Kons Old 12 AX7 based EQ&Overdrive Circuit

This article was submitted by KonZissis.

Kon's *Old* 12AX7 Based EQ & Overdrive Circuit for Hammond Organs

I have built a relatively simple but very good sounding 12AX7 based EQ / overdrive circuit into my 1963 C3. This bypassable circuit is connected after the G-G outputs and I have used the BobSchleicher bass and treble controls tone stack shown at http://www.tonewheel.com/Schematics/tonecontrol.jpg, as the pre EQ and the post EQ bass and treble controls.

Some people build valve overdrive circuits inside the AO-28 between the tone control wiper and the input grid of the 12BH7 and I originally did this as well, but after experimenting I discovered that the overdrive sounds better and more like that of an overdriven Leslie 122 valve amplifier with a chunkier bottom end growl if the 12AX7 circuit is placed outside the AO28 and straight after the AO28 GG outputs. My version of the bass and treble EQ circuit differs from that shown on Bob Schleicher's circuit in that on the Bob Scleicher circuit the first stage of the 12AU7 valve is wired as a unity gain cathode follower stage and then there are the actual bass and treble controls and then there is the second stage of the 12AU7 valve wired as a mild gain stage but on my circuit both stages of the 12AX7 valve are wired to have the full gain capability so that l can get maximum gain and overdrive levels with one 12AX7 valve, and the bass and treble controls go before the Grid 1 of the 12AX7 gain stage 1 and before the Grid 2 of the 12AX7 gain stage 2 instead of at the cathode of the stage 1 of the 12AX7. The direct signal from one of the AO28 _G_ outputs goes to the 12AX7 circuit _On / Bypass_ switch. The _G_ signal passes through the 10 K ohms potentiometer which is wired as the 12AX7 gain stage 1 input gain control and after this there is the switcheable Bob Schleicher bass and treble _pre EQ_ controls before the first stage of the 12AX7. The first gain stage of the 12AX7 is wired as a full gain stage. Because the bass and treble controls are passive, the gain of the 12AX7 is lowered when the bass and treble controls are switched in circuit and this minimises the amount of overdrive available on the 12AX7 gain stage 1. In order to be able to get a fuller overdrive capability in the 12AX7 gain stage 1, I have wired a 2.2 Mega ohms potentiometer between the HT high DC voltage supply and the 100 K ohms plate resistor of the 12AX7 Plate 1. The 2.2 M ohms potentiometer acts like a _variac_ that lowers the high DC voltage of the 12AX7 Plate 1. When the 2.2 M ohms potentiometer is turned fully clockwise, the full DC voltage goes to the 12AX7 Plate 1 so that the organ sound is stronger and only slightly overdriven if the passive bass and treble pre EQ is switched in circuit. By turning the 2.2 M ohm potentiometer anti clockwise, the high DC voltage going to the gain stage 1 of the 12AX7 is lowered and the Plate 1 of the 12AX7 is starved so therefore the organ sound distorts more easily so that even with the _pre EQ_ bass and treble controls switched in circuit and set low, the 12AX7 gain stage 1 can be still be fully overdriven.

After experimentation, a 220 ohms cathode resistor on the 12AX7 gain stage 1 cathode gave the best sound in both the clean and the overdriven settings and the cathode bypass capacitor is 470 mf for a full bass frequency responce.

The Plate 1 of the 12AX7 is connected to the HT high DC voltage via a 100 K ohms 1 watt resistor. The output of the 12AX7 gain stage Plate 1 is safely decoupled with a 10 mf 450 volt capacitor and this is followed by the 25 K ohms Gain stage 2 gain control. I used a large capacitance value of 10 mf for the decoupling capacitor so that there is no bass roll off caused by the lower impedance of the 25 K ohms Gain stage 2 gain level control, the 25 K ohms _Treble contour_ control and the _post EQ_ passive bass and treble controls. The _Treble contour control_ is wired after the Gain stage 2 gain control and it is a 25 K ohms potentiometer with a 2K2 resistor in series with a .1 mfd capacitor that can be helpful in lowering excessive squawkiness on the upper midrange and treble of the organ when the treble contour control level is turned anticlockwise a little below the full brightness position. After this there is the second switchable Bob Schleicher bass and treble _post EQ_ controls circuit and after this there is the second gain stage of the 12AX7 which is also wired as a full gain stage. You can set the Gain stage 1 for full overdrive and then set the Gain stage 2 control to produce a clean sound in the gain stage 2, with the Post EQ shaping the _post overdrive_ output sound, or you can overdrive the gain stage 2 for a heavier saturated overdrive. You can also have the gain control 1 for the 12AX7 gain stage 1 set lower so that the gain stage 1 is running cleanly and then set the Gain 2 control set high so that the 12AX7 gain stage 2 is overdriven. Because the 12AX7 gain stage 2 is output coupled with an output coupling transformer , the overdrive characteristic is affected by the output coupling transformer and this overdrive effect would be somewhat like that of an overdriven valve power output stage where the output transformer contributes to the overdrive sound

