(Also see Kon's latest project: Kons12AX7basedEQ&OverdriveCircuit.)
If you want to add some distortion to your Hammond's sound, Kon Zissis suggests the following circuit.
You can add a very nice warm sounding distortion to the Hammond tone wheel organs with AO-28 preamplifiers with the G-G outputs such as the models B-3, C-3, ?RT-3, A-100 series and the ?D-150 series, etc, without having to build a valve overdrive circuit or preamp into the organ, by wiring up a passive circuit using 3.3 volt IN4728 zener diodes wired back to back and a non-polarised capacitor wired in series with the zener diodes and connecting this circuit across the G-G terminals of the AO-28 preamp.
This circuit is appropriate for Hammond console organs which have valve preamplifiers with G-G output terminals, such as the AO-28 preamplifier of the B-3, C-3, ?RT-3, A-100 series and ?D-150 series organs. This circuit should also work with the older pre 1955 era organs such as the Models A, B, AB, BA, BC, BV, BCV, B-2, C, CV, C-2, C-2G, D, DV, G, Concert Model E, RT, and RT-2, etc, because the preamps on these organs also had the output coupling transformer with the G-G outputs.
PLEASE NOTE : This circuit should NOT be connected to the power amplifier output transformer speaker terminals of the power amplifiers in the organs such as the A-100 series and the D-150 series etc, it is only for the AO-28 preamp G-G outputs of these organs, like all the other console organs that have transformer coupled G-G output AO-28 preamps or the preamps with the G-G outputs of the earlier pre-1954 era console Hammonds. I have not yet had a chance to try this circuit out with the later era transistorised tone wheel console organs such as the R-100 series nor have I tried out this circuit out with the L series or the M series valve spinet organs so therefore I am not sure about how this circuit would be applied to these organs, but I would expect that placing the zener clipping diodes and the 100 uf series capacitor or perhaps the just the diodes without the series resistor somewhere between a signal stage and the ground on these organs might possibly work.
I was able to create an overdrive effect on a friend's 1970 T-300 spinet with the transistor amplifier in it by switching either one 3.3 volt zener diode or two 3.3 volt zener diodes back to back in parallel between the pin 8 of the power amplifier board and the ground. I wired a two pole / two position "centre off" switch so that I could select either one zener diode between the pin 8 and the ground for a milder overdrive effect or two zener diodes wired back to back in parallel between the pin 8 and the ground for a heavier overdrive effect. When the switch is set in the middle "centre off" setting, the diodes are switched out of circuit in order to restore the stock clean sound.
This passive circuit gives you overdrive "for free" in that you don't need to tap into the high voltage DC and you do not need build or add a valve overdrive circuit or preamp and it does not generate any hum nor do you risk damaging any valves, amplifiers or speakers.
This reasonably simple circuit produces a very good sounding overdrive effect and it has a six position Overdrive depth selector switch that allows for six levels of overdrive when the organ expression pedal is set at loud settings. The depth of the overdrive is determined by the number of zener diodes connected back to back in series across the circuit. The six levels of distortion available from the Overdrive depth selector switch are as follows:
I have installed this reasonably simple circuit onto the G-G outputs of my 1962 C3 and I have also built a 12AX7 based EQ / overdrive unit between the wiper of the Tone Control and the input grid of the 12BH7 output valve in the AO-28, and when comparing the overdrive sound character of the zener diodes based circuit to the overdrive sound of the 12AX7 EQ / overdrive circuit, and even the overdrive sound of my overdriven Leslie 122 amplifier, the overdrive sound from the passive zener diode based circuit sounds close and realistic. It really does have a valvelike overdrive characteristic especially when the sweetening capacitors are switched in, instead of having the harder unpleasant sounding distortion of an overdriven transistor amplifier power output stage or the "farty " sounding or the chain saw like distortion produced by a poorly designed valve overdrive circuit.
I have also added this passive zener diodes based overdrive circuit into a church owned stock wax capped 1960 RT-3 and the organist often likes using this overdrive sound during the church worship songs.
