Kons 12 AX7 based EQ&Overdrive Circuit

Kon's 12AX7 based EQ & Overdrive circuit for Hammond Organs

This 12AX7 based EQ / overdrive circuit is designed for the Hammond tone wheel console organs that have valve preamplifiers such as the B2, C2 , RT2 , B3 , C3 , RT3 , A100 series , D150 series etc. It might also work in the M , M2 , M3, M100 series , L100 series , Porta B etc organs but I have not tried this out. I think that it might also be adaptable for the earlier era console organs such as the Model a , B , BC , BV , CV , D , E etc but I have not tried this out either.

This circuit connects between the wiper of the preamplifier Tone Control potentiometer and the input grid of the output valve which is a 6SN7 on the AO10 preamp and a 12BH7 in the AO28 preamp, and it uses a 12AX7 valve to create the overdrive effect. The quality of the overdrive tone is very good and it does sound similar to an overdriven Leslie 122 or a Leslie147 etc valve amplifier depending on the settings of the EQ controls. Switching off the passive pre overdrive EQ

This circuit is reasonably simple in operation in that it uses one 12AX7 valve and a passive ’pre overdrive’ bass and treble tone stack EQ before the gain/overdrive stage and also a passive ‘post overdrive’ bass and treble tone stack EQ after the overdrive stage. Both EQ's can be independently 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 Bob Schleicher 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 Bob Schleicher'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. Both stages of the 12AX7 valve are wired as simple cascaded gain stages.

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 a muddy distortion because it is located after the gain/overdrive stage. With this you can simulate the deeper bass response of a Leslie connected together with a PR-40 Hammond tone cabinet or the fatter bass sound heard on some recordings where the miked up signal had a bass boost added via the recording console EQ controls or the final mix and mastering process.

Although the passive bass and treble tone stacks are similar to those shown on Bob Schleicher’s website , there is an additional modification to the treble capacitor of the pre overdrive and the post overdrive EQ treble controls. You can use a DPDT switch to select either the original 470 picofarad ( pf ) capacitor or a 1 nano farad ( 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 at around the 12 o'clock position and for the treble control the "flat" position is at around the 2 o'clock position. In actual use this might vary depending on the taper characteristic of the actual potentiometers that are used as the bass and treble controls. There might be variations in the taper between 1 mega ohm potentiometers from different manufacturers.

With the 470 pf capacitor, turning the treble control to the full position the higher treble frequencies are emphasized more resulting in more top end brightness and key click. When the 1 nf capacitor is chosen there is a slight 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 treble boost can be useful if you have a dull sounding organ or a dull sounding Leslie or a Hammond tone cabinet or an external amplifier or speakers.

With a stock organ, when 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 switched on , setting the gain/overdrive control to the fully clockwise position will produce a heavily overdriven 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 heavily 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 if the Master Volume control is set to a loud setting but if you do want to bypass the post overdrive EQ then I recommend that you at first set the Master Volume control to a very low setting and then set it to the desired setting .

If you switch off the pre overdrive EQ bass and treble tone stack you might then get a sagging or 'coughing' effect when the organ preamplifier Tone Control is set to the full brightness position and the expression pedal is set to full volume thus causing the first stage of the 12AX7 to be overdriven. You can minimise this ‘’coughing effect by wiring a 10 K ohms resistor between to two ‘bypass’ poles of the pre overdrive EQ ‘on / bypass’ switch which would normally be shorted out with a direct wire link. With this revised set up when the passive pre overdrive EQ tone stack is switched out of circuit, a 10 K resistor will be then activated in series with the input grid pin 3 of the 12AX7. This 10 K resistor will not affect the overall gain levels but there might be a subtle slight mellowing of the very high frequencies. When the passive pre overdrive EQ is switched on in circuit , the 10 K resistor will no longer be in circuit.

When the pre overdrive EQ is switched off , if you feel that he input stage of the 12AX7 is overdriven too much , you can then lower the gain by turning the 25 K “Cathode -A Bypass Internal Gain potentiometer to a slightly less than full gain setting until the ‘’coughing ‘’ effect is minimised or the excessive distortion is tamed down.

