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KZ Vibrato

MODIFICATION TO MAKE THE CHORUS EFFECT SOUND SIMILAR TO THE CHORUS EFFECT PRODUCED  BY THE VIBRATO SCANNER AND THE VIBRATO LINE BOX IN THE CONSOLE ORGANS AND THE OTHER SPINET HAMMOND ORGANS.

I have discovered a very simple way to modify the vibrato  preamp of my 1963 L102 so that the chorus effect sounds similar to the vibrato scanner chorus effect in the B3/C3 etc organs.

Here you can find the classic 888000000 + 3rd percussion sound of the mod performed by Magnus Enorson (thanks a bunch!)

This is an audio sample of the vibrato mod (thanks Nathan!)

The stock L100 series chorus effect sounds very wimpy and nowhere near as rich and shimmery as the chorus effect in the organs such as the B3, C3 etc which have the vibrato scanner. Most people have assumed that the L100 chorus effect sounds so inferior because the vibrato/ chorus effect is generated by the electronic valve oscillator circuit in the vibrato preamp instead of the mechanical vibrato scanner which is a variable capacitor.

However whist experimenting I realized that the real reason why the L100 chorus effect sounds so inferior is because the straight signal which is combined with the vibrato signal to create the chorus effect is electronically out of phase with the vibrato signal so therefore when the vibrato signal and the straight signal are combined to create the chorus effect, the chorus effect itself sounds inferior and nowhere near as rich and shimmery as the vibrato scanner chorus effect of the other Hammond organs.

When the Vibrato Chorus switch is set to the "Chorus" setting, the overall organ tonality thins out regardless of the settings of the Vibrato Small and the Vibrato Normal switches.

When the Vibrato Small and the Vibrato Normal switches are switched off to completely stop the vibrato/ chorus effect, the vibrato oscillator signal is switched out of circuit from the rest of the Vibrato amplifier circuit but the now non vibratoed organ signal is still coming from the same vibrato circuit so therefore when the Vibrato Chorus switch is set to the "Chorus" setting, the electrically out of phase straight signal coming from the junction of the C101 0.001uF capacitor and the R108 1.8M grounding resistor is still combined with the now non vibratoed signal coming from the vibrato circuit thus causing the organ sound to thin out.

Whilst experimenting I found out that the simple way to solve this problem of the straight signal being out of phase with the vibrato signal is by changing the position in the circuitry from where the straight signal is taken from.

Using the schematics from the L100 service manual or the easier to read schematics in the Porta B organ  service manual as a reference, in the stock L100 vibrato preamp circuit the straight signal is taken from the junction of the C101 0.001uF capacitor and the R108 1.8M grounding resistor which is  at the input stage of the vibrato amplifier. The straight signal taken from this junction goes to the BLU socket with the blue RCA plug on the Vibrato amplifier.

The straight signal on the cable with the blue RCA plug then goes to the Vibrato/Chorus switch. When this switch is pressed down to the "Chorus" setting, the switch contact then closes and this allows the straight signal to pass through to the other cable with the blue RCA plug that goes to the BLU socket on the Preamplifier / Percussion amplifier. The straight signal from this BLU socket then goes to the R227 680k resistor.

The Vibrato signal comes out of the Vibrato  amplifier through the C108 0.047uF capacitor.

The vibrato signal coming from the C108 then goes to the BRN socket on the Vibrato amplifier, and the cable with the brown RCA plug which is plugged into the BRN socket then sends the vibrato signal to the Preamplifier / Percussion amplifier where the other end of this cable with the other brown RCA plug is plugged into the BRN socket on the Preamplifier / Percussion amplifier next to the BLU socket with the blue RCA plug.

The vibrato signal then passes through the R228 470k resistor.

The other ends of the R227 and the R228 resistors join together so therefore this junction acts as the mixer stage where the straight signal combines with the vibrato signal to create the chorus effect when the Vibrato Chorus switch is set to the "Chorus" setting.

Because the straight signal and the chorus signals are electrically out of phase with each other, the lower frequencies become attenuated and the chorus effect  sounds nowhere near as good as the vibrato scanner chorus effect in the other Hammond organs.

However after some experimentation I unsoldered the end of the yellow wire (that sends the straight signal to the BLU output socket with the blue RCA plug) from the junction of the C101 and the R108 and I then soldered the end of this yellow wire to the junction of the C102 0.47uF capacitor, the C121 150pF capacitor and the R110 15k resistor.

The straight signal available here at this junction is electrically in phase with the vibrato signal so therefore this has drastically improved the Chorus effect to sound similar to the Chorus effect produced by the  vibrato scanner circuit in the other Hammond organs once I properly matched the volume level of the straight signal with the vibrato signal.

