Adding Gain control to electra distortion

[letsgocoyote]

How would I mod the basic Electra distortion circuit to have a gain control.  I have both tried on my own to figure out how or looked at how gain controls have been added to simple circuits (like the electra, or bazz fuss) and am still slightly puzzled.  The first time i tried replacing the resistor that comes off the collector to positive with a pot, which seemed to create some sort of control but im not quite sure i could call it a gain control.  Next i tried putting a 100k pot at the begging of the circuit to limit input but this jsut didnt work at all (maybe wrong pot value?).  I have seen other circuits recommened putting a 1k pot in place of the emitter resistor (if there is one) to ground ina  circuit to create a gain control.  Just looking for advice where to head, ims till learning how to mod simple circuit!

[Blues Lyne]

Try putting a 1k pot between the emitter and the 470ohm resistor.  It's worked for some.

[Tom9979]

I used a 10k log pot from emitter to ground, replacing the 470R. Works well but havn'y tried any other methods to compare.

[smallbearelec]

Here is a variant of the Electra that gives good control of the gain of the stage, and so of the distortion:

http://diy.smallbearelec.com/HowTos/BreadboardUrsaMinor/BreadboardUrsaMinor.htm

[antonis]

A pot (as variable resistor) instead of Collector resistor would also help (it's value depends on your needs for the maximum gain) but this will react to feedback loop...
(better choice is an Emitter trim pot..)

[induction]

A pot (as variable resistor) instead of Collector resistor would also help (it's value depends on your needs for the maximum gain) but this will react to feedback loop...
(better choice is an Emitter trim pot..)

The emitter pot (or trim) as a gain control works best with the emitter to lug 3, ground to lug 1, and a cap from lug 2 to ground, like in the Ursa Minor Steve linked to above (also seen in the Fuzz Face, etc.). This way the emitter sees a constant resistance to ground, so DC bias is set by the collector resistor and the emitter pot value (not setting), and the AC gain is set by the pot setting. In other words, this lets you adjust the AC gain without changing the DC bias.

If you can find a schematic of the Earthquaker Devices Speaker Cranker (hint: it's posted on the other forum), you'll see that it is basically an Electra with the emitter resistor replaced by a 330R in series with a 5k pot wired as described above with a 100u emitter bypass cap from lug 2 to ground. A few of the other component values are tweaked, including an increased collector resistor from (4k7 to 10k).

The  voltage gain of a common emitter amplifier with an emitter resistor can be approximated by -Rc/Re. (The negative sign means the signal is inverted.) So the stock Electra has a gain of -4k7/470 = -10. The Speaker Cranker has an AC gain between -10k/5330 ~= -2 and 10k/330 ~= -30. The emitter bypass cap trick gives a wider AC gain range to work with because the DC gain is held constant. If you try to get this much range in the gain control without an emitter bypass cap, I suspect your DC bias will hit the rails on one or both ends of the pot setting.  

[antonis]

The emitter bypass cap trick gives a wider AC gain range to work with because the DC gain is held constant.

I'd substitute the word "wider" with "frequency dependable"...

If you try to get this much range in the gain control without an emitter bypass cap, I suspect your DC bias will hit the rails on one or both ends of the pot setting. 

For sure, but isn't this a desirable behavior of a simple BJT distortion pedal..??

[Gus]

Two links to older threads that might give you an idea
http://www.diystompboxes.com/smfforum/index.php?topic=103688.0
http://www.diystompboxes.com/smfforum/index.php?topic=104006.0

[induction]

The emitter bypass cap trick gives a wider AC gain range to work with because the DC gain is held constant.

I'd substitute the word "wider" with "frequency dependable"...

I wouldn't. A 100u emitter bypass cap isn't going to be very selective about frequency. Let's assume we use the component values from the Speaker Cranker, For low settings of the gain pot, the gain will be mostly independent of frequency within the limits of guitar frequencies. For example, with the gain pot at 0, the voltage gain is 1.88 for all frequencies from 20 Hz to 20 kHz and beyond.  With the gain pot at 10, the voltage gain is about 25 at 20 Hz and increases to about 30 for all frequencies above 200 Hz. For guitar purposes, this arrangement gives a nice, flat frequency response at all settings. If you want a frequency-dependent boost, you can reduce the size of the cap. That will make the gain increase with frequency for higher gain settings, thought at lower settings the boost will still be frequency-neutral. Sim it and see.

If you try to get this much range in the gain control without an emitter bypass cap, I suspect your DC bias will hit the rails on one or both ends of the pot setting. 

For sure, but isn't this a desirable behavior of a simple BJT distortion pedal..??

No. If the DC bias hits the rails, you'll be lucky to get any sound out of the thing at all. For a strong enough input signal, you may get some glitches and scratches from the output, but it won't be very musical. If you try to make the boost range very wide without the emitter bypass cap, one or both extremes of the gain pot setting will be unusable.

[Mark Hammer]

Meh, a 1k variable resistor from the emitter, in series with a 22uf-47uf cap, and be done with it.

[brianq]

Have you heard of the Trotsky Overdrive? It's a modified Electra with gain control.
Check it out over @ Beavis Audio Research project page. Sounds pretty good.

[antonis]

@ induction: My bad...
(didn't read well enough your first post - about pot's lug and cap to GND...)

You're absolutelly right about DC bias stability and AC variable gain..!!!