NOTICE: Very HIGH Voltage inside.
Checking the Cathode Voltage for the 6550A Output Tubes.
The first step in servicing the Leslie Amp is to check the Voltages at various points to determine the amp’s operating condition.
There is one place I like to check first that gives you a fast indicator as to whether the Amp is operating correctly. Looking at the 6550′s, Pin 8 goes to the output transformer’s secondary. The center tap is connected to a resistor which is 150 ohms. This is the Cathode Resistor for the 6550 output tubes.
If the power supply is working right and the output tubes are conducting correctly, then this voltage will be right. The “top” of the 150 ohm cathode resistor should be 25VDC.
If this voltage is low;
- Some power supply voltages are weak.
- One or both output tubes aren’t conducting fully.
- The bypass capacitor which parallels the cathode resistor is shorted.
If this voltage is high;
- The power supply voltages may be high.
- One or both of the Output Tubes are conducting too hard.
- The cathode resistor has begun to open up.
Checking the Power Supply Voltages.
In checking the power supply remember that there are some LETHAL Voltages here!
The first voltage comes off the rectifier which may be a tube circuit on very old models or more likely a bridge rectifier made out of four diodes. This voltage should be around 430VDC. (Picture above shows it at 420Vdc, make it 430Vdc.)
The second voltage is found after the choke transformer. It should be around 420VDC.
The third voltage is after the OC3 regulator and should be 310VDC.
The fouth voltage is after a 10K 1watt resistor and supplies the preamp 12AU7a through the plate load resistors. It should be around 260VDC.
These voltages are general and depend highly on the line voltage and the circuit load. I routinely find them to be a little higher in the field.
If these voltages are low;
- The line voltage should be checked for 117VAC.
- The filter capacitor is weak.
- The load is over conducting. (Bad 6550 tube usually)
If these voltages are high;
- The line voltage may be high.
- The load is NOT conducting properly. (Bad 6550)
Checking the Regulator.
The OC3 tube is the regulator and is very important to the proper operation of the amp. Also, it practically never fails. It’s purpose is to provide a constant voltage drop or difference for the output tubes. This voltage drop should be around 105VDC.
The main job here is to provide the proper “screen” voltage to the 6550 tubes. You will notice from the schematic that the “plate” or pin 3 on the 6550 is 415VDC. The screen or pin 4 should be 105VDC lower. This voltage drop can vary somewhat and as long as it is close, you are OK. The 470 ohm resistor on pin 2 of the OC3 regulator acts as a current limiter and is a common failure part. Remember, if the 470 ohm resistor is bad, replace it with the same wattage (1/2 watt) because it also acts as a fuse. The regulated voltage should be around 310VDC which supplies the grid voltage for the 6550′s and the preamp tube voltage.
Preamp Tube Voltages.
The 12AU7a tube requires a voltage to the plates. This is a dual tube and actually works as a converter from the single ended input to a balanced output to drive the output tubes. The plate voltages should be checked on pin 1 and 6.
Both plates should be around 115VDC.
Usually the plate load resistors give very little trouble. Different tubes will affect the voltages depending on their conduction. Also, there may be some differences from one plate to the next due to the conduction of the tubes. If you have several tubes laying around, you might want to try to “match” the sections by measuring the plates and trying different tubes until the plates measure close to equal. I would say a 5 to 10 VDC difference isn’t a problem.
Last minute Tips.
- Replace output tubes as a pair.
- Always check the 6550′s cathode resistor and bypass capacitor.
- Always suspect the 470 ohm current limiter on pin 2 of the OC3.
- The four section Filter Capacitor is crucial and should be checked.
- Clean the tube pins and check the sockets.
- Clean the volume potentiometer.
One of the biggest complaints with Leslie Amps is distortion.
Here’s some things to check;
- The second section of the filter/power supply is 420VDC. If this capacitor section is weak, even though the voltage might read right, the stability of that voltage may be such that it cannot deliver a constant source to the plates of the output tubes and thus cause distortion at a mid level volume.
- A quick test is to measure the AC voltage here. It should be below .5 volts ac. Also, while monitoring this voltage, play sound through the Leslie. If the AC voltage rises dramatically (several volts), then this filter section is weak.
- Often over looked, the preamp tube should be checked and the plate voltages also checked.
- The 15″ woofer should be 16 ohm.
- Again the cathode voltage on the 6550 should be 25VDC.
- Remember these amps are only 40 watts and can easily over driven. If all things check out and you still have problems you might have a transformer problem. This is hard to check in many cases and might just require replacing it and looking for any improvement. Power Supply transformers are usually OK if the power supply voltages are right. The Choke transformer can be bypassed as a TEST to see if it’s opening up or having a short internally. The Output transformer can cause distortion but is not easily checked. You can measure the windings and test for a proper center tap through resistance checks. These transformers don’t usually give any problems and are still available from Hammond Suzuki.
- The voltages listed on these schematics may seem a little low in reality. That’s because the measurments were based on our power being closer to 117Vac. Power company voltages have risen over the years and is often closer to 120Vac or more. Any raise in input power will be multiplied by the step up power transformer.
One more tip
Sometimes the output tubes will work right at first but as they warm up they might begin to over conduct causing distortion and finally blow a fuse. One visual test is to look at the outputs after they have been running for a while and see if you detect any “glowing red” color on the dark metal (plates) in the middle of the glass.
With a sensitive meter, you can monitor the input grid of the 6650′s which is pin 5. This pin should be under 1VDC. If you see this voltage climbing quickly and/or passing 1VDC, then that tube may be bad.
Leslie 147 Switching.
Relay switching in a 147 is carried out by applying 117V AC to pins 2 and 5 of the Leslie Amp. This voltage is applied directly to the relay.
This is the simplest switching method compared to the 122 style switching. Here’s how it works. Pins 3 and 4 are the AC input to the amplifier. If you jumper pin 4 to 5, then you have the relay halfway connected. When you add your Leslie switch to pins 2 and 3, you complete the circuit. When the switch closes, the relay energizes. The Leslie kit for a 147 type amp is by far the simplest. All that is needed is a 6-pin socket, and the Leslie switch. Here’s the pin connections.
Pin 1 = Ground.
Pin 2 = Relay AC.
Pin 3 = AC power.
Pin 4 = AC power.
Pin 5 = Relay AC.
Pin 6 = Signal.
As you can see from the pinout list, you will have to short or jumper pins 4 to 5, and install the Leslie switch to the un-jumpered pins, 2 and 3. It was customary by Leslie to make the pins 4 and 5 permanantly jumpered while adding the switch to pins 2 and 3.
This information is for educational purposes only and no claims are made that this information will lead to any successful repair. Benton Electronics assumes no responsibility to its use.
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|Servicing the Leslie 122 Amplifier|
|Servicing the Hammond Vibrato Scanner|