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The stock V-21 Jensen horn driver is fragile. If you have a V-21 then don't drive the Leslie into distortion. In other words, this modification is probably not for you. There's a procedure in the HammondWiki submitted by Al Goff to the original Hammond mailing list describing how to adjust the Hammond preamp drive level to give either "classic Hammond Growl or a clean organ sound." There is a problem here though. While the procedure does allow you to set up the preamp to overdrive the 122 amplifier, the distortion produced by that amplifier is less than optimum. The 122 amplifier is conservatively designed and uses a very linear tube, the 6550. The 6550 is a good tube for high-fidelity service. It makes a very good bass amplifier (Ampeg SVT), and a also a good organ amplifier. When what you want is a clean, non-distorted organ sound then the 6550 the way to go. Unfortunately, it does not produce the most pleasant sounding distortion when overdriven. It has a rather hard, and to my ear unpleasant, clipping characteristic. With the Hammond chorus on, and with sufficient drive, you can hear the stock 122 clipping on the chorus-induced volume peaks, giving a sort of periodic crackle rather than a softer, more continuous distortion. Enter the EL34. The EL34 is also used for high-fidelity service, in the classic Dynaco Stereo 70 for example. When used in the Leslie, the sound of the EL34 is almost indistiguishable from the 6550, until the amplifier clips. The EL34 has a distortion characteristic with a softer onset than the 6550. As the volume is increased from low to high and clipping begins, the distortion increases from a barely perceptible level to an overdrive condition that exhibits none of the crackle and sharpness of the overdriven 6550. This becomes important when you attempt to set up the drive to the Leslie to maximize volume. The procedure referenced above has you adjust the trimmer while playing the organ to produce a maximum level signal, such as with full stops and big chords. The thing is, you might not play like that all the time so you might want to set it up to give maximum volume at a lower level of drive out of the organ. Then when you do play something louder, you'll get the crackly 6550 distortion. On the other hand, since the distortion out of the 6550 is much more noticable than the EL34 distortion, you can always just back off on the expression pedal a bit when you hear it. This modification allows you configure the 122 amp to use either 6550s or EL34s. The modification is relatively simple, requiring a rewiring of the Leslie output tube sockets, the addition of a 100 ohm / 5 Watt resistor, and a switch. Any tech should be able to do it in an hour or so. The usual caveats and disclaimers regarding this modification apply. Use at your own risk. |
