In the lab it is often useful to have a small “bench” amplifier to quickly check an audio signal, without having to power up the main system or set up a complete measurement chain. A palm sized monitor, with one input, a volume control, and a small loudspeaker, lets you immediately understand if a pickup is noisy, if a pedal distorts abnormally, if there is an intermittent contact, or if a circuit tends to oscillate. This project was born exactly with this goal: to build a micro single ended tube amplifier, simple, compact, and genuinely useful on the workbench.

It was a Sunday, the weather outside was bad, and I felt like building something quick but fun. I had in my hands a small low power transformer, salvaged from an ionizer I had built years earlier, and from there the idea for this “palm sized” tube amplifier was born. The goal was not to create a hi fi device in the strict sense, but a universal listener for guitar pedals under repair, turntable pickups, or any other audio signal. I could have achieved the same result with an integrated circuit like the LM386, but doing it with a tube is definitely more stimulating.

The result is an object that does one thing only, and does it well: turning a low level signal into something that can be listened to immediately, with a fairly neutral but at the same time “talkative” behavior, ideal for identifying noise, instability, or obvious defects.

For this project I chose the PCL84, a tube typically used in television sets, interesting because it integrates a triode and a pentode in a single envelope. This feature makes it possible to build a complete amplifier with very few components: the triode works as the voltage gain and impedance adaptation stage, while the pentode directly drives the output transformer in single ended configuration. It is a compact, economical solution that perfectly matches the idea of a bench audio monitor.

The schematic is deliberately essential and linear. The component values are not critical to the last percentage point, but the overall approach is: the tube must operate in a safe area, salvaged transformers should not be unnecessarily stressed, and the circuit must remain stable even with simple wiring. In devices this small, with short but not always shielded signal paths, high frequency behavior can easily cause trouble, so it is best to think pragmatically. Here is the schematic (click to enlarge)

The output transformer is a component salvaged from a dismantled tube radio, with a declared impedance of 13kohm on the primary and 12ohm on the secondary. As often happens with vintage transformers, these values should be considered indicative. When working with salvaged parts, it is normal to do some practical testing in combination with the available loudspeaker. In my case the amplifier drives a small 4 ohm speaker cabinet, a solution more than adequate for the intended use.

A certain impedance mismatch, in a service project like this, is generally tolerable. It is however important to observe the behavior of the output stage: if the sound becomes too compressed, if the tube overheats, or if the transformer shows abnormal vibrations, it is better to reconsider the matching. In a bench monitor it is preferable to give up some power in favor of stability and reliability.

The C4 capacitor of 560pico is connected in parallel with the primary of the output transformer. In vintage radio transformers this measure is often necessary to prevent self oscillation phenomena. This capacitor helps damp any high frequency resonances. In my case the value of 560pico proved sufficient to make the output stage behavior stable.

If, when replicating the circuit, whistles, instability, or noises appear that change when touching the wiring, the first check to make concerns the ground layout and the signal paths. Immediately after that, it is advisable to intervene precisely on C4, which in this type of build is not a minor detail but a functional element for overall stability.

The circuit was assembled on a piece of bakelite, a practical and robust solution, while the enclosure was made using a small plastic box printed in 3D. The goal was to obtain a compact, easy to handle, and safe object, with no accessible live parts. The knob, decidedly ugly and salvaged from who knows where, completes the whole and reinforces the spirit of the project: a working tool, not a display piece.