Lighthouse Electric Vacuum Tube Project

Build this Budget Two Tubes 4 Watt SE Amplifier

CIRCUIT DESCRIPTION
Fig. 1 shows the 4 Watts SE amplifier with fixed bias and a SRPP driver. The input stage is a SRPP 12AU7/12AT7 with very good linearity and low output impedance. The final stage is a grounded cathode single ended PA with a SV83 penthode. The SV83 is a Russian made power penthode, similar to the EL84. However, it is not directly interchangeable with the EL84, before some small changes are made. Its original design, intended for high-accuracy video amplification, gives the SV83 high transconductance and very high linearity. This quality tube is inexpensive and commonly available. There is a Negative Feedback signal taken from the secondary transformer wiring. Feedback is taken to the cathode of the lower input triode. A 12AT7 double triode may be used for a higher overall gain.
Fig.1 Amplifier Schematic

4W SE Amp Schematic
Fig.1

Specifications:
Frequency Response: 50Hz - 15kHz (-3dB)
Input sensitivity: 0.3 V
Input Impedance: 250k
Plate Voltage: 270 - 300 VDC/50-80mA
Filament Voltage: 1A 6.3VAC or 6VDC
Power Output: 4.8 Watt Max. @ 300VDC Supply
Tubes: 12AU7/12AT7, SV83

MAIN PARTS
REF. # DESCRIPTION QUANTITY PART No: SUPPLIER
NA Hammond Chassis 13.5" x 5" x 2" 1 P-H1441-18BK3 www.tubesandmore.com
P1 Potentiometer 250K audio w. switch 1 R-V250K-N2 www.tubesandmore.com
NA 9 pin tube socket, chassis mnt. 1 P-ST9-511 www.tubesandmore.com
NA 1/4" input jack 1 W-SC11 www.tubesandmore.com
NA terminal strip, 6 position 4 P-0602H www.tubesandmore.com
NA binding post 2 S-H266B www.tubesandmore.com
V1 12AU7 or 12AT7 vacuum tube 1 12AU7 www.triodestore.com/12au712ec615.html
V2 SV83 1 SV83 www.triodestore.com/sv83.html
TR2 Output Transformer SE 1 P-T31 www.tubesandmore.com
C5 capacitor 10uF/350-450V 1 C-ET10-450 www.tubesandmore.com

OTHER PARTS:
Resistors (0.25W 5% unless noted otherwise):
R1 = 10K/1W
R2 = 100R/1W
R3 = 820R
R4 = 180R
R5, R6 = 1K
R7 = 470K
R8 = 10K
Rs = series resistor 470 Ohms 2 Watts (see note* below)
 
Capacitors:
C1 = 0.22uF/250-400V film cap
C2 = 0.1uF/400-630V film cap
C3 = 47uF/16V electrolytic
C4 = 220uF/16-25V electrolytic
C5 = 10uF/350-450V electrolytic
C6 = 220pF/100-250V ceramic
 


WARNING POTENTIALLY LETHAL VOLTAGES MAY BE PRESENT!
Do not touch any part of the device with bare hands while under power. Filter capacitors may carry high voltage charge long after power was turned off. 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.

SKILL LEVEL: Intermediate.
Requires High Voltage, vacuum tube circuitry knowledge. Schematic reading, good soldering and wiring skills. Basic voltage and resistance measuring skills.

Construction
The amp circuit can be assembled using the traditional point-to-point soldering. Terminal strips (Radio Shack# 274-688 or tubesandmore.com)are 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. If at least one of the transformers is fully enclosed, then they may be mounted in a line, next to each other. Otherwise they need to be placed perpendicular to each other.
Use enough terminal strips, 1.25"-1.50" apart, to accommodate all power supply and amplifier components.
Start with power supply assembly first. Install the series resistor 'Rs' on a separate terminal strip (not shown in picture). This resistor may get warm. 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 V1b control grid (pin 2). Ground only one side of the shielded cable.
Solder the penthode grid-resistor R6 directly to the tube socket. Run the other end to the strip terminal. Install a short wire jumper from pin 6 to pin 3 on the V2 (output)tube socket.
After installing all components, double-check your wiring.

Set-Up and testing
Install a 0.2 A MDL main fuse and 0.1 (100mA) AGC HT fuse. Install the tubes. 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)= 4.2 Volts
Penthode plate (pin 6)= 255 Volts
Penthode screen (pin 7)= 190 Volts
Triode cathode (pin 3)= 3.3 Volts
Triode plate (pin 1)= 92 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
This type of configuration applies to a transformer without center tap.
Conventional Power Supply
Fig.3

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

NOTE:
*the series resistor [Rs] determines the B+ output voltage and should be sized accordingly. Alternatively, the series resistor may be replaced by a 60mA choke.
*Make sure, that the penthode screen voltage (pin 9) does not exceed 200 VDC!

The power transformer TR1 must provide at least 60mA continuous DC plate current and 1000mA 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. For a 300V plate supply, a 4.5V BIAS (cathode) voltage is required. Alternatively the series resistor should be increased or decreased in order to adjust the voltage to the correct value.
R20 is the bleeding resistor.

POWER SUPPLY PARTS
REF. # DESCRIPTION QUANTITY PART No: SUPPLIER
TR1 Power Transformer, Fig.4 1 P-T269EX www.tubesandmore.com
TR1 Power Transformer, Fig.3 1 P-T261G6 www.tubesandmore.com
C20, 21 capacitor 47uF/350V axial 2 C-ET47-350 www.tubesandmore.com
R20 bleeding resistor 150k 1W 1 R-E150K www.tubesandmore.com
L1 Choke (optional) 1 P-T154M www.tubesandmore.com

Other PSU Parts:
BR1 = rectifier bridge 1000V/1A (Fig.3)
D1, D2 = 1N4007 (Fig.4)
Misc.: fuse holder, Fu1 0.3A MDL, Fu2 0.1A AGC, rubber feet.

MAXIMUM TUBE RATINGS:
penthode plate current: 40mA
penthode plate voltage: 300 VDC
penthode screen voltage: 200VDC.
penthode screen current: 7.5mA.
Exceeding these ratings will damage or shorten the life of the tube.


TROUBLESHOOTING
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 R2.
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.


DISCLAIMER OF LIABILITY:
THESE PROJECTS ARE PROVIDED 'AS-IS', WITH NO WARRANTY OF ANY KIND.
USE OF THESE CIRCUITS SHALL BE ENTIRELY AT THE USER'S OWN RISK.
The publisher of this web site does NOT assume any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights.

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