A few years ago I got the chance to have a set of MFA Luminescence Preamplifier. I call it a set because it is a two-chassis preamp; one power supply and one amplifier. In the power supply chassis there are two tubes, one EF184 (6EJ7) and one 6L6GC. In the amplifier chassis there are ten tubes: four 5691(6SL7), two 6EM7, two 6SN7 and two 6DL7.
The circuitry is hand-wired on soldering frame. If you search the internet, you can see the internals of the Luminescence. It is a very "hand-made" preamp as far as I know; since my Luminescence is different from the internet pictures of the amp. The schematics online also have many versions, and they are different from mine. I don't think it was "modified" in the middle since some differences are big. However, their circuitries are pretty much the same.
The power supply chassis has four transfomers, three for heaters and one for high voltage. All the elctrolytic caps in my heater supply are gone. The markings are 10000uF and 15000uF, all dried out and need replacing. There is a big 6.3V zener for 5691 heaters, that is gone, too. The high volatge regulator is using EF184 as error amplifier and 6L6 as current pass device, it gives 310V output. Apperently the HV works fine, but there is another transistor regulatorin series after the tube regulator, This transistor regulator is using two resistors and two capacitors as volatge divider and just one transistor as pass device. I removed that. In the tube regulator there is a zener as reference voltage. The rating is 82V but it measures 42-43V. It is sitting under the cathod of EF184. I wonder the current for the zener is too low to turn-on the zener so it is not working as a reference voltage. Since the output has a VR to adjust the voltage, the output voltage is alright.
Checking the plate resistor of EF184, I found the current going through EF184 and zener is about 0.25mA. It is not enough the turn-on the zener. Although bigger plate resistor can help on the gain of the error amp (EF184), the zener beneath it is not working properly. I reduce the plate resistor to 300k and the zener voltage goes back to 82V. Of course, I adjusted the output voltage resistor to make it 310V as before.
I removed the big 6,3V zener for heater. It is a TO-3 device but mine doesn't work. The heater voltage was very low, around 5.1V. after adjusting the resistance in the heater circuitry, I raised the voltage to 5.9-6V.
The heater voltage is wired negatively, I should say it is -6V, The negative voltage is used as bias voltage supply for 5691(6SL7). I replaced the resistors and capacitors in the bias dividers, just for the peace of mind.
By the way, I don't have four 5691s for this monster. I am using 6SL7 and the heater current is half of 5691.
It works fine now. I am using it to enjoy the music. No hum, no buzz. I am very happy with the sound.
