Brief Tutorial of Leslie 122 Switching

[muckelroy]

The regularity of 122-type speed switching posts on this forum makes me wonder if there is a better way to help "explain" how the switching system works. There are so many well-written resources out there on how the switching system works, and how to troubleshoot / fix problems with the system. I thought - why not contribute to the knowledge pool?




All of this is a paraphrasing of a mixture of sources, a lot of which is described in the Leslie 122 service manual. Refer to the Service Manuals for original source material, and always call a qualified professional for doing any work mentioned here safely.




History:

• The earliest Leslie speakers had a toggle switch mounted to the side of the Leslie cabinet itself that directly turned the motors on, or off. This is as easy as it gets, but you had to physically be standing next to it, or have freakishly long arms to reach this switch while playing.
• The earliest form of a "remote" motor control on a baffle-type speaker tremulant was that found in the early Hammond tone cabinets (CX-20, DX-20.) Those had a mercury contact switch, with a roller connected to it meant to be turned on or off by your foot. This was all in a brown bakelite enclosure, and was mounted next to the swell pedal.
• Leslie speakers began their own form of "remote" motor switching not long after they began to make their speakers, and this employed the use of a relay inside of the Leslie amplifier to switch the motors on, and off. When off, an optional brake accessory would cause the motors to stop more quickly, rather than coast to a stop. In later two-speed Leslies, there is no brake (by default), but switching instead turns off the fast motors and turns on the slow motors.

What is a relay, and why are they used?

• A relay is a very small electrical switch that is switched by means of a small electromagnet coil (more on this later.)
• Relays come in many shapes and sizes, but Leslie speakers use a single pole, dual throw relay. This means there is only one pole (one actual "switch" inside that conducts power, sometimes also called a "flapper") and that pole moves in between two possible "throws" (or "positions".) This is the physical switch that physically sends power directly to either the slow motors, or the fast motors. (Or, in the case of early single-speed Leslies, it switches between Fast and Brake/Off).
• So why is a relay used, and why don't we just run long extension cords from the motors to the organist and back, and switch it there? The reason a relay is used is that the Leslie motors require a high amount of electrical current to operate. If that electrical current demand were to run the full length (30 feet or more) of Leslie cable, as well as at a switch at the organist's console, it would cause excessive heat, and would wear out the wiring, and the speed switch. It would also be more likely to cause excessive electrical noise in the organ and Leslie audio circuitry, and would also increase the safety risk to the organist.
• So, why is the relay located under the amplifier chassis, hidden away? The relay does make a small amount of noise, and this noise is suppressed by hiding it under the chassis. You also will see a rubberized grommet mount that quiets this noise. Also, placing the relay under the chassis prevents electrical shock risk, as there are exposed terminals on the "open pot" relays that Leslie speakers employed.

How does a relay do its job?

• It has a small coil of wire inside it, and a small metal rod runs through that coil. When the coil is energized, it creates an electromagnet that "pulls" the metal rod downward. This rod is attached to the "pole" discussed earlier that makes up the actual switch that sends power to the motors.
• So, the easiest way to think of the relay is this: Imagine that the Leslie motors are connected to a light switch on the wall, and you must use your hand to move that switch up or down on the wall. The relay coil takes the place of your hands and does that up or down movement for you.
• The relay coil is very small, and only requires a fraction of the electrical current demanded of the Leslie motors. For this reason, it is relatively safe to send that small electrical current through long lengths of Leslie speaker wiring, and run back to the organ console.
• The speed switch at the organ console does not directly send power to the motors, but instead sends power to the relay coil. Note that in 122 Leslies, the movement of the speed switch does not directly send power to the relay, but on a Leslie 147 type, it does. (More on that later.)
• The relay pole has a small spring on it that "locks" it into a "normal" position when the coil is not energized. On almost all Leslies, when the coil is not energized, that is "fast" speed. So, when you do energize the coil, the rod inside the coil "pulls" on the pole, with just enough force to overcome the spring, to switch it to the other contact, thus switching to slow speed. Once you turn "slow" back to Fast, OR turn off the organ altogether, the relay switches back to fast, because of the spring holding tension on the pole itself.

So now, you hopefully understand what the relay is for. It is the physical switch that sends power to the Leslie motors. It hides under the Leslie chassis, but it does make a small audible "click" when it moves back and forth. The thing that moves that hidden switch is the relay coil. Ultimately, the speed switch at the organ controls whether or not to energize this coil to make this job happen successfully.