As well as that, the organ tonality no longer thins out when I set the Vibrato Chorus switch to the "Chorus' setting.

This result of the organ tonality sounding full and deep both in the "Vibrato " setting and in the "Chorus" setting is a huge improvement on the thin sounding organ tonality of a stock L100 series organ when the Vibrato / Chorus switch is set to the "Chorus " setting.

With this new  repositioning of the end of the yellow wire to the junction of the C102, the C121 and the R110 there was a noticeable switching thump noise created when the Vibrato Chorus switch was switched between the Vibrato setting and the Chorus setting so therefore I unsoldered the end of  the yellow wire from this junction and I then wired up a 0.47uF capacitor in series with the end of the yellow wire and I wired another short piece of yellow wire to the other end of the 0.47uF capacitor and I covered this with heat shrink tube and I then soldered the other end of this new yellow wire to the junction of the C102, the C121 and the R110.

The 0.47uF capacitor added in series with the yellow wire has eliminated this switching thump noise.

In order to create the most accurate sounding simulation of the  vibrato scanner style chorus effect, the volume level of the straight signal needs to be properly matched in relation to the vibrato signal so therefore the value of the R227 needs to be altered.

The required resistor value range to create the right sounding chorus effect depth might possibly vary with each L100 series or Porta B organ depending on the actual volume level of the vibrato signal.

On my 1963 L102 which I have modified to produce the boosted main organ signal levels and the boosted percussion signal levels, I have shorted out the R227 680k resistor which passes the straight signal and I then wired up a 500k linear taper potentiometer in place of the R227.

For convenience I wired up this 500K potentiometer between the end of the new yellow wire and the  BLU socket terminal on the Vibrato amplifier and I drilled a hole on a convenient location on top of the chassis of the Vibrato amplifier and I then attached the 500k potentiometer here so that I can easily adjust it without the need to have to unscrew and open up the Vibrato amplifier.

This 500K potentiometer now functions as the "Chorus Depth" control.

Alternatively the R227 680k resistor in the Preamplifier / Percussion amplifier could have been removed and then simply replaced with the 500k trimpot or potentiometer ”Chorus Depth” control over  there instead of in the Vibrato amplifier because this is all part of the same signal path so therefore the results are identical.

After some experimentation I then wired up a 100K resistor in parallel with the R228 470k resistor which carries the vibrato signal in the Preamplifier / Percussion amplifier.

I chose the 100k resistor because after experimenting with switching between the Vibrato effect and the Chorus effect and also with the Vibrato Small and the Vibrato  Normal switches being switched on or off and I then found out that the combination of the 100k resistor wired in parallel with the R228 680k resistor and the appropriate setting of the 500k "Chorus Depth" control potentiometer produced the same overall volume levels and tonality in all the different switch settings except that the high treble response is slightly brighter when the Vibrato Chorus switch is set to the "Chorus" setting regardless if the Vibrato Small and the Vibrato Normal switches  are switched on or off.

I then adjusted the setting of the 500k potentiometer to set the volume level of the straight signal to be properly matched with the vibrato signal in order to get the right sounding chorus depth which is as close as possible to the chorus depth produced by the vibrato scanner circuit in my Hammond C3 organ.

The 500k "Chorus Depth" control potentiometer can be set to produce either the shallower sounding pre 1959 style chorus effect produced by the 22k resistor or deeper sounding post 1959 style Chorus effect produced by the 12k resistor in the vibrato scanner circuitry of the B3, C3 etc organs.

I have wired up the 500K "Chorus Depth " control potentiometer so that when the dial is turned to the full anticlockwise setting ( position "0" ) the outer pin 1 and the middle wiper pin 2 of the potentiometer are shorted out thus allowing the straight signal to be heard at it's full volume level which overpowers and  drowns out the vibrato effect, and when the dial is set to the fully clockwise setting (position "10" ), the straight signal's volume level is at it's weakest  thus causing the vibrato signal to be stronger in the chorus effect mix.

I have also wired up a 0.0047uF capacitor in parallel with the pins 1 and 2  of the 500k "Chorus Depth" potentiometer so that the treble frequencies in the straight signal are slightly emphasised thus resulting in the shimmery effect to the chorus effect similar to the shimmery effect on the chorus effect produced by the vibrato scanner circuit in my Hammond C3 organ.

Because of this modification to the vibrato circuit in my 1963 L102 organ there now is truly no longer any need for me to seriously modify the L102 organ by adding a vibrato scanner and a vibrato line unit and an appropriate pre vibrato scanner boost circuit and a post vibrato scanner recovery circuit because the modified wiring of the chorus circuit in my 1963 L102 as described above more or less sounds almost identical to the vibrato scanner chorus effect in my Hammond C3 organ.

