Lighthouse Electric Vacuum Tube Project

Build this Budget 3 Watt SE Amplifier with one 6BM8 Tube

The heart of the Amplifier is a single ECL82 tube. Enclosed in it's envelope is a triode alongside a penthode, the latter making up the output stage of this single-ended circuit. Input level control potentiometer at the input is optional and can be replaced with a 1 Meg Ohm fixed resistor to ground. The final stage utilizes the penthode driving the output transformer. The ECL82 is operated purely in cathode bias mode. There is a Negative Feedback signal taken from the secondary transformer wiring. Feedback is taken to the cathode of the triode.
Power Supply requirements are 250-280 VDC/45mA and 6.3V/800mA AC or DC filament voltage.
Fig.1 Tiny Amp Schematic

3W SE Amp Schematic

Frequency Response: 50Hz - 20kHz (-3dB)
Input sensitivity: 0.3 V
Input Impedance: 250k
Supply Voltage: 250 - 280 VDC/35-45mA
Power Output:3 Watt Max.
Tube: ECL82/6BM8

NA Hammond Chassis 13.5" x 5" x 2" 1 P-H1441-18BK3
P1 Potentiometer 250K audio w. switch 1 R-V250K-N2
NA 9 pin tube socket, chassis mnt. 1 P-ST9-511
NA 1/4" input jack 1 W-SC11
NA terminal strip, 5 position 4 P-0501H
NA binding post 2 S-H266B
V1 6BM8/ECL82 vacuum tube 1 T-6BM8
TR2 Output Transformer SE 1 P-T31
C6 capacitor 4.7uF/350V 1 C-ET4D7-350

Resistors (0.25W 5% unless noted otherwise):
R1 = 2.2K
R2 = 220K
R3 = 820K
R4 = 560R/1W
R5 = 33K
R6 = 33K/0.5W
R7 = 1K
Rs = series resistor 470 Ohms 2 Watts (see note* below)
C1 = 0.22uF/100-250V film cap
C2 = 0.033uF/400V film cap
C3 = 100pF/100-1000V film or ceramic
C4 = 220uF/25-50V electrolytic
C5 = 0.01uF/400-630V film
BR1 = rectifier bridge 400V/1A (Fig.3)
D1, D2 = 1N4007  (Fig.4)
Misc.: fuse holder, Fu1 0.2A MDL, Fu2 0.1A AGC, rubber feet.

Above parts you may allready have in your junk box or you can buy at your local Radio Shack store.

WARNING Potentially lethal voltages can be present. Do not touch any part of the device with bare hands while under power. After disconnecting power, wait until filter capacitors are discharged. Measure voltage with voltmeter to be sure. If you do not know how to handle high voltage circuits, do not build this device. Get help from a qualified technician.

The amp circuit can be assembled using the traditional point-to-point soldering. Terminal strips (Radio Shack# 274-688 or used for this type of assembly. A metal chassis can be purchased to accommodate the amplifier and power supply as well.
First, install all mechanical components, like input-output jacks, tube socket, power switch, fuse holders and controls. Two separate fuses, main power fuse and HT B+ fuse, are recommended. Next install power supply transformer, output transformer and terminal strips. Use enough terminal strips, 1.25"-1.50" apart, to accommodate all power supply and amplifier components. Start with power supply assembly first. Follow proper grounding techniques to avoid oscillation and hum. Grounding should follow one path to a central chassis grounding point (e.g. one of the terminal strip mounting screws).
Use a twisted stranded 20Ga wire for the filament connection. Run this wire close to the chassis and away from the input jack and volume pot. The speaker return (GND) wire should also run separate to the central grounding point. The metal chassis should also have only one (if possible)connection to the central grounding point. Use one conductor shielded wire from volume pot to control grid (pin 1). Ground only one side of the shielded cable. Example of component placement and point to point wiring is shown in Fig.2 (Bottom View). Install the series resistor on a separate terminal strip (not shown in picture). This resistor may get warm. After installing all components, double-check your wiring. Install a 0.2 A MDL main fuse and 0.1 (100mA) AGC HT fuse. Install the tube. Turn the volume pot fully CCW for no signal. Connect your voltmeter test leads to the HT B+ power supply output and ground. Mini-clip test leads (Radio-Shack# 278-1160) are very useful for this measurement. Switch your meter to 500 VDC. Now turn on the power and watch the voltmeter. Your instrument should read at least 240 VDC instantly. If is not indicating any voltage or very little, turn off the power immediately. There is a wiring error. Correct the problem and try again. HT fuse may be blown. Do not apply power longer than is necessary to take the measurement. Next, measure the filament voltage. When everything turns out O.K., apply power and check plate, screen and cathode voltages. Compare measured voltages to values shown below. Allow +/- 10% discrepancy depending on tube brand and condition.
DC Voltages after tube warm-up (3-5 min.) based on B+ 270V
Penthode cathode (pin 2)= 18 Volts
Penthode plate (pin 6)= 267 Volts
Penthode screen (pin 7)= 262 Volts
Triode cathode (pin 8)= 1.3 Volts
Triode plate (pin 9)= 100 Volts

If measurements turn out O.K, let the amp warm up for 10-15 minutes. Watch the tube for any signs of overheating. Only the filament and the cathode (center part of the tube) should glow orange. Any other part of the tube should remain dark.


Point to point wiring
Fig.2 Wiring example

Power supply shown in Fig.3
Conventional Power Supply
Fig.3 This type of configuration works with a transformer without center tap. R20 is the bleeding resistor.
*NOTE: the series resistor [Rs] determines the B+ output voltage and should be sized accordingly.

A center tap transformer is used in the power supply shown in Fig.4
Conventional Power Supply

The power transformer TR1 must provide at least 60mA continuous plate current and 800mA filament current. Cathode and screen resistors of the output tube are calculated for 270 Volts plate voltage. If your power supply has different plate voltage, the resistors should be re-calculated accordingly. Alternatively the series resistor should be increased or decreased in order to adjust the voltage to the correct value.

TR1 Power Transformer, Fig.4 1 P-T269EX
TR1 Power Transformer, Fig.3 1 P-T261G6
C20, 21 capacitor 47uF/350V axial 2 C-ET47-350
R20 bleeding resistor 150k 1W 1 R-E150K

penthode plate current: 40mA
penthode plate voltage: 300 VDC
penthode screen current: 8mA.
Exceeding these ratings will damage or shorten the life of the tube.

Condition: upon power-up a loud high pitch noise or motoring sound comes out of the speaker.
Solution: Negative Feedback is at wrong phase. Reverse the output (speaker) wires on the transformer.
Condition: sound is distorted.
Solution: check all tube voltages, compare to values indicated above. Adjust your input voltage if its to high. Replace the tube if unsure of its condition.
Condition: tube outside (anode) glows dark red.
Solution: BIAS voltage to low. Increase cathode resistor R4.
Condition: tube outside (anode) glows dark red. BIAS is O.K.
Solution: output transformer plate winding resistance too low or shorted. Replace transformer.
Condition: amp is dead, no sound. Voltages checkout O.K.
Solution: wiring error, no input signal or input shorted. Bad output transformer or speaker.

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