It is important that zener diodes be used in this circuit. I tried out germanium diodes, silicon diodes and combinations of germanium / silicon / zener diodes but the zener diodes on their own wired back to back in series produced the most realistic Leslie valve amp-like overdrive sound.
This circuit uses 3.3 volt 1N4728 zener diodes for the distortion. The diodes and either a 100 uf or a 10 uf ( two 4.7 uf capacitors switched in parallel ) or a single 4.7 uf capacitor non polarized capacitor in series are wired across the AO-28 preamplifier G-G outputs when the overdrive effect is switched on. The 100 uf and the two 4.7 uf series capacitors are normal 25 volt non-polarized capacitors .The 100 uf or the two 4.7 uf capacitors are wired in series with the zener diodes in order to suppress an unpleasant gnarly sounding intermodulation distortion characteristic that occurs when the zener diodes are directly connected across the G-G outputs. There is a two pole /two position "centre off" switch so that you can choose either a 100 uf capacitor ( switch position 3 ) or two 4.7 uf capacitors in parallel to make up approximately 10 uf ( switch position 2 ) or just one 4.7 uf capacitor ( "centre off " switch position 1 ) . This switch is labelled as " Bass compression level " on the schematic.
The 100 uf capacitor allows the full bass to be pass through to the diodes and therefore be distorted and compressed similar to what happens with the overdriven Leslie 122 or 147 valve amplifier.
The two 4.7 uf capacitors switched in parallel to make up 10 uf block the lower bass from going to the diodes so therefore the lower bass is not distorted and compressed as much with the result that you can hear a more ballsy sound with louder but not as dirty sounding bass along with the overall distorted organ sound.
The single 4.7 uf capacitor blocks even more of the bass from going to the diodes and being overdriven thus resulting in an even louder and less dirty sounding bass along with the rest of the distorted organ sound. This effect is most noticeable in the more heavily distorted Overdrive depth rotary switch position 5 and especially the position 6 settings . All of this is a matter of personal taste. The 10 uf or the 4.7 uf setting can also be useful if the Tone wheel generator has been recalibrated to produce a strong bass output and that when using the overdrive effect this strong bass output is drowning out the other frequencies when you are using the 100 uf capacitor.
There is a switch labelled as " Jensen V21 clipping threshold" and a 1K linear potentiometer labelled as " Jensen V21 dirt level" on the schematic. With these controls you can approximate the dirty sound of a Jensen V21 treble driver that distorts even at lower volume levels because it has been overdriven so many times that the voice coil is deteriorating or that the voice coil has become misaligned. This effect is created by switching in parallel with the main circuit a 0.330 mfd capacitor and a 1.65 milli Henries, 0.04 ohms inductor in series with either one 3.3 volt zener diode or three 3.3 volt zener diodes in series. The 0.330 mfd capacitor and the 1.65 mH o.04 inductor in series act as a passive band pass filter so that the dirty V21 effect is applied to the upper midrange and lower treble frequencies.
The 0.330 mfd capacitor blocks the bass and the lower midrange frequencies from passing through the diodes so that only the upper midrange and the treble frequencies pass through and the 1.65 mH inductor coil blocks the higher treble frequencies from passing through thus simulating the tendency of a dirty sounding Jensen V21 to predominantly distort in the 800 Hz to 2 kHz region ( the TG notes 57 to 73 ). The inductor is a small square shaped transformer coil with a normal metal E-I core similar in size to the little transformers in portable transistor radios or on the cassette tape head amplifier circuit boards. I chose this particular inductor because it worked well with the 0.330 mfd capacitor . You will probably need to experiment with a number of inductors so that you can find out what works best in producing the right simulation of the dirty sounding Jensen V21 predominantly distorting in the TG notes 57 to 73 region.
The " Jensen V21 clipping threshold " switch determines whether the Jensen V21 style distortion begins only at louder settings of the expression pedal ( ( switch position 2 ) or even with the expression pedal set at lower volume levels ( switch position 3 ). The "centre off" switch position 1 completely removes the dirty V21 treble driver simulation from the circuit.