In order to allow the highest treble frequencies to pass through and also  the full volume level to pass through to the input stage of the vibrato amplifier I shorted out the R101 220k resistor and the R102 820k resistor (the R102 is 180k in the later L100 series organs), and I disconnected the R108 grounding resistor to remove any possible high treble cut effect and I removed the R135 330k grounding resistor in order to allow the lowest bass frequencies to be fully passed through to the input of the vibrato amplifier.

In order to allow the full bass response to fully pass through without being attenuated through all the various stages of the vibrato circuitry inside the Vibrato amplifier, I have wired up 0.1uF capacitors in parallel with the C119, the C103, the C105 and the C107 capacitors which are all 0.01uF capacitors.

I have also wired up a 0.47uF 630 volt capacitor in parallel with the C108 0.047uF output capacitor so that the full bass response of the vibrato signal can pass through to the Preamplifier / Percussion amplifier circuitry when the vibrato signal coming from this output stage is combined with the straight signal and the percussion signal inside the Percussion amplifier where the impedances have changed now that I have modified the wiring inside the Preamplifier / Percussion amplifier to produce the main organ signal volume level boost  and the percussion volume level boost modifications which will be explained in detail further down below.

I have wired up a 0.47uF 630V capacitor in parallel with the C108 0.047 uF capacitor so that the full bass response of the vibrato signal passes through to the Percussion amplifier and also that the bass response is not attenuated in any way by the changed impedances created by the main organ signal volume level boost and the percussion volume level boost modifications that I did to the circuitry inside the Percussion amplifier.

Despite the fact that I have modified the Vibrato amplifier and the Preamplifier / Percussion amplifier to noticeably boost up the volume levels of the straight signal, the vibrato signal and the percussion signal, the resulting sound is still clean and undistorted and there is no abnormal or objectionable increase in the background hum or hiss noise levels.

WHAT TO DO:

    VIBRATO AMPLIFIER:

  • Disconnect the end of the yellow wire from the junction of the C101 capacitor and the R108 resistor
  • Wire up a 0.47uF capacitor to the end of the yellow wire and then connect another piece of yellow wire to the other end of the 0.47uF capacitor and then cover this with heat shrink tube and then connect the end of the new yellow wire to the junction of the C102 capacitor, the C120 capacitor and the R104 resistor
  • Remove the R108 and the R135 grounding resistors
  • Short out the R101 and the R102 resistors.
  • Wire up  0.1uF capacitors in parallel with the C119, the C103, the C105 and the C107 capacitors
  • Wire up 0.47uF 630V capacitor in parallel with the C108 output capacitor
  • PREAMPLIFIER / PERCUSSION AMPLIFIER:

  • Wire up a 100k resistor in parallel with the R228 resistor.
  • Remove the R227 resistor and replace it with a 500k linear taper potentiometer or trimpot . The 500k potentiometer will act as the "Chorus Depth" control
  • Wire up the 500k "Chorus Depth" control potentiometer so that when the dial is turned to the full anticlockwise setting (position "0") the outer pin 1 and the middle wiper pin 2 of the potentiometer become shorted out
  • Adjust the setting of the 500k "Chorus Depth" potentiometer to set the  volume level of the straight signal to be properly matched with the vibrato signal in order to get the right sounding vibrato scanner style chorus depth
  • If desired, wire up a 0.0047uF capacitor in parallel with the pins of the 500k "Chorus Depth" potentiometer so that the treble frequencies in the straight signal are slightly emphasised thus resulting in the shimmery effect to the chorus effect similar to the shimmery effect on the chorus effect produced by the vibrato scanner circuit in the B3, C3 etc organs. If you want you can experiment with different capacitor values such as 500pF , 220pF 100pF etc according to personal taste

 

CHORUS EFFECT MODIFICATION BUT WITHOUT ANY OF THE OTHER VOLUME LEVEL BOOSTS OR ANY OF THE BASS RESPONSE MAXIMISATION MODIFICATIONS:

Please note that the various modifications to the Vibrato amplifier and to the Preamplifier / Percussion amplifier circuits which are discussed throughout this article noticeably boost the overall gain/volume levels and they maximise the bass response of the main organ signal and the volume levels of the percussion signals so that the power amplifier can be overdriven when the expression pedal is set to high volume levels.

