In recent years there has been a real rediscovery of valves originally developed for the television industry. Many electronic tubes designed to operate in the horizontal deflection stages of color televisions from the 1960s and 1970s actually have extremely interesting electrical characteristics for audio use as well. The PL508 is one of them. Although it is almost unknown in the traditional hi-fi world, this valve offers a very unusual combination of ruggedness, current capability and low internal resistance, making it ideal for building powerful, dynamic valve amplifiers with a very distinctive sonic character.
This project was born from the desire to build a different kind of push pull amplifier, avoiding the usual EL34, KT88 or 6550 valves that are now seen everywhere. The idea was to use a forgotten but technically excellent valve, operating it in triode connection to obtain a more natural, linear and refined sound. The final result is a stereo push pull amplifier of about 20W RMS per channel, capable of combining excellent measured performance with a very musical and surprisingly modern tonal balance.
The PL508 was designed to operate as a line output valve in color television receivers from the 1960s, practically the smaller sister of the better known PL519 and PL504. It had to drive the convergence coils required by the early shadow mask picture tubes and the frame coils used on wide neck tubes. It is therefore an extremely rugged valve, designed to withstand very high pulse voltages and heavy working currents for thousands of continuous hours.
The PL508 can withstand positive pulses of up to about 2,500V and its heater was designed for 300mA series connection, typical of universal televisions without a mains transformer. Although its original purpose was completely unrelated to audio, this valve was already classified by the manufacturer also for AF use and shows very interesting characteristics when used in hi-fi amplifiers.
I was asked for a premium schematic and, since the person had an open mind and was not interested in yet another already seen amplifier, I decided to use one of the many forgotten valves that have excellent electrical characteristics but are systematically ignored by DIY builders simply because they are not well known.
In terms of size and dissipation, the PL508 vaguely resembles a 6V6, but electrically it is a completely different valve. It has a much lower internal resistance, a far greater current capability and an internal structure designed to work under very demanding conditions. In triode connection it can dissipate about 15 watts, showing very linear and predictable curves. In an audio amplifier, these qualities translate into better control, dynamics and drive capability.
One of the most interesting aspects of this valve is precisely its behavior in triode connection. Many television valves, once triode connected, become difficult to manage or retain characteristics that are too “hard” for refined hi-fi use. The PL508, on the other hand, behaves surprisingly well and makes it possible to obtain an amplifier with a very balanced character, controlled bass, present mids and an extremely extended high range.
The circuit developed is therefore a push pull amplifier using triode connected PL508 valves, capable of delivering about 20 watts RMS. The goal was to keep the circuit relatively simple, but without pointless compromises, using a proper input stage able to drive the output valves correctly.
The input stage consists of a long tail cascode phase splitter built with two ECC81 / 12AT7 valves. This solution provides excellent symmetry and good voltage swing capability without resorting to overly complicated circuits or excessive global feedback. It is followed by a buffer stage made with the two sections of an ECC82 / 12AU7, necessary to correctly drive the input capacitance of the output valves and maintain stability even at high frequency.
The amplifier uses a total of 10 valves. The output valves work with fixed bias, adjustable by trimmer, a solution that allows the operating point to be optimized and the PL508 characteristics to be used to their full potential. The power supply section is deliberately classic but robust, with a bridge rectifier followed by the indispensable CLC filter, ensuring clean anode voltage and very low ripple.
Below you can see the premium schematic of the project.
First photos of the unfinished build made by “S.C.”
From the first bench tests, the amplifier already showed very promising behavior. The measured power was 19.7W RMS undistorted, perfectly in line with the initial expectations. The most interesting aspect, however, is not so much the maximum power, but the ease with which the circuit maintains cleanliness and stability even close to clipping.
Later, S.C. brought me the completed build so that I could carry out the final measurements and the last optimizations around the global feedback network. At this stage the work focused mainly on the balance between frequency extension, stability and damping factor, trying to preserve the musical character of the amplifier without turning it into something sterile or too “laboratory like”.
After several months, the project was finally completed with its definitive enclosure.
The final appearance deliberately recalls classic custom-built valve amplifiers, leaving the valves and transformers clearly visible. The result is a device with a very personal look, far removed from the usual standardized commercial kits.
The measured performance figures are:
Maximum undistorted power: 19.7 Watt RMS
Overall THD @ 1 watt: 0.16%
Bandwidth @ 1 watt: 10Hz – 110khz -1dB
Damping factor DF: 5.0
The measured performance is definitely remarkable, considering that this is a valve push pull amplifier using triode connected television output valves. The extremely wide bandwidth demonstrates the excellent quality of the output transformers and the overall stability of the circuit. The damping factor is also higher than that of many traditional amplifiers, allowing better control of the loudspeaker, especially in the low range.
In listening tests, the amplifier shows a very fast and dynamic character, with dry and well controlled bass, detailed mids and a very airy high range that never becomes aggressive. The triode connected PL508 valves retain part of the “strength” typical of television valves, while maintaining an extremely pleasant musical presentation.