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This article was submitted by KonZissis.
My ?previous 12AX7 based EQ and overdrive circuit was located after the G-G output terminals of the AO-28 preamp because I was able to get a better bass growl and crunch from this location compared to the lesser bass growl and crunch that was previously available if the circuit was placed between the tone control wiper and the input grid of the ?12BH7 output valve.
One major problem with the previous EQ/overdrive circuit that was located after the G-G output terminals was that an expensive output coupling transformer was required that would split the output signal into a dual balanced line output so that the output signal could be properly matched with a balanced line Leslie such as the 122 or a HammondToneCabinet.
More recently I have experimented with and I found a way to get the nice bass growl and crunch with the EQ/overdrive circuit being located between the tone control wiper and the input grid of the 12BH7. The reason why the bass response was being rolled off in my earlier 12AX7 EQ/overdrive circuit design that was located between the tone control wiper and the input grid of the 12BH7 was that the impedance to ground of the input gain control that fed the signal into the first gain stage of the ?12AX7 valve would cause the lower bass frequencies to be rolled off from the signal coming from the tone control wiper. The tone control wiper needs an extremely high impedance in order to allow the full bass response to pass and even using a 1 Meg ohm pot as an input gain control causes some of the lower bass frequencies to roll off. I overcame the bass roll off problem by not wiring any input gain control between the tone control wiper and the input grid of the 12AX7 and just having the passive bass and treble EQ wired between the tone control wiper and the 12AX7 input grid.
I did a careful A/B listening test with the 12AX7 EQ/overdrive circuit switched in and out of circuit and with the gain/overdrive control set to a low level to produce the pure clean sound and the master volume control set to the full volume setting so that the volume level of the organ through the EQ/overdrive circuit was identical to that of the fully bypassed organ signal. After doing this careful A/B listening test, I verified that with the EQ controls set to the flat setting, the tonal response of the organ is identical and without any bass roll-off. Because of this the newly revised circuit is now simpler and cheaper to build than the version that was located after the GG output terminals and that needed the output coupling transformer. The quality of the overdrive tone is very good and it does sound similar to an overdriven Leslie 122 or a 147 valve amplifier depending on the settings of the EQ.
This circuit is reasonably simple in operation in that it uses one 12AX7 valve and a passive bass and treble EQ before the gain/overdrive stage and also a passive bass and treble EQ after the overdrive stage. Both EQ's can be activated or bypassed with a 2-pole / 2-position (DPDT) switch.
The passive bass and treble EQ's are _Baxandall_ type EQ's and I have simply copied the bass and treble tone stack that BobSchleicher
displays on the 
schematics section of his website (see 
http://www.tonewheel.com/Schematics/tonecontrol.jpg),
however I have disregarded the low gain cathode follower circuitry that BobSchleicher's circuit uses and I have simply wired the EQ tone stack
before the input grid and after the master volume control of the EQ/overdrive circuit.
The pre-overdrive EQ affects the tonality of the signal going into the 12AX7 valve. This is useful if your organ has a weak bass response and you want more of a bass growl and grind or if you want more distortion from the organ treble frequencies.
The post-overdrive EQ shapes the sound after it has passed through the overdrive stage. This can give you a nice fat bass but without distortion because it is located after the gain/overdrive stage. You can simulate the deeper bass response of a Leslie connected together with a PR-40 Hammond tone cabinet.
There is an additional modification to the treble capacitor of the post overdrive EQ treble control. You can use a DPDT switch to select either the original 470 picofarad (pf) capacitor or a 1 nanofarad (nf) capacitor. The 470 pf capacitor produces the true flat response sound when bass and treble are set to the _"flat"_ position. For the bass control, the _"flat"_ setting is the 12 o'clock position and for the treble control the _"flat"_ position is at around the 2 o'clock position.
