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Cheap tube amp9/12/2023 ![]() ![]() I have designed and built many tube amps over the years from push-pull ultra linear to single-ended triode, using home-wound transformers. Luckily, the HT fuse did its job and no further damage occurred. I am pleased to say that, during the eight years since I built this amplifier, only one fault has occurred, which was an internal short-circuit in one of the output tubes. This gave 0.14% THD at 2W with an 8Ω load without feedback, or 0.007% with 26dB feedback. I could only measure the distortion without feedback (by injecting the signal directly into the grid of the input tube) because the distortion with feedback was less than that of the signal generator. If you can increase the load to between 40 and 100Ω, then you can easily obtain 50W of pure class A power. This enables the amplifier to deliver 25W into an 8Ω load or 40W into a 16Ω load, such as my full-range reflex-port loaded Lowthers. The obvious choice of output tube was the Russian designed 6C33C triode, because a single pair can deliver 2.5A into an 8Ω load from a moderate 150V rail. The simplest way to reduce the feedback to 11dB is to omit the coupling capacitors between the first and second stages.įinally, in order to drive normal loudspeakers, I decided that I needed a power rating of at least 20W. However, the advantage of a DIY amplifier is that you can adjust the feedback to suit your own taste. I have applied 26dB of feedback, which is a similar amount to most classic tube designs and sets the output impedance to 0.4Ω for well-controlled bass. However, zero feedback in this design would result in audible noise and an output impedance of 8Ω, which would severely affect the tonal balance of most loudspeakers. I know that signal feedback is a controversial issue and there are those who maintain that the ultimate goal should be 0dB. Similarly, the DC bias needs hardly any adjustment over time. To this end, I incorporated ample loop DC feedback, which - after initial adjustment - keeps the offset voltage within 20mV between tube replacements. In a long-tailed pair, the supply current is virtually DC so that the power supply is effectively removed from the signal path.Ībove all, I wanted a stable, reliable design that would not need constant readjusting. Push-pull amplification not only cancels even harmonic distortion, but also provides good rejection of power supply ripple. The pentodes provide the current drive as well as greater voltage swing than triodes.Ī general aim of the design was to have as simple a circuit as possible with a minimal number of components in the signal path as well as push-pull operation throughout (Fig. This configuration has more in common with the Futterman circuit 2, except that a long-tailed pair of pentodes is used for the driver stage instead of the concertina phase splitter. Instead, I designed a non-complementary totem-pole output stage using a novel combination of local feedback and current drive in order to achieve good symmetry and cancellation of even harmonics, as confirmed in subsequent measurements. One option was to adopt the “circlotron” circuit1, which was invented by Cecil Hall in 1951, but that precluded the use of natural current limiting and would have greatly complicated the power supply configuration. Another problem was how to realize a symmetrical output stage when tubes do not come as complementary NPN and PNP pairs as with transistors. First, in order to protect the loudspeakers in the event of a fault, it needed to be naturally current limiting without using auxiliary protection circuits. However, this concept is not easy to realize in practice, otherwise there would be more of these around. ![]() An alternative is the output transformer-less (OTL) tube amplifier. Hence, a lot of iron and copper are required in order to minimize these problems. ![]() Also, the winding resistance typically wastes 10% of the output power. The main causes of this are iron core saturation distortion and the winding inductance which bypasses the loudspeaker at low frequencies. Photo 1: Front View of the author's original OTL amplifier. ![]()
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