A customer came to me with a preamplifier purchased used on eBay, from a seller in Germany. This curious device, with a rather unusual internal design, required a thorough inspection. Before bringing it to me, the customer had asked another technician for an opinion. His behavior, as I recount in the little story that follows in this article, deserves a humorous tone to lighten the situation.

How to Trick an Audiophile with a Tube Replacement

Once upon a time, in a small mountain village where sound had a soul, there lived a man with a big heart and a passion for music named Peppino. One day, while wandering through the market of rare sounds, Peppino purchased a heavy, extremely heavy preamplifier produced by a legendary Italian company. This machine, whose weight suggested that it might contain a black hole inside, was the pride of anyone who wanted to listen to music “the proper way”. Peppino, however, being a perfectionist, decided to have it checked…

The Meeting with the Guru

So Peppino went to the famous Guru Magician, a mysterious man known by everyone as Baffone Valvolario, because of his moustache shaped like a sine wave and his reputation as a wizard of tubes. Baffone stared at the preamplifier with a gaze that looked like an X-ray scan, stroked the left side of his moustache and declared: “Dear Peppino, the tubes are all worn out! They must ALL be replaced, without exception. Price? A neat 1000€.”

Baffone pronounced the amount with the air of someone selling the secret to capture the stars. But Peppino, despite his love for music, still had a healthy dose of common sense, and a wallet not particularly fond of sudden disasters.

The Journey to the Honest Technician

So he decided to seek a second opinion and turned to SB-LAB, a humble and honest technician, a master at dealing with machines that seemed to come straight out of Tesla’s dreams. When Peppino arrived with his preamplifier, a forklift was needed to unload it. After taking a moment to massage his back, SB-LAB got to work with the trusted uTracer, a device capable of reading the condition of tubes with the precision of an oracle.

To great surprise, all the tubes turned out to be perfect, except for a lonely EF184 that was a little “tired”, probably because it was playing hide and seek with the audio signal. SB-LAB replaced it with a brand new tube whose price, 8€, was so modest that Peppino could have bought it with the coins from the coffee machine.

The Guru’s Trap

It was then that SB-LAB realized the Machiavellian plan of Baffone Valvolario. His goal was not only to empty Peppino’s wallet, but also to keep all those perfectly working NOS tubes and resell them to the next unfortunate customer. A real tube merry-go-round, a “tube circus” that would have made even the most shameless dealer blush.

The moral of the story: “Not every moustache hides wisdom, and not every tube needs replacement. Trust the one who measures, not the one who performs theatrics.”

The Problems Reported by the Customer

When the preamplifier arrived at my workshop, it presented three main issues:

1. Poor sensitivity to the remote control signals. The device could not detect the remote from more than 30 centimeters away.

2. One audio channel completely silent.

3. Request for a complete tube check.

Checking the Tubes

After testing the tubes with the uTracer, I confirmed that they were all in excellent condition, except for one EF184 that was slightly tired, which I promptly replaced. This check was the first step to ensure that the problem did not lie in the tubes themselves.

The Curiosity About the Capacitors

During the analysis of the preamplifier, my attention was drawn to the huge battery of capacitors mounted inside. Out of curiosity, I decided to remove one to examine it more closely. The capacitor, strangely enough, had been painted black. With some thinner, I removed the paint to see what was hidden underneath. I will leave the rest of the story to the photos. Anyone who wants to better understand the measurement I show on the LCR bridge can read this other article…

Investigation of the Silent Channel

The silent channel showed a lack of voltage. To identify the problem, I had to analyze the entire circuit to understand the general operation. During the inspection I discovered several technical peculiarities:

Tubes used in an eccentric way: Four power tetrodes similar to EL34 were used as diodes, with all the grids connected to the anode. In addition, two noval double triodes were used only to power two LEDs, a task that could have been accomplished with a simple resistor. I decided to remove them because they were unnecessary. In fact, the presence or absence of these tubes did not affect the operation of the device.

Separate dual power supply for the two channels: Each channel had its own voltage regulator consisting of a 6AS7g tube used as the series element, a gas regulator used as the voltage reference, and another noval power pentode used as the error amplifier. Using a power output tube in this role is rather unusual.

Fault in the voltage regulator: One of the four regulators was defective. During the repair I accidentally received an electric shock, fortunately without consequences, and discovered that the electrolytic capacitors had no discharge resistors. I discharged the capacitors using the appropriate tool, and the power supply returned to proper operation.

The Mystery of the Burned Resistor

Following the power supply path of the silent channel, I found a burned resistor. After replacing it everything seemed to work, but the question remained: what had caused the failure? The answer emerged only when I reinstalled the cover. One of the mounting screws touched an uncut component lead, causing a short circuit. I removed the board, cut the excess leads, and permanently solved the issue.

The Remote Control Problem

Once the main issues were solved, the remote control problem remained. After careful inspection, I discovered that the phototransistor responsible for receiving the signals was placed under the bottom panel of the device, pointing toward the table. I desoldered the phototransistor, routed the wires inside the device, and drilled a 3 mm hole on the front panel. Placing the phototransistor behind the hole restored perfect remote control operation even at a greater distance.

Grounding

The device had a very thin ground wire, probably around 0.5 mm in cross section, coming from the VDE inlet. This wire was not even connected to the chassis, but instead went to a switch that allowed the ground to be connected or disconnected. When the ground was connected, a resistor of several tens of ohms was inserted in series. However, the construction of the device made grounding not optional but mandatory for safe and proper operation. To improve this situation, I removed the resistor and the switch and connected the ground wire directly to the chassis using a new 2.5 mm wire, secured with an M6 screw, toothed washers, and a self-locking nut.

A small lesson. If you hear hum coming from a preamplifier or amplifier, look for the problem elsewhere, in your electrical installation. It is essential to check the grounding rod with the help of a qualified electrician. Remember that you should never disconnect the ground of your equipment.

Channel Balance

After the repair, the preamplifier still showed about a 1.5 dB imbalance between the two channels, even though the tubes measured on the curve tracer were all in perfect health, practically like new. However, manufacturing tolerances can never be ignored. This brings us back to the issue of zero feedback circuits. Adding 2 or 3 dB of feedback to a circuit can often solve problems like this without compromising sound quality or musicality in any way. But the “tube gurus” will always speak badly about feedback and will claim that by spending enormous amounts of money they select ultra precise tubes, often reusing tubes taken from other customers and resold again and again, promising perfectly balanced channels. The reality is that tubes remain tubes, and such matching can only give good results for a limited time. Moreover, many of these devices do not even include a balance control, which further complicates the situation.

Conclusions

In this article you will find only a few photos, all close-up and modified with a pixel effect to make the device unrecognizable. This is because I do not wish to promote it in any way. In addition, I want to express my opinion freely, since in just over a year I have had at least three devices of this type on my bench and have developed some reflections.

In Italy there seems to be a particular trend in the audio sector. Devices with very high prices, often in the tens of thousands of euros, that do not always offer quality proportional to their cost. They are often very large and heavy devices with a design that appears poorly optimized and with questionable technical solutions. Some of these products show problems such as hum, grounding issues, or sound performance that does not stand out compared to much cheaper equipment.

These products are sometimes praised by specialized magazines, but I wonder how much of this enthusiasm is based on objective evaluation. Personally, I believe that the price to performance ratio is often more balanced in the mid range of the market, where it is possible to find very good devices without chasing absurd prices. I encourage everyone to think carefully before being influenced by price or advertising and to evaluate products based on the real quality they offer.