However if you want to maintain the normal stock volume levels and the stock bass response of the L100 series organ tonality but that you still want to get the rich and shimmery  sounding vibrato scanner style chorus effect, you can ignore the all of the modifications that boost the overall gain/volume levels and maximize the bass response of the main organ signals and the modifications that boost the volume levels of the percussion signals and you can only just do the modification to the vibrato circuit that allows the straight signal to be electrically in phase with the vibrato signal and you can simply replace the R227 680k resistor with the 500k "Chorus Depth" control potentiometer so that you can set the chorus depth exactly to where you want it.

As explained previously, this involves disconnecting the end of the yellow wire from the junction of the C101 and the R108 and then connecting a 0.47uF capacitor to the end of the  yellow wire and then connecting another yellow wire to the other end of the 0.47uF capacitor and covering this with heat shrink tube and you then connect the other end of the new yellow wire to the junction of the C102, the C129 and the R104 inside the Vibrato amplifier thus allowing the straight signal to be electrically in phase with the vibrato signal.

Because you want to maintain the stock volume levels of the L100 series organ, you can then leave the R228 470k resistor in place as it is in the Percussion amplifier. The R228 carries the vibrato signal and you only need to modify the value of the R227 680k resistor that carries the straight signal in the Percussion amplifier.

You remove the R227 and replace this with the 500k trimpot or potentiometer which will henceforth function as the "Chorus Depth" control.

You can then adjust the 500k "Chorus Depth" control potentiometer to set the  volume level of the straight signal to be properly matched with the vibrato signal in order to get the right sounding chorus effect.

Depending on personal taste you can wire up a small value capacitor such as a 100pF , a 220pF , a 330pF , a 470pF, a 680pF , a 0.001uF etc in parallel with the 500k "Chorus Depth" control so that the treble frequencies of the straight signal are emphasized and when these enhanced straight treble frequencies are mixed with the vibrato signal, the chorus effect will then have an even more spicy shimmering quality similar to that of the Chorus settings on the console organs which have the vibrato scanner.

Depending on personal taste or on the actual setting of the 500k "Chorus Depth" control that produces the best sounding chorus effect, it might be necessary to wire up a resistor in series with the small value capacitor in order to get the right kind of shimmer effect that will help the chorus effect to sound as close as possible to the vibrato scanner style chorus effect.

Please note that because of the different impedances involved after the R227 and the R228 resistors in the stock circuit as opposed to the impedances after the R227 and the R228 of the "beefed up" modified circuit in my 1963 L102 which is described in detail further down below, I am not sure if the volume levels and the tonality of the organ will end up being similar regardless of whether the Vibrato Chorus switch is set to the "Vibrato" setting or to the "Chorus " setting.

On my modified 1963 L102 organ the volume levels and the tonality of the organ are more or less the same with the same full bass and midrange response regardless of whether the Vibrato Chorus switch is set to the "Vibrato " setting or to the "Chorus" setting, except that there is a slightly brighter high treble response when the Vibrato Chorus switch is set to the "Chorus" setting.

I have not had a chance to try out the chorus modification on an otherwise  stock circuit because I had already done the volume level boost modifications before I discovered the Chorus modification.

If there in fact is a volume level difference or a tonality difference between the "Vibrato" setting or the "Chorus" setting of the Vibrato Chorus switch, then it might be best to simply accept this because the benefits of the modified chorus effect far outweigh any potential discrepancies between the volume level differences or the tonality differences  between the "Vibrato" setting or the "Chorus" setting of the Vibrato Chorus switch.

I assume that those who do this modification will probably most likely use the "Chorus" setting more often than the "Vibrato" setting anyway, and there will in fact not really be any change in the volume levels or the tonality when the Vibrato Small and the Vibrato Normal switches are set to the off or to the on settings so therefore you can then leave the Vibrato Chorus switch set to the "Chorus" setting.

WHAT TO DO:

    VIBRATO AMPLIFIER:

  • Disconnect the end of the yellow wire from the junction of the C101 and the R108 and then connect a 0.47uF capacitor to the end of the yellow wire
  • connect another yellow wire to the other end of the 0.47uF capacitor and then cover this with heat shrink tube
  • connect the other end of the new yellow wire to the junction of the C102 capacitor, the C120 capacitor and the R104 resistor
  • PREAMPLIFIER / PERCUSSION AMPLIFIER:

