Building A Vacuum Tube Amplifier

THE STEREO 6T9
Building a Vacuum Tube Amplifier
Copyright © 2004 by Spare Time Gizmos. All rights reserved.

Photo 1 - The Stereo 6T9 with small MP3 player

There's a magic in building audio amplifiers that I've never found in a thousand microprocessor projects. And if that amplifier happens to glow in the dark with the soft light of vacuum tube filaments, so much the better. This projectis a simple introduction to vacuum tube amplifiers. No one will accuse it of being HiFi, but it's easy to assemble using a printed circuit board, uses only a handful of readily available components and, best of all, the vacuum tubes and transformers used are cheap enough not to break the bank even if you have to buy them brand new. Output power from this stereo amplifier is approximately 4W perchannel. If you mount it in a nice enclosure and give it a pair of efficient speakers to work with it makes a pleasant amplifier for a PC sound card, or a portable CD or MP3 player. Yes, it is anachronistic to use a vacuum tube amplifier with a PC and your coworkers will probably think you're slightly off balance, but that's part of the appeal.

4/8/2004

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WarningsUnlike battery operated transistor or IC circuits, vacuum tubes can be dangerous. Not only is this project powered from the AC power line, but it contains voltages as high as 500V. These voltages can be lethal! The safest way to work on this circuit is to always disconnect the power before making any changes or connecting any test probes. Make sure that you are well clear of the circuit and anytest leads before applying power. If you must work on it while it is on, always think about where both your hands are! Follow the old radio man’s rule of keeping one hand in your pocket at all times. Remember that high voltage capacitors such as C1 and C2 can store lethal voltages for hours after the power is removed. This circuit contains a bleeder resistor, R3, which normally drains the chargefrom these capacitors within a few seconds, but never assume that it’s working. Resistors can fail, so always check the voltage on these capacitors or short them out before working on the circuit. Finally, power tubes such as the 6T9 get very hot in normal usage. You can easily get burned by touching the 6T9 envelope!

Circuit Description
Figure 1 shows the schematic for one channel of theStereo 6T9 amplifier; the other channel is identical. With the exception of V1/V2 and T1/T2, all parts numbered 1xx (e.g. R101, C104, etc) are part of channel 1 and parts numbered 2xx (e.g. R201, C204) are the corresponding parts for channel two. The circuit used is a classic two stage amplifier. The triode section of V1 serves as a voltage amplifier with a gain of approximately 10-15x, and the pentodesection is a power amplifier with an audio output of around 4 watts. If you know nothing about how vacuum tubes work, you might want to read the sidebar, "How Tubes Work," before reading further. Capacitors C101/C201 AC couple the signal to the grid of the triode section, and block any possible DC offset from the signal source that might affect our bias. Resistors R101/R201 are "grid leak"resistors - they ensure that the grid has a DC path to ground. Of all the electrons that pass from the cathode to the plate of the tube, a few will strike the grid by pure chance. Unless these electrons have some place to go, they will accumulate on the grid and cause it to build up a steadily

Figure 1 - One Channel of the Stereo 6T9 Amplifier

4/8/2004

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2increasing negative voltage. Eventually enough electrons build up on the grid to completely block any current flow to the plate, and the tube stops working. The grid leak resistors give these extra electrons a path to ground. Resistors R102/R202 develop approximately 1.4V drop because of the plate current of around 0.6mA that always flows thru the triode. By raising the voltage of the cathode to +1.4V we...