When troubleshooting a speed switching issue, find out whether or not you hear the "click" of the relay. Hearing the "click" means most likely, all the stuff leading up to the relay is doing its job of energizing the relay coil. If you don't hear the click, it could mean that the stuff leading up to the relay coil is NOT energizing the relay properly.




What energizes the coil, really?

• On a 147 type Leslie amplifier, it's very simple. The relay coil is rated at 117 volts AC, and is switched directly with the speed switch at the organ console. It accomplishes this via pins 2 and 5 of the 6 pin connection. In this sense, the speed switch at the organ is acting just like a light switch, turning the coil on or off. That's all there is to it.
• On a 122 type Leslie, it's much more complex. We'll start with the coil and work our way backwards from there.

Coil, and current

• The coil has two ends, or connections. You can think of it as an input and an output, but for our purposes, it does not matter which is which The coil will only create a magnetic field when there is enough current flowing through the coil to do so. "Electrical current" means that there is a flow of electrons, (not unlike a river of water), traveling through the wire of the coil. The only way that current can flow through the coil is if the current has a place to go to. Imagine filling a jar with water. When it is full, the water inside the jar won't flow through the jar, it just sits there. If you want the water to flow through the jar, you must create a path for the water to leave the jar.
• In the Leslie 122 amp, one side of the coil is connected to a source of electrical voltage. This is a 12k 2-watt resistor, coming from the 310 Volt DC power supply of the Leslie power supply. The other side of the coil is connected to the cathode of the 12AU7 switching tube.
• At first glance, you may think that the coil being connected to the cathode of the switching tube makes no sense. Is this a "place" that the current can "flow" to? Yes, actually it is.

Continued in part II...

[muckelroy]

Tubes, and current

• Every Leslie 122 has two small 12AU7 tubes near the front. The outer one closest to the gain pot is responsible for the audio signal. The other (inside) one has only one job - it is a gate of sorts. From this point forward, we refer to this 12AU7 tube as the "switching tube".
• Inside the switching tube is actually two tubes in one envelope, but we are only focusing on one of these, and it is known as a "triode". In a triode vacuum tube, there are 3 components: Cathode, grid, and anode (aka "plate").
• The cathode is very hot, because it is heated up by a tube heater, similar to a light bulb filament. This is why tubes glow. You are seeing the heater filament glowing, and it is making the nearby Cathode very hot.
• The plate (or anode) always has about 25 volts on it. This is very unusual as tubes go (it is usually a much higher voltage for a 12AU7) but in this application it is the perfect voltage.
• The grid sits in between the cathode and the anode. If the grid was not there at all, the tube would be a diode, and, current would flow from the cathode to the anode in a steady stream. However, the grid IS there, and when there is no voltage on the grid, the grid "blocks" the flow of current from the cathode to the anode. This is what the switching tube is doing when the Leslie is in "fast" position. There is very little current flowing from the Cathode to the plate, because the grid in between them is blocking almost all of that current. So, one side of the relay coil has a somewhat high voltage from the Leslie's power supply, but the other side of the coil is connected to a "dead end" - the cathode, that does not flow much current through the coil, because there is no place for that current to go.
• The act of switching to Slow speed at the console sends a positive voltage to the grid of the switching tube. Once the grid of the switching tube receives this positive voltage, this allows current to flow inside the switching tube from the cathode to the anode. Suddenly, the "floodgates open" and current is now flowing from one side of the relay coil (the power supply), through the coil, and through the cathode of the switching tube. This flow of current through the coil (by way of the switching tube giving the current a place to go) makes a magnetic field that moves the rod inside it, and does the deed of changing speeds.
• Other types of tubes (6J5, 6SN7, etc) are used for this purpose in earlier Leslie models, but the concept is the same for those tubes as it is for the 12AU7 tube. The switching tube is not being used to pass an audio signal, but is being used as a gate to open or close the flow of current, which thus opens or closes the flow of current through the relay coil, which thus switches which motors get power.
• So, anything that prevents the switching tube from operating may cause a malfunction of the relay. A weak switching tube, dirty tube socket, or lack of plate or grid voltage from upstream, could all cause this part of the circuit to not work correctly.

Getting voltage to the switching tube grid

• The way that the 122 type Leslie gets a DC (direct current) voltage to the switching tube grid is perhaps one of the most quirky and hard to understand aspects of the design. At the same time, when it works, it works beautifully and reliably.
• Don Leslie wanted to be able to control the Leslie relay remotely somehow, but in the 1940s, he lacked a spare wire in the Hammond organ connector to do so. Hammond consoles and tone cabinets used 6 pin connectors, and all 6 pins were being used. There was not a spare unused pin. Don Leslie could have designed his amplifiers with a 7 pin connector (using one for switching voltage), and forced his customers to retrofit 7 pin connectors on their organs, and buy new cables with 7 conductors inside them. Instead, he devised a system that used the existing 6-pin connectors and cables. He wanted his speaker to be as "plug-and-play" as possible.