The Overdrive depth rotary switch position 1 is the best setting to hear the dirty Jensen V21 effect on its own without the main overdrive diodes being in circuit because there are no main overdrive diodes connected to the Overdrive depth switch position 1. This setting is good if you want to simulate the effect of a Leslie amplifier running cleanly but that the Jensen V21 is dirty sounding. The dirty Jensen V21 effect is mainly noticeable in the Overdrive depth rotary switch positions 1 to 3.The positions 4 to 6 drown out the dirty Jensen V21 effect because there is so much distortion on the overall organ tone at these two settings.
Apparently Gregg Rollie from the early Santana line up and some other Rock organists liked using and recording with old beaten up Leslies that were overdriven so many times that the Jensen V21 treble drivers had become partially damaged and they would have a dirtier sound. Al Goff has mentioned that some Rock organists wanted him to deliberately overheat new V21 voice coils so that they would be burned in and sound old and dirty.
The circuit also features a sweetening switch which is a six position rotary switch with the following capacitors on it:
Position 1) 0.068 mfd
Position 2) 0.1 mfd
Position 3) 0.150 mfd
Position 4) 0.220 mfd
Position 5) 0.330 mfd
Position 6) 0.470 mfd
These selectable '"sweetening" capacitors are switched across the zener diodes and series capacitor in order to cut out the fizzy sounding high treble harmonics of the distortion in order to create a more pleasant sounding sweeter or a more "vintage" sound. The 0.330 mfd and the 0.470 mfd capacitors darken the overall treble response for a noticeably darker , mellower tone. The capacitors used here are a matter of personal taste and there is a one pole / two position switch that switches the sweetening capacitors completely out of circuit so that the full brightness of the distortion effect is heard.
Because the Jensen V21 in my 1971 Leslie 122 has a brighter sound than the V21's in other Leslies that I have played through, I prefer the way the overdrive sounds with the 0.1 mfd or the 0.150 mfd or the 0.22 mfd sweetening capacitors in circuit instead of the brighter overdrive sound produced when there is no sweetening capacitor in circuit. With Leslies that have a mellower sounding V21 you may not need to use the sweetening effect. This is all a matter of personal taste. The 0.330 mfd capacitor darkens the treble and this can approximate to some extent the dark sound heard on some very old Hammond recordings of the 1940's or 50's such as the Jimmy Smith album "Midnight Blues" .As well as this if your amplifier or speakers are excessively bright sounding , the sweetening capacitors can be helpful in removing the excessive brightness for a mellower sound.
lf you are happy with the sound of a particular sweetening capacitor value and you expect to always have this sweetening capacitor value in circuit , then you can wire that capacitor permanently in the overdrive circuit and then omit the sweetening capacitor selector switch. On the other hand, if you like the way the overdrive sounds without the sweetening capacitors and you don't expect to use the sweetening capacitors, then you can totaly omit the sweetening capacitors and the sweetening capacitor selector switch.
A 10 K ohms dual ganged potentiometer is wired after this circuit as a master volume control. Because the overdrive effect is passive, there is a some loss of volume when you set the Overdrive depth selector switch to the heavier overdrive positions 4 , 5 , and especially the position 6 and then have the organ expression pedal at full volume to produce the maximum distortion. When the expression pedal is at a lower volume setting, there is no volume loss regardless of the overdrive depth selector switch position. It is only when the sound distorts that the volume level compresses, similar to how it would when you overdrive a valve amplifier. If you are playing the organ at a comfortable listening volume such as in a house or an apartment , then the volume loss is not really a problem because you can set the volume control of the Leslie or whatever amplifier that you are using to the loudest level that you are happy with and then you can play around with the overdrive master volume control to get the output volume that you want. On the other hand , the volume loss might be a problem if you are playing un-miked at live gigs and you want the maximum volume levels coming from the organ and the Leslie.
