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This is a page about the vacuum tube amplifier I spent most of January and early February building. This is the very first vacuum tube amplifier I've ever built. Based on a design featured in the January 1995 issue of Popular Electronics, this seemingly simple design uses two 60FX5 vacuum tubes wired in a push-pull configuration. It is powered by line voltage alone (no special high-voltage power transformers, just two 120V to 12.6V 3 amp transformers wired back-to-back for isolation purposes), which is converted to DC using a silicon rectifier, and then filtered using a NOS (no leakage) multisection 100 microfarad electrolytic capacitor rated at 350 volts (an old Cornell-Dubilier, I plan to replace it in the future with a new-manufature LLC multisection cap). From there, 120VDC goes to the 60 volt filaments of the tubes (wired in series), while a 330 ohm 5 watt power resistor (actually a 300 ohm 5 watt resistor and a 30 ohm 10 watt resistor wired in series) lowers the voltage to 92VDC for the plates/screens of the tubes. The amplifier uses four resistors (one 20 ohm 5 watt potentiometer for hum-balance control; two 47 ohm 1/2-watt fixed resistors , and a 220 ohm 1 watt fixed resistor) to lower voltages further. The original design called for the amp to be built on a large baseboard, and the power supply to be separate and wired to the amplifier using Fahnestock clips. I built my amplifier on a traditional aluminum chassis (12" x 6" x 2") with the power supply on the same chassis (a good distance away from the tubes), and added a power switch and an indicator light. I built it, and it worked on the second try (I had to resolder some connections and bend the socket pins away from each other). I think it sounds teriffic for a first project. For a journal I wrote while building this amp, click here.
Tube: (2) NOS Sylvania 60FX5 pentodes, $2.20 each
Output Transformer: Hammond 1608 8,000 ohm 10 watt transformer, $36.82
Input Transformer: AES 157 Nickel-Core Interstage Transformer, $15.95
Power Transformer: (2) Radio Shack 120 volt to 12.6 volt CT 3 amp transformers, $9.99 each
Rectifier: 4 amp 400 PIV silicon full-wave rectifier
Capacitor: N.O.S. Cornell-Dubilier four-section 60-100 microfarad capacitor (CDE#DD0474.2A) three sections rated 350 volts, fourth section rated 50 volts
Hum-Balance Potentiometer: Radio Shack (?) 20 ohm 5 watt potentiometer (schematic calls for 25 ohm 2 watt)
Fixed Resistors: (2) 47 ohm, 1/2-watt; (1) 220 ohm, 1-watt; (1)330 ohm, 5-watt (used 300 ohm 5 watt and 30 ohm 10 watt in series)
TOTAL COST: $90.87
Here's a schematic of my amplifier:
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