• Early designs (known as Type 1) took advantage of the B+ current, going to the preamp, but I won't go in to that, because it was a very unreliable design fraught with problems. Nearly all Type 1 Leslie amplifiers get converted to Type 2 (the one we use today), because of how much more reliable it is.
• Type 2 switching is done by superimposing a DC voltage mixed in with the organ's balanced signal. Inside the organ is a Leslie tremolo kit. That kit contains a transformer. When you move the speed switch to "Slow", that switch applies a DC voltage to the center tap of the transformer inside the Leslie tremolo kit. That transformer feeds the Leslie a mixture of organ signal and DC voltage. You can't "hear" this DC voltage. You can only "hear" the AC voltage (signal) from the organ.
• When the DC voltage travels down the signal wire, it also travels to the grid of the 12AU7 switching tube. (There's a resistor in between, but that's the idea.) The 12AU7 signal tube is protected from this DC switching voltage with some capacitors, which bock the DC voltage from damaging the signal tube and affecting any other part of the amplifier.

The Speed Switch, and Tremolo Kit

• The most popular switch is the "half-moon" switch, which is actually just a rotary style switch. Some refer to it as a "Telecaster" switch, but that is an over-simplification. While it is similar to many rotary switches that go in to guitar bodies, there are characteristics unique to the Half-moon enclosure that make it challenging to find just the right switch from a guitar parts supplier.
• Modern day Leslie kits sold by Hammond-Suzuki use white capped 3-position switches that move up and down. Lots of harsh words are uttered about these kits, but they do work fine.
• Plenty of performers use foot switches, cheek block switches, etc. Any switch that is well insulated and that is rated for 0.5A 125V or 0.25V 230V will work. The switch is usually a latching, single pole single throw switch, which either makes or breaks the connection. Some performers prefer for tremolo to only activate while holding the switch down, and to go back to chorale when let go (aka "momentary" switch). There are many flavors of switches.
• The typical 8000 kit simply grounds (turns off), or does not ground (turns on) the DC switching voltage that is sent to the transformer.
• The most common vintage kit for 122 (6H) amps is the "8000" kit, also known as the "428" kit. It usually gets its high voltage source from the organ preamp, but if you have an early organ, it might get its high voltage source from the Leslie amp pin 5. The voltage source is very high, so there is a 33k resistor that lowers it down. A common way of obtaining this voltage is with a tube socket adapter that plugs in to the rectifier tube socket of the organ preamp. On these, there is a 33k resistor molded in to the adapter, and you can't see it. A lot of times, that resistor begins to fail, and a reduced voltage (or no voltage) makes its way to the Leslie kit, causing erratic or failed speed switching.
• You can either replace these tube socket adapters with a donor, or you can attach a 33k resistor and a wire to the appropriate pin of your organ preamp rectifier tube socket under the chassis. This should be done with care and may require a small hole be drilled into the chassis in a discreet location, with a good strain relief. High voltage is no joke, and you do not want this to short to the chassis by having insulation rub off of the wire.
• The most common component that fails is the 10uF 450V capacitor inside the kit. When that capacitor begins to leak, you may have an audible "pop" on speed switch, or speed switching will start to be erratic.
• Original kits consist of carbon composition resistors. They are usually robust and rarely fail altogether, but they do drift, and these kits are not terribly difficult to rebuild if wanted. Be mindful of the power rating for all resistors replaced.

After the relay, then what?

• The motor sockets always have one of their legs wired to AC voltage (pin 3), and the other leg gets switched on or off via the relay pole.
• There are 0.1 uF 600V capacitors connected across these sockets. These are a form of "snubber", which dampens the effect of arcs and other transients that occur when switching AC voltage on or off.
• If these capacitors are original to the amp, they should be replaced, almost without hesitation. The exception would be recently-built Leslie amps that already have orange drop capacitors here, though even orange drop capacitors may fail.
• If the relay is definitely "clicking" over when you switch speeds but the Leslie is stuck on fast or slow, it is entirely possible that one or both of these capacitors are to blame.
• Motors themselves can also cause a "stuck on slow" symptom if they are misaligned, dirty, lacking oil, or gummed up. Well serviced and lubricated motors are a must for all Leslie installations. I won't go into that in great detail here.

