Lighthouse Electric Vacuum Tube Project

Build This Low Budget 17 Watts Guitar Amp
This three tube amplifier will generate at least 17 Watts output power into 8 Ohms speakers. The input stage is built on one half of the 12AT7 dual triode (V1a)and provides voltage amplification. The output voltage of this first stage is directly coupled to the phase splitter built on the second half (V1b) of the dual triode. The signal is then fed (180 deg.) phase reversed to each of the (V2, V3) power tubes. The output stage operates in "Push-Pull" AB1 setup. The output penthodes share a common cathode resistor (fixed BIAS). Negative feedback (NFB) is taken from the secondary winding of the output transformer (OT1) to the cathode circuit of the input stage. The amount of the feedback applied is only 40dB. Despite this low NFB the circuit is very stable. The input sensitivity (700mV) was kept low in order to use effects pedals without over driving the amp.

The power supply uses separate transformers for plate current and filament supply. The filament circuit uses anti-phase wiring where the center tap of the secondary wiring is grounded. The benefit of this method is reduced 60Hz hum. In fact, our prototype amplifier did not generate any detectable hum.

No Frills Guitar Amp
Fig.1 Guitar Amp Circuit

NF Power Supply
Fig.4 Power Supply Circuit.

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 wiring. Terminal strips are used for this type of assembly. A metal chassis 8" x 12" x 2", will accommodate the amplifier and power supply as well.
First, install all mechanical components, like input-output jacks, tube socket, power and/or stand-by 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. Run a 16 GA bare copper wire in the center of the chassis as a ground bus wire. Follow proper grounding techniques to avoid oscillation and hum. Grounding should follow one path to a central chassis grounding point.
Use a twisted stranded 20Ga wire for the filament connections. Run this wire close to the chassis and away from the input jack and volume pot. The speaker return (GND) wire should be connected close to the PSU common return (GND) connection. The metal chassis should have only one connection to the central grounding point. It can be trough the grounded input jack or any other hardware (screw).For safety reasons, run 18GA 600V rated wire for all line voltage (120AC) connections. Use one conductor shielded wire from input jack to volume pot and also from volume pot to control grid (pin 2). Ground only one side of each shielded cable. Example of component placement and point to point wiring is shown in Fig.3 (Bottom View). Install the cathode resistor R4 away from other parts, as this resistor will get hot. After installing all components, double-check your wiring.

Chassis Assembly
Look at the finished amp here:
"Coke Bottle 6L6CG visible
"Top View

Testing Turn over the chassis on one side, in order to make measurements under power. Support it, so it will not flip over while taking measurements. Install a 0.5 A MDL main fuse and 0.25 (250mA) MDL HT fuse. Install the tubes. Turn the volume pot fully CCW for no signal. Make sure the stand-by switch is in stand-by (off) position. Connect your voltmeter negative test-lead to the ground bus. Mini-clip test leads (Radio-Shack# 278-1160) are very useful for this measurement. Switch your meter to 200VAC. Turn on the power switch only (stand-by switch off). Measure the filament voltage on tube V1 (pin 4 & 5),V2 (pin2),V3 (pin 7).Your instrument should read at least 6VAC. 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. If this voltage is OK, turn off power and install the tubes. Turn on again the power switch with the stand-by in off position. Let the tubes warm up for two minutes. Switch your voltmeter to 500VDC. Place your positive probe on B+ (C1). Now turn on the stand-by switch and watch the meter. Do not touch anything else. Your meter should indicate instantly at least 300 VDC . If the is not the case, turn off the stand by switch immediately. There is a wiring error. Correct the problem and repeat the test. HT fuse may be blown. Do not apply power longer than is necessary to take the measurement. When everything turns out O.K., compare measured voltages to values shown below and in the circuit diagram. Allow +/- 5% discrepancy depending on tube brand and condition.
DC Voltages after tube warm-up (3-5 min.) based on B+ 307VDC
Triode V1a pin 3 = 1.5 Volts
Triode V1a pin 1 = 108 Volts
Triode V1b pin 6 = 185 Volts
Triode V1b pin 8 = 109.5 Volts
Penthode V2 & V3 pin 3 = 19.5 Volts
Penthode V2 & V3 pin 9 = 295 Volts
Penthode V2 & V3 pin 7 = 305 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. The amp is ready for listening tests.


NA Steel chassis Hammond 12" x 8" x 2" 1 P-H1441-22
P1 Potentiometer 500K audio w. switch 1 R-U5-500KA
NA 9 pin tube socket, chassis mnt. 1 P-ST9-511
NA 8 pin octal tube socket, chassis mnt. 2 P-ST8-800
NA 1/4" input jack 1 W-SC11
NA Terminal strip, 8 lug, package of 5 1 P-0802H
NA binding post 2 S-H266B
V2, V3 6L6GC power penthodes matched pair 1 T-6L6GC-MP-PV
V1 Dual Triode 1 T-12AT7-China
OT1 Output Transformer 25W P-P 1 P-TF41318
C3 Capacitor 4.7uF/350V 1 140-XAL350V4.7
C4 Capacitor electrolytic 47uF/35V axial 1 140-XAL35V47
C5 Capacitor electrolytic 470uF/35V axial 1 140-XAL35V470
C8, C9 Capacitor 47nF/630V poly-film axial 2 C-TD047-630
R4 Power resistor 240 Ohms 5W 1 280-CR5-240

Resistors (0.25W 5% unless noted otherwise):		Fuses:
R3 = 150k/0.5W						F1 = 0.5A MDL
R5,6 = 1K						F2 = 0.25A MDL
R7,8 = 470K
R9/10 = 56k/0.5W
R11 = 22K
R12 = 100k
R13 = 820R
R14 = 150R
R15 = 15k
R16,17 = 68k (optional)
Misc. parts: fuse holder, stand-by switch SPST, knob, power cord, rubber feet.

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

TR1 Power Transformer 1 553-VPS-230190
BR1 Bridge Rectifier 1000V 2A 1 583-RS207L
TR2 Filament Transformer 1 553-F6-12
C1 Capacitor 100uF/350V axial 1 C-ET100-350
C2 Capacitor 47uF/350V axial 1 C-ET47-350
R1 Power resistor 1.5k 5W 1 280-CR5-1.5K
R2 Power resistor 3.3k 5W 1 280-CR5-3.3K

penthode plate dissipation: 19 Watts
penthode plate voltage: 360 VDC
penthode screen dissipation: 2.5 Watts.
Exceeding these ratings will damage or shorten the life of the tube.
Final Notes:
If only one input is used, R16 and R17 resistor may be omitted. In that case, connect the input jack directly to the P1 potentiometer.
To improve the output tubes BIAS balance, two cathode resistors, one for each tube may be used. In that case, the resistor value must be increased to 470 Ohms. Each cathode resistor must have own 470uF blocking capacitor.
Instead of a potentiometer with a switch, a separate SPST (lighted) switch may be used.
The specified power transformers use split bobbin isolation and are class B UL recognized. With very careful wiring, a grounding with a three prong power cord is not necessary. However, for your own safety it is highly recommended.

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: power tube anode (outside) glows dark red.
Solution: BIAS voltage to low or missing. Check cathode resistor R4 and capacitor C5
Condition: power tube anode (outside) glows dark red. BIAS is O.K.
Solution: output transformer plate winding resistance too low or shorted. Replace output 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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