necessary. Haven't gotten around to it yet. May also swap out that 4558 with a TL072 since that supposedly cleans it up. I have some laying around. I just haven't had any big objections to it as it is so far.  
Nice sounding flanger! Bright and rich. Very flexible. Still miss that old ADA though. =(
4/27/2000 3:40 AM

Re: swapping the 4558 for a TL072: this was one of the first mods I did to my re-issue, and it made the effect noticeably stronger, to my ears, anyway. The noise level was reduced slightly, but like you said, even with the 4558 it's not objectionable (for this type of flanger).
4/27/2000 1:35 PM
Mark Hammer

It appears that everything which was available for tweaking in 1979 is still available, despite whatever board changes were undertaken during reissuing. The trimpot locations are changed somewhat, and I thank you for that info.  
The recommendation to pop a couple of holes in the chassis and use pots was simply because these undiscoverd tweakables are the kind of things that people normally like to play with in real time, or in between songs while gigging. As noted, a "good" flanger will have mix and initial delay as front panel controls.  
Unless it has drifted, it is unwise to tamper with the bias control. BBD chips need to see a signal riding on a DC bias voltage to function right, and the bias trimpot sets it just right for that chip with *that* power supply and *those* component tolerances. Some companies replace this with a resistor pair set up as a voltage divider, and for some units this works fine. Other chips and designs are fussier and need to be biased individually.  
In contrast, the clock null can easily be replaced with a pair of matched resistors, no matter unit it is on, since it simply mixes the two BBD chip outputs in equal proportion. Much like the reversed coils in a humbucker pickup, these two outputs carry opposite phase versions of the annoying clock signal. When combined in equal measure, the annoying clock signals from each BBD output cancel each other, the same way the annoying hum from each coil on a humbucker PU is cancelled by what's coming from the other coil. If you wanted to really go nuts, I suppose you could replace those two resistors with a 10-turn trimpot, scope the darn thing, and tweak until there was absolutely no clock signal coming from the BBD output mixer (in this case, the wiper of the trimpot). Given that most designs will have at least SOME clock signal bleedthrough throughout the circuit (e.g., being picked up by the unshielded wire going to the output jack), overkill at the BBD mixer stage can be largely wasted effort.
4/27/2000 6:59 PM
Re: Electric mistress mods?
IŽll open it up this weekend so wish me luck.  
I was thinking, maybe if I wanted an extra knob then IŽll put the pot in the "direct out"-hole.  
I never use it anyway.  

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