The classic console Hammonds (B-3, C-3 and A-100) plus valve (tube) Leslie speaker cabinets will always be first choice in an ideal setting, but back in the real world of tricky get-ins and less-than-ideal acoustics, there are plenty of alternatives which in many cases are a better option....

For some time now, when it's not been feasible to use the full vintage Hammond rig (which is quite often!) I've been using a Crumar Mojo organ, through either my Leslie 145, or via its internal rotary (Leslie) simulation into full range powered cabinets (currently a TC Electronics bass combo amp and a Yamaha DXR10 top). The latter amplification gives me more bottom-end EQ flexibility and SPL than the Leslie on it's own, at the expense of not throwing the sound around the way that a real Leslie does. Running another top cab in stereo helps, however.

For severely space-restricted gigs or where public transport is necessary I use a Nord Electro 4D, which has the organ sound of the C2D plus pianos and electric pianos in a single manual package. In my opinion it sounds good, though not as good as the Mojo in terms of organ sound. However as a swiss-army knife for all occasions it is great.

The Neo Instruments Ventilator pedal set the bar for realistic Leslie simulation when it came a few years ago, and I was so impressed that I did a couple of demo videos at the time which can be accessed via Youtube or on the Neo Instruments website. Recent improvements (via software updates) of the Mojo's in-built rotary simulation have led me to abandon the Ventilator when using the Mojo, but it works wonders for other clones which have weak Leslie simulations.

Why It's Sometimes Better NOT To Use A Real Vintage Hammond And Leslie

I'm writing this in response to certain people one encounters on gigs who feel mortally cheated if they see there is no "real Hammond" on the stage. "Nothing sounds like a real Hammond.." is the sentiment that they're implying. Whilst this was undoubtedly true in the past, things have so improved lately, that listening back to recordings I've made recently, it's all but impossible to tell if the organ was vintage or digital. With a bit of care and knowhow, authentic results can be had, and a great balanced sound achieved using the modern stuff...

Ignoring the obvious logistical challenges of manoeuvring several hundred kilos of antique equipment, there are good reasons why in many circumstances a more pragmatic (and modern) solution is a better idea.

1. Sound
If there's a good PA system and the room or hall is large, then no problem mic-ing up the Leslie and EQ-ing the front-of-house as necessary. (This of course relies on the FOH engineer being familiar with how to mic/mix a Leslie. In some cases, they'll be less for them to mess up if you can just provide them with a stereo D.I.!!)

If however, the room is small (or tiny) and the Leslie can only go in a corner and will be obscured by other musicians and their instruments, or architectural obstacles or the audience (e.g. if the band is set up at the same floor level as the audience this is inevitable) and the space is less than acoustically ideal.... THEN.... with a vintage Hammond console and Leslie there is only a limited amount of adjustment available. The bass end (bass and low mids) tend to boom, and the treble can get stifled/absorbed. Neither the Hammond nor leslie is equipped with tone controls, and Only so much can be done via the drawbars without compromising ones playing style.

Modern clones are equipped with bass and treble controls at the very minimum, and can interface effectively with a vintage Leslie to get the best of both worlds.

Where amplification of the Leslie isn't available (to basically get the treble sounds above ear height of the audience) it may be better to use an electronic simulation and a high quality small full-range amp system (e.g. a powered sub and top on a pole). This enables proper control of eq, and stereo can be used and/or multiple speakers to direct the sound where it's needed.

2. Preservation!

If there's any chance that an audience member (or any other individual) may mistake one's pride-and-joy instrument for a table, I tend to not risk it. One of the best ways to "kill" a vintage Hammond is to spill alcoholic or sugary drinks on the manuals. An awful corrosive reaction will ensue on the hundreds of tiny resistance wires inside the keyboards, which it is all but impossible to remedy . It's not worth the risk.

3. Physical reasons

It sometimes feels like you've done one gig already by the time you've unloaded the van, carried everything in and set it all up. Gripping/lifting/carrying heavy objects isn't really compatible with the finer muscular demands of playing at challenging tempos or achieving a relaxed posture on the gig.
Also, however many people offer to help "move the organ" only you know the best way to do it, and how not to damage it. In my case, there's a definite technique involved in negotiating the ramp into and out of the back of my van. If the bass pedal contact strip "grounds-out" on the lip of the floor, it can do some damage, so it's a one-man job. I have developed strategies over the years to deal with backache, but now and then I get caught out, and it's no fun having to dep out gigs because you can't even stand up straight...