There is an 820 ohms resistor wired between the “Cathode –A Bias Internal Gain” control and the actual Cathode- A pin 3 of the 12AX7 and there is an 820 ohms resistor wired between the Cathode – B Pin 8 of the 12AX7 and the ground. The 820 ohms value resistor is commonly used as the cathode resistor in many valve circuits. Many valve circuits also have a 330 uf or a 470 uf capacitor wired in parallel with the 820 ohms resistor as a cathode bypass capacitor and this results in the maximum gain levels in the 12AX7 valve and originally this overdrive circuit also had the 470 uf cathode bypass capacitors wired in parallel with the 820 ohms cathode resistors, however after doing some experimenting I noticed that the distortion sounded smoother and less harsh when the 470 uf cathode bypass capacitors were disconnected. Disconnecting the 470 uf capacitors resulted in slightly less gain so that there was not as much extreme distortion available but the slight loss of gain still allowed for a full heavy level of distortion but the valve “breathes’” better now that I have disconnected the 470 uf cathode bypass capacitors. This is a matter of personal taste and if you do like the absolute maximum gain levels and a harsher sounding distortion then you can wire up 470 uf capacitors in parallel with the 820 ohms cathode resistors.

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 labelled 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. Using the “Variac simulator” to lowering the HT voltage going to the plate B will result in a more distorted sound but it will also result in lowered output levels when you set it to produce a very squashed sound so therefore if you do use the Variac Simulator control you might then need to switch off the Post Overdrive EQ in order to have a reasonably loud output level.

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 1962 C3 organ 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.

By the way, although I have specified a 22 nf capacitor on the output warming control , you can experiment with other capacitor values to get the type of sound that you like. I have added a 1 pole , 12 position rotary switch with the following capacitor values: 1 nf, 2.2 nf. 3.3 nf, 4.7 nf, 5.6 nf, 8.2 nf, 10 nf, 15 nf, 22 nf, 33 nf, 47 nf, and 82 nf. 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 might be able to use the output warming control to help compensate for this because the output warming control will tame down the excessive upper midrange response..

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 level 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 organ preamplifier. Even though the 12AX7 EQ/overdrive circuit is not connected to the G-G output terminals of the organ preamplifier, 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 “vintage” sounding 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.

I am the Hammond tech and roadie of the Dutch born Australian Progressive Rock organist Ray Vanderby and I have built the 12AX7 based EQ / overdrive into Ray's 1959 Hammond C3 organ. Because of the work that I have done with Ray's C3 and Leslies , Ray calls me Professor Kon and he refers to the12AX7 based EQ / overdrive unit as the ?ProKon EQ / overdrive unit.

Ray Vanderby is the band leader of the Australian Progressive Rock band “ Cosmic Nomads” , and in May 2007 Ray Vanderby added me into his band as the rhythm guitarist of the Cosmic Nomads. The tone wheel 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 Progressive Rock 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 Jon Lord / Ken Hensley – like heavy overdrive sound heard on the songs is from the 12AX7 based EQ / overdrive unit.

In late August 2007 the Cosmic Nomads began to record new songs for the next album and this again prominently features Ray’s 1959 C3 with the 12AX7 based EQ / Overdrive unit providing all the overdriven sounds.

The Cosmic Nomads My Space link is : www.myspace.com/cosmicnomads

Ray Vanderby sometimes posts audio samples of the Cosmic Nomads songs on his Cosmic Nomads my space page and he also sometimes posts live film footage or promotional film clips of the Cosmic Nomads on ?YouTube. Go to ?YouTube and type the words “Cosmic Nomads” in the search bar and there might be some Cosmic Nomads clips available on ?YouTube. Any Cosmic Nomads clips dated as of 2006 and after will feature Ray’s C3 organ with the 12AX7 EQ / Overdrive unit creating the overdrive sounds and the C3 is being played through his Leslie 925.

Here below is the schematic of the 12AX7 based EQ / Overdrive circuit.