To add this overdrive circuit to your organ, you need to unsolder the two wires from the G-G output terminals of the AO-28 or similar preamplifier. These wires are at the left side of the AO-28 near the tone control. The first (brown) wire is the earth ground. The next (black) wire is the first G wire. The second (red) wire is the second G wire. You unsolder these two wires from the G-G terminals. You then wire up a twin core shielded cable to the G-G outputs and then solder the shield of this cable to the earth ground (brown wire) terminal. Solder the original black and red G-G wires on to another shielded twin core cable. Seal the new soldered connection with heat shrink tubing or with reliable good quality electrical sticky tape. Solder the shield of this cable to the earth ground (brown wire) terminal. Both of these two cables should be at least two or three metres long so that the cable is long enough to reach the front side of the organ. It is much better if the cable is too long instead of being too short because you can cut out the excess length later on. You can drill a hole on the wooden shelf of your organ so that you can bring the new cables on to the underside of the wooden shelf so that you can place the new overdrive circuit control box on either the lower left or the lower right side of the organ underside, so that you can easily reach the control box when you are seated at your organ. Do not place the control box in the middle of the organ underside because it may get in the way of your legs when you are seated at the organ.
The metal control box will contain the following control switches and dials :
The bypass / overdrive on switch.
The rotary six position diodes selector (overdrive depth) switch.
The Jensen V21 dirt level control and the Jensen V21 clipping threshold switch.
The rotary six position sweetening capacitors selector switch and sweetening capacitors 'on / off' switch.
The master volume control. The master volume control is wired in only when the overdrive circuit is switched on.
I have used a four pole / two position switch for the bypass/overdrive on switching so that the overdrive circuit is completely removed from the organ and the original stock wiring can return to the G-G terminals when you bypass the overdrive circuit.
The master volume control is a dual gang 10 K ohms linear potentiometer ( or log depending on which produces a smoother , more even level of attenuation along it's travel path. I have used a dual gang potentiometer because the G1 and G2 outputs feed into the dual balanced line input of the Leslie 122 amplifier, so this means that both balanced lines need an independent volume control. The 10 K ohms dual ganged potentiometer master volume control is basically wired up as a stereo volume control.
The input Pins 1 of the 10 K ohms dual ganged potentiometer go the signal coming from the GG outputs AFTER the point where the zener diodes based overdrive circuit has been added. One wire from the G connection goes to one of the input Pin 1 , and the wire from the other G connection goes to the other input Pin 1 of the 10 K dual ganged potentiometer.
The middle Pins 2 of the 10 K ohms dual ganged potentiometer go to the output signal wires and when the overdrive circuit is switched on , these output signal wires send the output signal to whatever Leslie connection kit circuit or Hammond tone cabinet is used , or to the direct connection to the power amplifier in the A100 series and the D150 series etc organs, or to a line out connection for an external amplifier or effects feed.
Pins 3 of the 10 K ohms dual ganged potentiometer go to the earth ground. Use the earth grounded shield wires from the new cables to ground out the pins 3 of the 10K ohms dual ganged potentiometer and the metal box. If there is some hum audible when the master volume is set to a lower than maximum volume position , then you can minimise or eliminate this hum by wiring up an optional 1 K ohms resistor between the pin 1 and the pin 2 of the 10 K ohms master volume control. Because this is a dual ganged 10 K ohms potentiometer , you need two 1 K ohms resistors , one each for both sets of the pins 1 and the pins 2 of the 10 K ohms dual ganged master volume control .
This is a relatively simple circuit to build. You can hand build a simplified version by omitting the switches and the master volume control and just wire up the diodes, the series capacitor and the parallel sweetening capacitor and connect them to the GG outputs of a AO28 and other similar preamps with two alligator clips for an on the spot overdrive test on any Hammond Tone wheel organ with AO28 style G-G outputs from the preamp. I did this when I went to an organ shop and took a Tone wheel Generator output levels reading of a 1965 C3 and a 1966 C3 and a mid 60's A105 . When I connected the simplified circuit to the G-G terminals of these three organs, the overdrive sounded very nice and natural.