The output plate of the second gain stage of the 12AX7 is wired up with an output coupling transformer similar to how the 12BH7 output valve is wired up in the AO28 preamplifier. I decided to output couple the Plate 2 of the 12AX7 gain stage 2 with the output coupling transfoirmerin order to get balanced G-G outputs that are directly compatible with the balanced inputs of the Leslie models such as the Leslie31H, 21H , 22 , 122 125 or 222 etc as well as the various Hammond Tone cabinets such as the PR20 or the PR40. On the Plate 2 of the 12AX7 gain stage 2 there is a .1 mfd 630 volt decoupling capacitor that safely isolates the high voltage DC, and after this .1 mfd 630 volt decoupling capacitor there is the _Output sweetening capacitors_ circuit that is 1 pole / twelve position rotary switch with twelve different mfd value capacitors on it that are grounded from one end.This sweetening circuit sharply rolls off the fizzy sounding treble harmonics of the overdrive sound, so that the distortion sounds sweeter and more like that heard on the overdriven Hammond recordings from the 1960' / 70's era. I got the idea for the sweetening capacitors connected between the ground and the plate of the 12AX7 gain stage 2 because on the Leslie 122 valve amplifier circuit there is a high frequency suppression caused by the two .0047 mfd capacitors that are wired between the plates of the two ^[?6550] power valves and the ground. There is a .0047 mfd capacitor as one of the twelve selectable capacitors wired on to the 1 pole / 12 position rotary switch ( switch position 4 ). The twelve different selectable capacitors produce various types of sharp treble roll off for a sweeter characteristic to the overdrive treble harmonics without the raspy fizziness of the raw unfiltered sound of the overdriven 12AX7 valve. The sweetening capacitors can be fully switched in or out of circuit. Because I like the sweetening capacitors effect, I always have the sweetening capacitors switched in circuit. The sweetening capacitors are especially needed if I listen to the organ via headphones late at night. The sweetening capacitors are also essential if the organ is connected to a brighter sounding external amplifier or if the organ is directly connected to a PA or if it is directly fed into a recording console.

Another wire is also connected to the decoupled end of the .1 mfd 630 volt decoupling capacitor where the sweetening capacitors _on / bypass_ switch is connected to, and this wire is used as a variable localized feedback to the 12AX7 gain stage 2 grid via a 500 K ohms potentiometer and a .0047 mfd capacitor in series. This circuitry is part of the switchable _Vintage Leslie amplifier treble roll off_ simulation. The Leslie 122 valve amplifier has a 390 K ohms resistor wired in series with a .0047 mfd capacitor as a localised feedback high frequency treble roll off between the grid and the plate of the ^[?6550] output valves, and I have tried to simulate this Leslie amplifier treble roll off with the 500 K ohms potentiometer in series with the .0047 mfd capacitor. The earlier Leslie amplifiers such as the model 31H and 21H also had some treble roll off so l have tried to approximate this with the 500 K ohms potentiometer variable treble roll off. I have never heard a Leslie31H or any pre 122 / 147 era Leslies, so therefore I do not know how accurate the treble roll off effect with the 500 K ohms potentiometer is. Setting the 500 K ohms potentiometer at the upper end of its range to get to the 390 K ohms region will approximate the Leslie 122 localised feedback treble roll off wired between the grid and the plate of the ^[?6550] valve. As well as this, on the Leslie 122 valve amplifier there is a treble roll off at the balanced inputs due to the series input 10 K ohms resistors and the .001 mfd capacitor wired across the grid end of these two 10 K ohms input series resistors. When the _Vintage Leslie amplifier treble roll off_ switch is turned on, the input signal from the AO28 GG outputs passes through a 10 K ohms resistor wired between the input of the 12AX7 EQ / overdrive circuit and the input Grid 1 of the 12AX7 gain stage 1, and at the grid end of the 10 K ohms resistor there is a .001 mfd capacitor that goes to the ground in order to produce a similar treble roll off characteristic of the balanced input stage of the Leslie 122 valve amplifier.

After experimentation, a 820 ohms cathode resistor on the 12AX7 gain stage 2 cathode gave the best sound in both the clean and the overdriven settings and the cathode bypass capacitor is 470 mf for a full bass frequency response.