[muckelroy]

Part III....

The 122a and 147a

• The relay coil does exactly the same thing on the 122a and 147a as their vintage brethren. The only difference is that the pole doesn't switch between two 120 V motor socket voltages, but instead the pole either grounds or doesn't ground a small control board voltage (8V I think). This tells the motor control circuit board to run the single motor at either fast or slow speeds. Everything else about the system leading up to the relay is exactly the same.

Solid state relays

• They're out there. Read the installation directions very carefully. When working right, they are silent and don't "click" on operation.




Easy DIY?

⚠️⚠️⚠️A lot of this is component-level stuff that, if it fails, requires the right components, tools, and soldering skills to replace properly.

• Even the most qualified technicians CANNOT DIAGNOSE A FAILURE in the speed switching system without a voltmeter, or other basic tools to take readings throughout the system.
• When a failure happens, the best thing the organ owner can do is unplug the motors from the amp and test plugging them directly in to an extension cord. Verify they work as-is. If not, fix the motor (oil them, adjust them, etc.)
• On a 122 amp, swap the 12AU7 tubes, or replace the switching tube with a new one.
• Clean the 12AU7 tube contacts with a small amount of DeOxit, CRC QD, or other high quality solvent and reinsert the tube.
• If the issue persists, test with a different Leslie speaker cable if one is available.
• Does the relay audibly click when switching speeds? (A small 1 - 2 second delay is normal on 122 Leslies. Longer delay could be a problem)
• Test connecting the Leslie to a different organ (make sure it is configured for a 122 Leslie), if one is available.
• Give your technician the answers to the results of the above steps, to aid him or her ahead of their service visit.

[Wes]

Holy moly. GREAT POST, Callie! Thank you!

I'll note that cleaning the 12AU7 pins to diagnose switching is only useful on 122-type Leslies.

[muckelroy]

Slightly off-topic segway. Some people may ask why the 122XB is an 11-pin and not 6-pin hookup, and about adapting between the two. I'd like to comment on that here.

The 122XB is a totally different front-end than the other 122 types. It has an 11-pin hookup. This hookup is unbalanced, and it does not carry AC power voltage. Instead, these amps have a dedicated built-in power cable coming straight out of the front of the chassis. Speed switching on the 122XB is not done with a relay. Rather, just like other 11-pin type Leslies, when pin 7 or 8 are attached to ground at the organ, that is what switches to slow or fast respectively. When neither pin 7 nor 8 are attached to ground, the motors stop.

No 122XB amps have a speed switching tube.

If you come across a 122XB amp, some do have a relay inside them, and some don't. After about 2005, changes were made to the amp that added a relay which does only one thing. When the amp is powered on, the relay closes and sends 15V of power from the control board's power-on circuit to the rest of the motor control board to thus turn on the motors. That relay coil is energized by the screen voltage of the 6550 tubes after a 20k 7W resistor.

Pre-2005 122XB amps do not have this relay. They send the cathode voltage of the amp up to the motor control board to be used for its power.

122XB amps are obvious by the presence of a power cable coming out of the front, as well as the 11 pin connector. If you see a 122XB amp chassis, without even viewing the underside you can identify its version. Get a good look at the 9 pin molex connector. If all 9 wires are used, it's a 2005 or later version. If only 8 are used, it's the pre-2005 version.

Needless to say, you can't send DC-superimposed old style 122 signal and speed switching to the 122XB and expect it to work. You have to build an 11 pin interface on the organ side and go from there.

[rockandrollsupplies]

Thank you!!
Great info

[bnelson218]

Wow! Welcome to Leslie Theory of Operation 101. Thank you, Muckelroy, for this vast information which could just as easily be considered a primer to basic electronics. It's always appreciated when someone takes the time and effort to provide us such an informative topic. And while you're at it, perhaps at your next convenience you could demistify the purpose of the OC3? :)

[geoelectro]

I had been wanting to do something like this to direct people to when asking about switching problems. I would like to expand on one thing said above.

"The plate (or anode) always has about 25 volts on it. This is very unusual as tubes go (it is usually a much higher voltage for a 12AU7) but in this application it is the perfect voltage."

Actually, the Cathode has 25Vdc on it while the Plate is connected to the relay coil. The Plate will have a much higher voltage on it when the tube is off coming from the relay because there is little current flow. When the grid is energized, the tube goes into conduction (on) and the plate voltage drops causing enough current flow to switch the relay.