With the 470 pf capacitor, turning the treble control to the full position results in the higher treble frequencies being emphasised thus making the distortion more raspy sounding. When the 1 nf capacitor is chosen there is a discernible increase in the lower midrange warmth when the treble control is set at the "flat"_ 2 o'clock position and when you turn the treble control to the full position the upper midrange as well as the treble is emphasized for more presence and _"scream". This can be useful if you have a muddy sounding Leslie or a Hammond tone cabinet or an external amplifier or speakers. If you want to you can add this same capacitor switching option to the pre overdrive EQ treble control.
With the pre-overdrive EQ switched on, you can get a fully clean sound even with the expression pedal at the full volume position when the gain/overdrive control is set at a low setting. With the pre overdrive EQ swithed on , setting the gain/overdrive control to the fully clockwise position will produce a heavily ovedriven Leslie 122 / 147 simulation when the expression pedal is at the full volume position. With the pre overdrive EQ switched off ,there will be some distortion in the first gain stage of the 12AX7 when the expression pedal is at the full volume position regardless of the setting of the gain/ overdrive control and
setting the gain/ overdrive control to the fully clockwise position will produce a heavy level of saturated overdrive similar to the early 1970's Deep Purple Mark 2 albums such as 'Machine Head' and Made in Japan ' which featured Jon Lord's Hammond C3 organ played through a heavilly overdriven Marshall valve amplifier.
I recommend that the post overdrive EQ is normally left on because when it is bypassed the signal levels from the EQ / overdrive unit will be extremely high and they will overload the final stage of the organ preamp.
BY THE WAY after some more recent experimenting I noticed that the two 10 K resistors that are shown in series with the grid pins 2 and 7 in the schematic are not really necessary after all and you can get a subtle increase in the very high treble if you omit these two resistors because the 10 K resistors in series produce a subtle roll off in the very high treble. However if you switch off the pre ovedrive EQ bass and treble tone stack you will then get a sagging or 'coughing' effect when the first stage of the 12AX7 is overdriven and a 10 K ohms resistor wired in series with the grid pin 2 will help to stop this effect. If you want you can wire this 10 K ohms resistor to be on the bypass section of the 'pre overdrive EQ' on / bypass switch so that the 10 K series resistor is in circuit only when the pre overdrive EQ is switched off.
The 10 K ohms resistor is not needed in series with the grid of the second stage of the 12AX7 ( pin 7 ).
The two cathode bias internal gain pots determine the total internal gain level of the two gain stages of the 12AX7 valve. In my own circuit in my 1962 C3 l have found that the best sounding overdrive tone is created when the 25 K ohms cathode gain control of the 12AX7 gain stage 1 is set at slightly less than the full position. I have the 10 K ohms cathode gain control of the 12AX7 gain stage 2 set at the full gain position. If both cathode gain controls are set to the fully clockwise position for maximum gain then the distortion has a harder sounding quality to it but when the cathode gain control pot of the 12AX7 gain stage 1 is set at slightly less than the full position such as between the 3 o'clock and the 4 o'clock position , the valve breathes better and the distortion sounds more natural. The trick is to set the cathode bias internal gain controls to the setting that produces the maximum amount of distortion that you want when the gain / overdrive control is set to the fully clockwise setting. If you only like a mild distortion , then you can set the 25 K ohms cathode bias gain control of the gain stage 1 to a lower setting so that when the gain / overdrive control is set to the fully clockwise position there will only be a milder level of distortion.
Next to the 100 k ohms plate resistor of the second gain stage of the 12AX7, there is a 1 M ohms pot wired in series and it is labeled as "Variac simulator". This 1 M pot can lower the HT voltage going to the second stage of the 12AX7 valve in order to starve the plate of the second gain stage for a very squashed and dirty sound if this is desired. When it is set to the fully clockwise position, the full HT voltage goes to the plate B for the loudest output level. Even with this setting you can still get a full overdrive sound when the gain overdrive control is set to the fully clockwise position.
