Lighthouse Electric Vacuum Tube Project

Build this OTL Hi-Fi Stereo Headphone Amplifier

G10 OTL Headphone Amplifier

Features:

CIRCUIT DESCRIPTION
The two-stage amplifier is built on a SRPP input stage employing a 12AT7 medium gain double-triode and a powerfull pure class A cathode follower, employing the russian made 6C19P power triode. Negative Feedback is optional and may be switched on for even less THD and lower output impedance. The 6C19 power triode is biased with two cathode resistors connected parallel. BIAS current is set for aproximately 72mA. However, this small beast is capable of delivering up to 140mA cathode current for short periods of time. Output stage has true 235 Ohms output impedance. This headphone amplifier was designed towards the Bayerdynamic DT770 Pro headphones with 250 Ohms impedance.

G10 LH-Electric Headphone Amp
Fig.1 Headphone Amplifier Circuit (one channel shown).

The most significant part of this project is the power supply, as it requires three different voltages. It has to deliver 230 VDC for the pre-amp, 92 VDC for the PA and 6.3 VDC for the tube filaments. Total power demand is 34 Watts where aproximately 32 Watts is dissipated as heat. Two versions of the PSU are suggested below. The first version requires a custom made transformer but is otherwise strait forward.

The second PSU version uses separate units for the plate voltage and filament voltage. Most of the components are ready available except the voltage regulator, which is DIY. The plate transformer TR1 is generating U1 for the PA and U2 for the pre-amp stage. A voltage doubler is producing the higher U2 voltage. For the filament supply a modified computer PSU may be utilized as shown here.
As the PSU dissipates a lot of heat, installing a cooling fan is recommended.

Power Supply Version 1
Fig.2 Power Supply Circuit Version 1.

ower Supply Version 2
Fig.3 Power Supply Circuit Version 2.


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.

Construction
The two channel amplifier circuit and the PSU can be assembled using the traditional point-to-point wiring method. However, for our prototype, we designed a PCB. We have some PCB's available for purchase (see below).
We are still working on an electronic voltage regulator PCB which may be available in the future.

Testing
It is highly recommended to test the PSU voltages with load resistors first before connecting it to the OTL Amp. Use the following load resistors for PSU testing:

U1=92V output: 635 Ohms 15W or 2 x 1.270k Ohms 7W ( for PSU version II 1350 Ohms 8W or 2 x 2.7k Ohms 4W)
U2=230V output: 47k Ohms 2W
U3=6.3V filamenrt output: 5 Ohms 10W or 2 x 10 Ohms 5W

Be prepared to do the tests rapidly as the load resistors will dissipate a lot of heat. The accuracy of the output voltages is important for the OTL performance and tube longevity. Voltages should be as follows:

92V +/- 1 Volt
230V +/- 15 Volts
6.3V +/- 0.1 Volt

Connect the load resistors to the appropriate outputs. Make sure that the correct fuses are in place. Apply power and measure the output voltages. If measurements are within limits, the PSU is ready to be connected to the OTL amp. Otherwise the voltages must be corrected.

OTL Amp Testing
Apply power and let the tubes warm up for 3-5 minutes (Stand-By ON). Next switch Stand-By switch to "operate". Immediately measure the cathode voltages of each output tube. Cathode voltage should be 17V DC +/- 1V. If not within limits, turn off power quickly and look for wiring errors, solder shorts. Otherwise watch the tubes 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.

 

AMPLIFIER MAIN PARTS
REF. # DESCRIPTION QUANTITY PART No: SUPPLIER
P1 Potentiometer 100K A audio 2 any na
NA 9 pin tube socket, PCB mnt. 4 P-ST9-222 www.tubesandmore.com
NA 1/4" phono output jack - stereo 1 S-H601C www.tubesandmore.com
NA RCA input jack 2 any www.tubesandmore.com
NA PCB stand-off 4 any na
V1, V2 Dual Triode 2 12AT7 www.tubesandmore.com
V3, V4 Power Triode 2 6C19P www.ebay.com
na PCB 1 G10 see below

	COMPONENTS: (numbers in brackets = right channel)

	Resistors (0.25W 5% unless noted otherwise):			Capacitors
	R1 (R7)  = 820k							C1  (C5)  = 0.22uF 100V plastic film
	R2 (R8)  = 180R							C2  (C7)  = 0.15uF 630V plastic film
	R3 (R9)  = 680R 0.5W						C3  (C8)  = 47uF 16V electrolytic
	R4 (R11) = 820R 0.5W						C4  (C9)  = 4.7uF 350V electrolytic
	R5 (R12) = 220R							C10 (C11) = 220uF 50V electrolytic
	R6 (R10) = 330K							C12 (C13) = 1uF 250V plastic film
	R13 R16 (R19 R22) = 10K						C14 = 10uF 350V electrolytic
	R14 R15 (R20 R21) = 470R 3W-4W
	R18 (R24) = 15K 0.5W
	R18 R16 (R19 R22) = 10K
	R30 = 4.7k 0.5W


POWER SUPPLY PARTS VERSION 1
REF. # DESCRIPTION QUANTITY PART No: SUPPLIER
TR1 Power Transformer 1 custom order na
L1 Hammond Filter Choke 1 159Q www.tubesandmore.com
BR1 Bridge Rectifier 400V 2A 1 583-RS207L www.mouser.com
BR2 Bridge Rectifier 50V 4A 1 583-RS207L www.mouser.com
C101, C102 Capacitor 220uF/200V 2 C-ET100-350 www.tubesandmore.com
C103, C104 Capacitor 47uF/350V 2 C-ET47-350 www.tubesandmore.com
C105, C106 Capacitor 4700uF/25V 2 C-ET47-350 www.tubesandmore.com
R30 Power Resistor 8.2k 1W 1 NA NA
TR1 Specification
Primary Voltage Secondary 1 Rating Secondary 2 Rating Secondary 3 Rating SUPPLIER
115V/230V 176V CT 0.2A (2 x 88V) 220V 0.04A 8V 3A Custom Order

POWER SUPPLY PARTS VERSION 2
REF. # DESCRIPTION QUANTITY PART No: SUPPLIER
TR1 Power Transformer 1 553-VPS190-230 www.mouser.com/
TR2 Filament Transformer 1 553-FD7-16 www.mouser.com
BR1 Bridge Rectifier 400V 2A 1 583-RS207L www.mouser.com
BR2 Bridge Rectifier 50V 4A 1 583-RS207L www.mouser.com
C101, C102 Capacitor 220uF/200V 2 C-ET100-350 www.tubesandmore.com
C103, C104 Capacitor 47uF/350V 2 C-ET47-350 www.tubesandmore.com
C105, C106 Capacitor 4700uF/25V 2 C-ET47-350 www.tubesandmore.com
R30 Power Resistor 8.2k 1W 1 NA NA
TR1 Specification
Primary Voltage Secondary 1 Rating Secondary 2 Rating Secondary 3 Rating SUPPLIER
115V/230V 176V CT 0.2A (2 x 88V) 220V 0.04A 8V 3A Custom Order


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.
All rights reserved. You may build these circiuts privately for your own use.
You may NOT build or sell them for commercial profit without our permission!

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