Because the Cathode sits on 25Vdc, the switching voltage to the grid must be higher than 25Vdc. So why would they make the Cathode sit on 25Vdc when they could have set it to ground?

If the Cathode was at ground, the actual audio signal would trigger the tube and relay. They needed to set the threshold high enough so the loudest signal wouldn't interfere with the switching. There is a ready available 25Vdc source already in the amp so that was used.

Geo

[muckelroy]

Hi Geo, you are correct, and I apparently had the cathode and plates mixed up while explaining where the 25V was on the switching tube! I hope this didn't muddy the waters too much.

[David Anderson]

Once -- and only once -- I ran into a 122 amp with very strange problems that turned out to be a switching 12AU7 with heater-cathode leakage. The 12AU7 cathode is directly connected to the 6550 cathodes, so 12AU7 heater-cathode shorts or leakage will end up in parallel with the 150 Ohm cathode resistor via the heater's connection to ground. I can't remember the symptoms, but it was a head-scratcher.

[muckelroy]

Random thought. Whenever I get confused about whether closing or opening the tremolo switch produces fast or slow, just remember it like this...

122-type switching:
Switch Closed = fast
Switch Open = slow
--> This is because the switch either grounds, or doesn't ground the control voltage in the kit.

147-type switching:
Switch Closed = slow
Switch Open = fast
--> This is because the switch energizes, or doesn't energize the Leslie relay coil directly.

[Terza]

Thank you Muckelroy for writing so extensively about slow/fast switch. I read all three parts and everything you wrote makes sense, however I am still confused about what is going on with my gear (A100 + Leslie 122RV).
When I start the organ, 122 goes to fast for about 5-10 seconds, after which I hear discrete 'click' (relay) which makes it go to slow. The sound is OK and Leslie stays in chorale (slow) as it should. It doesn't switch to tremolo (fast) on its own. When I turn half-moon switch to tremolo (fast), another audible 'click' is heard, Leslie speeds up but becomes unresponsive when I want to switch back to chorale (slow), it stays in tremolo. Based on my understanding of basic electronics, I suspect that the relay itself is faulty and will need to be replaced, bu I could be wrong. I would appreciate if you can share your thoughts and eventually give me an idea how to sort out this issue and fix it. Thank you very much !

[David Anderson]

I suspect that the relay itself is faulty and will need to be replaced, bu I could be wrong.

They relays do sometimes go bad, but I've seen more people replace relays when they weren't the problem than I can count. In fact, the fact that you are hearing clicks are are getting both speeds suggests that it's not the relay.

Fast/tremolo is effectively the default position of the relay. That's its position when not energized.

There has to be a positive voltage on the grid of the 12AU7 used for relay control for the relay to switch to Slow/Chorale. The point where switching will happen is 30-32 Volts. So, if your Leslie kit sources switching voltage from the organ's preamp, you should be able to measure the switching voltage at the female end of the Leslie cable, unplugged from the Leslie, between pins 2 (Ground) and 1 or 6. It should be zero on Fast and somewhere between 32 and 90V for Slow.

If your Leslie kit sources control voltage from the Leslie, it becomes somewhat trickier to diagnose.

[David Anderson]

To put it another way, I think sometimes people want a 122 switching problem to be a bad relay because the relay is something you can visually identify and access fairly easily.

[Terza]

Thank you very much David for your comment. I will measure the voltage as you suggested and see if that identifies the problem. I know that 122 goes fast when turning the power on, then the relay clicks and switches to slow when it warmed up (half-moon switch is in chorale position). When I switch it to tremolo, the relay clicks, Leslie goes fast but it stays in tremolo. Will try to replace 12AU7 tube as well. Thank you kindly!

[Terza]

I must admit it David, I am unable to solve this puzzle. The problem still remains, however I checked a few things out. Firstly, the Leslie kit is powered directly from organ's amp. I checked half-moon switch and it seems to be fine, as the woofer produces very quiet, subtle hum when switching it back and forth between chorale-tremolo. Also I have removed the 12AU7 tube (further away from the volume knob) which controls Leslie's speed, and naturally without it the Leslie stayed in tremolo mode after powering it up. I am not sure if I did something wrong when measuring the voltage at female end of Leslie cable. I assumed that the terminal 2 is where the black wire is located. The only thing that comes to mind is that the 30/30/30/10 capacitor is at fault and the relay doesn't receive needed voltage to switch back to slow (chorale).
Any comments / suggestions are most cordially appreciated. Thank you.

[Terza]

I am unsure if it is related but also the reverb amp (I have 122 RV) blew the fuse...Thank you again