  • Leave the R228 resistor in place as it is inside the Preamplifier / Percussion amplifier
  • Remove the R227 resistor and then replace it with a 500k trimpot or linear taper potentiometer which will now act as the "Chorus Depth" control
  • Adjust the 500k "Chorus Depth" control potentiometer to set the  volume level of the straight signal to be properly matched with the vibrato signal in order to get the right sounding chorus effect
  • Depending on personal taste you can wire up a small value capacitor such as a 100pF , a 220pF , a 330pF , a 470pF, a 680pF, a 0.001uF etc in parallel with the 500k "Chorus Depth" control so that the treble frequencies of the straight signal are emphasized so that the chorus effect will then have an even more spicy shimmering quality similar to that of the Chorus settings on the console organs which have the vibrato scanner
  • Depending on personal taste or on the actual setting of the 500k "Chorus Depth" control that produces the best sounding chorus effect, it might be necessary to wire up a resistor in series with the small value capacitor in order to get the right kind of  spicy shimmer on the treble frequencies

 

SETTING THE VIBRATO WIDTH TO BE SIMILAR TO THAT OF THE VIBRATO SCANNER VIBRATO WIDTH:

The earlier L100 series organs including my 1963 L102 organ do not have  the "Vibrato width control" that the later L100 series organs, instead the vibrato width on the earlier L100 organs is set by the R130 330k resistor. If you want to be able to adjust the vibrato width you can remove the R130 resistor and replace it with a 500k trimpot which will then function as the "Vibrato Width control". After installing the 500k trimpot you should then turn on both the "Vibrato Small" and the "Vibrato Normal" switches and then adjust the 500k trimpot until the vibrato width is the same as the vibrato scanner generated vibrato /chorus effect on the Hammond console organs when the "Vibrato and Chorus" rotary switch is set to the V3 or the C3 setting.

On the later L100 series organs which do have the R131 500k "Vibrato Width" control potentiometer you can simply adjust this  until the vibrato width is the same as the vibrato scanner generated vibrato / chorus effect  on the B3 / C3 etc organs when the "Vibrato and Chorus" rotary switch is set to the V3 or the C3 setting.

WHAT TO DO:

  • On the earlier L100 series organs without the "Vibrato width control", the vibrato width is set by the R130 resistor
  • To adjust the vibrato width, remove the R130 resistor and replace it with a 500k trimpot which will then function as the "Vibrato Width control". After installing the 500k trimpot you should then  turn on both the "Vibrato Small" and the "Vibrato Normal" switches and then adjust the 500k trimpot until the vibrato width is the same as the vibrato scanner generated vibrato /chorus effect on the Hammond console organs when the "Vibrato and Chorus" rotary switch is set to the V3 or the C3 setting
  • On the later L100 series organs which do have the R131 "Vibrato Width" control potentiometer you can simply adjust this control until the vibrato width is the same as the vibrato scanner generated vibrato / chorus effect  on the B3 / C3 etc organs when the "Vibrato and Chorus" rotary switch is set to the V3 or the C3 setting

 

SETTING THE VIBRATO SPEED RATE TO BE SIMILAR TO THAT OF THE VIBRATO SCANNER VIBRATO WIDTH:

The vibrato scanner speed rate is approximately 6.87 cycles per second ( 412 cycles per minute according to the Hammond B3, C3 etc service manual ).

The vibrato/ chorus speed on my 1963 L102 was noticeably slower than the vibrato/ chorus speed of my Hammond C3 organ. The vibrato speed rate can be increased so that it is the same as that of the vibrato scanner by decreasing the value of the R126 1.5M resistor which sets the vibrato speed rate. On my own L102 I used the odd combination of two 10M resistors and a 900k resistor wired in parallel with the R126 so that the vibrato speed rate is more or less identical to that of my Hammond C3 organ.

The schematic of the later L100 series organs does not specify an exact value for the vibrato speed rate resistor which is labelled as R138, instead there is a note that "this resistor selected to provide proper vibrato rate 6.6-7Hz", however in my mid 1960’s L143 organ the R138 is a 1.5M resistor which is the same value as the R126 in my 1963 L102 organ.

If the vibrato speed rate on the L100 series organ is either too slow or too fast then remove the R126 (earlier L100's) or the R138 (later L100's) and replace this with either a 1M or a 2M trimpot that will allow the vibrato speed rate to be set exactly the same as that of the vibrato scanner.

If you do not have access to an organ such as B3, C3 etc that has the vibrato scanner in it, you could then carefully listen to recordings which feature B3's, C3's etc with the Chorus C3 setting and then use these as a guide to set the "Vibrato Width " control to sound as close as possible to the Chorus C3 setting sound and you can also set the vibrato speed rate trimpot to be at the same speed as the vibrato scanner speed rate as heard on the recordings.

WHAT TO DO:

  • If the vibrato speed rate on the L100 series organ is either too slow or too fast, you then remove the R126 (earlier L100's) or the R138 (later L100's) and replace this resistor with either a 1M or a 2M trimpot that will allow the vibrato speed rate to be set exactly the same as that of the vibrato scanner.