The secondary winding of the output coupling transformer has dual balanced G-G outputs and a grounded centre tap, so that it can be directly connected to the balanced line input of the Leslie kit or the Leslie 122 amplifier or the Hammond tone cabinets as well as the single channel Leslie 147 amplifier or a line out for an external amplifier. After the output coupling transformer dual G-G outputs there is a 10 K ohms twin ganged pot wired as a master volume control for the 12AX7 EQ / overdive circuit. In order to produce a full frequency responce and signal output levels as loud as that of the AO28 output coupling transformer, the output coupling transformer for the 12AX7 EQ / overdrive circuit should have similar specs to that of the AO28 preamplifier output coupling transformer with the balanced GG outputs and centre tap on the secondary winding. I think that the AC impedance of the transformer should be as follows~:

Primary winding	5 K ohms to 10 K ohms
secondary winding	around 500 ohms.

I have measured the DC resistance of an AO28 output coupling transformer of a 1960 RT3 organ and the primary winding DC resistance measured at around 1 K ohms and the secondary winding DC resistance measured at aound 24 ohms ( 12 ohms between each G output and the grounded centre tap.) I do not have a spare proper AO28 spec output coupling transformer so therefore I have had to improvise with one of the transformers that I had in my pile of transformers, and the output levels of the transformer in my own 12AX7 EQ / overdrive circuit in my 1963 C3 are not as loud as that of the AO28 type transformer, but I do have a full frequency response and a good sound. Depending on what actual output coupling transformer is used, you may need to experiment with the mfd values of the output sweetening capacitors for the best effect and you might possibly need to experiment with the ohms value of the 12AX7 gain stage 2 cathode resistor in order to get the best sounding solid clean sound and the smoothest overdrive without the intermodulation distortion and the anemic _farty_ sounding distortion that occurs when the cathode is not biased properly.

With the particular wiring of this 12AX7 circuit featuring the output coupling transformer at the 12AX7 plate output of the gain stage 2, this is somewhat like a miniature power amplifier and the overdrive sound mimics that of an overdriven valve power amplifier. With the fully controlable gain settings of the 12AX7 circuit, I can go from pure clean sounds to all the way up to the early 70's Jon Lord Hammond C3 through an overdriven Marshall amplifier level of overdrive. The EQ controls are useful and with the output sweetening capacitors the overdrive can be made to sound pleasantly warm and natural rather than harsh and brittle.

The HT high DC voltage comes from a tap in the AO28 power supply near the 12BH7 output valve and the 6.3 volt heater filament voltage comes from the 6.3 volt filament voltage output of the AO28 power transformer. If you want to, you can build an independent dedicated high voltage power supply for the 12AX7 valve circuit.

Here below is a simplified block diagram of my 12AX7 Pre EQ / overdrive / Post EQ set up with my 1963 C3~:

1. Recapped and fully recalibrated TG . Organ through AO28 preamp. Normal output through the GG outputs
2. Bypassable 12AX7 Pre EQ / Overdrive / Post EQ circuit. Input section featuring stage 1 gain control and bass and treble _Pre EQ_ controls which are set to the desired position or switched out of circuit for more overdrive capability on the gain stage 1.
3. 12AX7 valve gain stage 1 set to either fully clean or overdriven level with Gain Control 1. Variable high DC voltage for _Variac_ type plate voltage reduction control and threshold of distortion for the 12AX7 gain stage 1.
4. Secondary stage bass and treble _Post EQ_ controls with the bass and treble controls set to the desired position or switched out of circuit.
5. 12 AX7 valve gain stage 2 set to clean level or higher gain for more _post EQ_ overdrive. Switchable vintage Leslie amp treble roll off simulation.
6. Switchable variable output sweetening capacitors on the plate of the 12AX7 stage 2 to remove any raspiness or brittleness from the overdrive sound.
7. Master volume. Output to direct Leslie kit, Leslie 122, or Hammond Tone cabinet or to any other amplifier.

PLEASE NOTE: If this 12AX7 EQ / overdrive circuit is intended to always be used with a normal external amplifier instead of a balanced input amplifier such as the Leslies or the Hammond tone cabinets , the output coupling transformer can be omitted and the output signal can be taken directly and safely from the 12AX7 Plate 2 via 10 mf 450 volt or a 47 mf 450 volt decoupling capacitor and the direct output signal can then go to a single ganged 10 K ohms master volume control. If the output coupling transformer is omitted, then you need to add a 100 k ohms 1 watt resistor between the HT high voltage DC supply and the 12AX7 Plate 2, similar to other more conventional 12AX7 based preamp circuits. If the output coupling transformer is omitted, then the output directly from the Plate 2 will be significantly louder so you would need to initially set the Master volume very low and then set it to the desired level in order to prevent accidentally severely overloading the external amplifier.