After the signal has passed out of the second stage of the 12AX7 it goes to the output warming control. This is basically a passive high cut control similar to a passive tone control but there is a sweet spot setting at lower end of the range at around 9 o'clock where there is the effect of a bottom boost which adds depth to the sound and the top end of the distortion is smoothed out for a sweeter, more vintage sounding overdrive effect because this output warming control brings a grounding 22 nf capacitor to the signal coming from the plate of the 12AX7 valve and it affects the way that the treble distorts when the second stage of the 12AX7 valve is overdriven. I normally play my 1963 C2 in my room at a comfortable volume level and at these mild volume levels , setting the output warming control to the 9 o'clock position produces a very nice sounding chunky bottom end combined with a sweeter overdrive tone that sounds very pleasant. If you do set the output warming control to the 9 o'clock position then setting the post overdrive EQ bass control down to around the 9 o'clock position will produce a more or less flat bass response. All of this is a matter of personal taste, the actual Leslie or external amplifier that the organ is being played through and the room acoustics.
This variable capacitors rotary switch adds a lot of flexibility to the output warming effect of the overdrive sound.
If you have a modern treble driver such as the Hammond Suzuki 100 watt ferro fluid driver that is too bright sounding in your Leslie, you can use the output warming control to help compensate for this.
After the output warming control there is the master volume control and after this there is the passive post overdrive EQ bass and treble controls. Set the amount of overdrive that you want with the setting of the expression pedal and the gain/overdrive control and then set the master volume to what ever listening volume level that you like.
An optional wiring modification is to add a sweetening switch that brings either a 100 nf or a 150 nf or a 220 nf capacitor across the G-G output terminals of the AO-28 preamp. Even though the 12AX7 EQ/overdrive circuit is not connected to the G-G output terminals of the AO-28, the audio signal in the organ passes through the G-G outputs before going to the Leslie or the tone cabinet and consequently the sweetening capacitor wired across the G-G outputs will result in a sweeter, more rounded out treble response which will make the treble harmonics of the overdrive effect sound sweeter because any fizzy sounding higher harmonics of the overdrive effect are rolled off. The sweetening switch and the actual capacitor nf value chosen is also a matter of personal taste and also a matter of what Leslie or other amplifier and speakers are used. The stock Jensen V21 in my 1971 Leslie 122 is brighter sounding than the V21's in other Leslies 122 cabinets that I have played and I like the sweetened sound when the 150 nf or the 220 nf capacitor is switched across the G-G output terminals. On other Leslies with mellower sounding V21's ,the overdrive sounds nice without needing the sweet switch.
November 2006 update. If you want to hear recorded examples of what this EQ / overdrive unit sounds like through a 1959 C3 organ played through a Leslie 925 cabinet , you can hear it on the songs that have been posted on the My Space website of the Australian Progressive Rock/ Psychedelic Rock band Cosmic Nomads who are led by the Hammond organist Ray Vanderby. The Cosmic Nomads My Space link is : www.myspace.com/cosmicnomads Click on this link and listen to the songs from the album ''Vultress' that have been posted there.
I am Ray Vanderby's Hammond tech and roadie and I have built this 12AX7 based EQ / overdrive into Ray's 1959 Hammond C3 organ. The tonewheel generator of Ray's 1959 C3 has been recapped with new capacitors in order to more or less restore the original stock sound and Ray plays his C3 through two solid state biamped Leslie 925 cabinets which have a JBL bass speaker and a JBL 2482 treble driver. In October 2006 'Cosmic Nomads' recorded their new album Vultress at Atlantis Sound Studio in Melbourne , Australia and this album prominently features Ray's 1959 C3 through the Leslie 925 and the 12AX7 based EQ / overdrive unit is used on all the songs . The 925 was played reasonably loud but still clean sounding in the studio and all the ovedrive heard on the songs is from the EQ / overdrive unit. Because of the work that I have done with Ray's C3 and Leslies , Ray calls me Professor Kon and he refers to the EQ / overdrive unit as the ?ProKon EQ / overdrive unit.
Here below is my hand drawn schematic of the 12AX7 based EQ / Overdrive circuit.
Also see ?KonsZenerDiodeBasedOverdriveCircuit.
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