PLEASE NOTE *: Steffan Diedrichsen recommended to me that two 200 ohms resistors should be wired in series between the GG outputs and the overdrive circuit. Steffan wrote that these resistors act as current limiting protection in case if there is ever a short circuit in the overdrive circuitry that would cause the G-G outputs to short out. I tried this on a friend's C3 and this did not seem to have any adverse effect on the overdrive sound or on the output levels so therefore it should be OK to add these two optional 200 ohms resistors if you want to. lf you do decide to add these 200 ohms limiting resistors , try shorting them out to see whether they affect the overdrive character or the volume level of the sound.
Because the actual signal AC output voltage from the AO28 GG terminals is only around up to 6 volts , l dont think that that voltage level is strong enough to cause the overdrive circuit components to malfunction and short out , but just the same it is worth wile mentioning Steffan's recommendation for the 200 ohms 1 watt resistors here.
PARTS LIST Four pole / two position switch (bypass / overdrive on switch). x 1
Rotary two poles / six positions switch (diodes selector, overdrive depth and sweetening capacitors selector). x 2
Two pole / two positions centre off switch (Bass compression level and Jensen V21 clipping threshold selectors ) x 2
Single pole / two position switch ( Sweetening capacitors on or off ) x 1
1K ohms linear potentiometer ( Jensen V21 dirt level control ) x1
10 K ohms linear ( or log depending on which has a more even volume tapering response along it's travel path ) dual ganged potentiometer (master volume) x 1
1.65 milli Henry, 0.04 ohms DC resistance inductor or little transformer to block the higher treble from passing through to the Jensen V21 dirt simulator. The actual inductor mH value may need to be experimented with in order to find the mH value that works the best. x 1
Knobs for the six position overdrive selector switch and the sweetening capacitor selector switch and the Jensen V21 dirt level and the Master volume control x 4
3.3 volts IN4728 zener diodes (overdrive clipping diodes and Jensen V21 dirt simulation ) x 9
100 uf 25 volt non polarised capacitor ( Capacitor in series with the diodes to prevent an unpleasant gnarly sounding inter modulation characteristic to the overdrive sound and also to allow the full bass distortion ) x 1
4.7 uf 25 volt non polarized capacitors ( Smaller mfd value capacitors in series with the diodes to prevent an unpleasant gnarly sounding inter modulation characteristic to the overdrive sound and also to block the lower bass frequencies from passing to the diodes thus resulting in less bass distortion and compression and a louder but cleaner bass to be heard ) x 2
0.068 mfd MKT or greenback capacitor ( sweetening capacitor 1 ) x1
0.1 mfd MKT or greenback capacitor ( sweetening capacitor 2 ) x1
0.150 mfd MKT or greenback capacitor . ( sweetening capacitor 3 ) x 1
0.220 mfd MKT or greenback capacitor . ( sweetening capacitor 4 ) x 1
0.330 mfd MKT or greenback capacitor . ( sweetening capacitor 5 and Jensen V21 dirt simulator capacitor ) x 2
0.470 mfd MKT or greenback capacitor . ( sweetening capacitor 6 ) x 1
OPTIONAL 1 K ohms 1 watt resistors (Only needed for the Master Volume potentiometer if there is some hum when the master volume is set to a low volume position . If there is hum , the 1 K resistors are wired between the Pins 1 and the Pins 2 of the 10 K ohms dual ganged master volume potentiometer.) x 2
OPTIONAL 200 ohms 1 watt resistors for current limiting protection in the event of a short circuit between the GG outputs x 2
Six metres of twin core shielded cable (two or three metres each for the two new cables)
Metal box big enough to comfortably contain all of the overdrive circuit components x 1
Here below is my hand drawn schematic of the zener diodes based overdrive circuit:
A higher resolution PDF version of the circuit diagram, suitable for printing, is available
here.
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