Solid State Leslie Modifications
The legendary 122 Leslie Amplifier - here one sitting on my bench - is famed for his growl, bite and soul. This makes tube leslies very sought after and expensive.
What makes the "Tube Leslie Sound"?
The 122 is "the" Leslie to have. Equipped with big 6550 tubes, a mysthic glowing stabilizer and lots of iron, it adds the characteristic growl and distortion to your organ. Since 122 and 147 Leslies are pretty expensive nowadays, Hammond players wonder if they could achive the same sound from their solid-state Leslies (700, 800 and 900 series, in particular).
Before thinking about certain modifications of a transistorized Leslie, let's talk about some issues that contribute to the Leslie's sound. First, the speakers, rotors and the cabinet do most of the work. Also, the Jensen V-21 treble driver has a heavy rolloff from about 8 kHz up, so it smoothens distortion coming from the amp. I also think that the narrow louvers in the wooden cabinet contribute to the sound. In fact, the stage Leslies sound more bright than their wooden counterparts. So, if you own a genuine 770, you're on the right way.
The Tube Deal
Without driving it into distortion, differences between tube and transistor amplifiers are subtile - the audience won't hear it, if you don't tell. The 122 amp has some distortion in the 3% range when putting out around 10 Watts and an almost flat frequency response from 30 Hz to 10 kHz (there is a slight rolloff for frequencies from 8 kHz up). However, there are differences in their output damping: Transistor amps do have very low internal impedances in the Milliohms range, so they control speakers very exactly. Tube amps may have internal impedances in the 2 Ohms range, especially when overdriven (loss of negative feedback), so speakers connected to them tend to live their "own lifes". This emphasises the speaker's/driver's characteristics. BTW: We are NOT talking about speaker impedances here!
Emulating the relatively high internal impedance of a tube amplifier is pretty easy: Just put a 1.8 Ohm/11 Watt resistor (for 8 Ohm drivers) in one of the Jensen's speaker leads and a 1 Ohm/11 Watt resistor in one of the woofer's leads. That marginally cuts off the amp's power, but it makes the speaker act like connected to a tube amp. As the audiophools would say: "You will definitely hear a difference", not only from the lowered power output (loss is in the 5 Watt range). Give it a try, it won't cost you a fortune.
When it comes to an overdrive condition, differences are way more big. Solid state amps just cut anything above their margins, so a sine wave becomes a sharp clipped trapezoid shape. That sounds harsh and very unpleasant, because it adds lots of higher odd harmonics (those kill your treble driver fast). Overdriven tube power amps also clip signal shapes, but with a decent rounded behavior (see picture below), so higher odd harmonics are present at a way much lower level. To achive the distorted tube Leslie sound, you have to add a limiter that imitates the push-pull power stage exactly.
The Solution: MIG-LR
I've made up a little gadget that "tubifies" your transistorized Leslie. It works well in solid-state Leslies with an active crossover board as seen in the picture below. First, by putting a real miniature tube (russian 1SH18b) in the signal path that adds those (triode-like) pleasant soundig even harmonics (k2, k4), second, by adding an overdrive stage that works with integrated field effect transistors (FETs) instead of silicon diodes (as seen in many guitar stomp boxes) to add the odd harmonics (k3, k5) produced by saturated 6550s. FETs act much more like tubes than transistors, since they use the electrical "valve" principle.
The MIG-L(R) is compatible with all genuine Leslie speakers equipped with the 760 type crossover board found in 760, 710, 770, 925 for example, pictured above (open your Leslie back panel to make shure).
It may be installed in an 860 Leslie as well. The 860 crossover PCB looks different and has no master volume control, but it is electrically equivalent to the 760. It will work well, but has to be fastened a little different (stock screw holes won't fit). C66 is located near the black cable. Other applications are possible if your Leslie has the common 1V RMS input level. If the particular model has mo input attenuator pot (master volume), it may be inserted directly behing the Leslie input plug (Pin 1 for 11-pin Leslies, for example).
Check my Leslie schematics folder or visit Captain Foldbacks Leslie Schematics Page for details on particular models. If you are unsure what type of Leslie you have, check the Tone Cabinet Database from Maryland Organ Service (USA).
Wiring is very easy (only four wires), but some soldering necessary. Here you find the MIG-LR installation instructions (PDF) in English. You need Adobe Acrobat Reader 5.0 (free) to open it.
Hier ist die deutschsprachige MIG-LR Dokumentation (PDF) zu finden. Sie benötigen den Acrobat Reader, um das Dokument zu öffnen und zu drucken.
Important - please readUnder rough use (vibrations due to transport or high volume levels) you should secure the tube with a thick bead of silicone sealant to the MIG-L circuit board. This also minimizes microphonic effects.
There is a version available with remote bypass switching input (pin in upper right corner, see picture), the MIG-LR. Connect a latching footswitch (like Boss FS-5L) to the remote pin to toggle between effect on or off.
The MIG-L is intended for use in a genuine Leslie speaker with transistor amplification; it needs about 1 Vrms input for full distortion. This is true for Leslies controlled by a Combo Preamp or an organ high-level speaker output. For other purposes, i.e. Leslie clones of other brands or even Leslie simulators with audio line inputs (1/4" jack), I've made up the MIG-LRL version. It yields full distortion at 100 mVrms input, so it can be used with line-input amplifiers often used on Leslie clones.
The MIG-LRS (next picture) is designed for stand-alone use or to build into a Leslie Simulator like the famous Dynacord CLS-222. It has its own power supply, wired for 230V AC. It also has a remote control input for switching between tube limiter and bypass mode. When properly adjusted, distortion will occur only in the MIG-LRS, not in the CLS-222. The MIG-LRS will limit the level before nasty transistor clipping occurs in the Leslie simulator.
Here you find the Addendum for the MIG-LRS (PDF).
If you have a Leslie clone, it will need some work to achive the smooth sound of a Jensen V-21 driver with it's phenolic diaphragm and characteristic frequency response. Leslie clones often use metal-dome high-range drivers (tweeters) and a 1500 Hz crossover network, while the Jensen works from about 700 Hz up (official crossover point at 800 Hz). Typical tweeter drivers used by brands like Elka, Solton, Allsound, Echolette, Dynacord sound shrill and aggressive compared to the V-21, so think about replacing the crossover network (or simply doubling the hi-range filter capacitor) and the tweeter by a mid-range PA horn driver like the Monacor KU-516 when you own a non-Leslie Leslie. For two of my Leslie clones, I used old military surplus midrange PA drivers from a command truck speaker. They are loud as hell and need hefty attenuation to match up with the woofer, but they come very close to the V-21's frequency response and sound way much better than the stock tweeter driver.
A stock Leslie clone with stock tweeter driver will sound even worse with distortion, so first modifiy your horn driver and crossover network before thinking of an MIG-LRL.