The Vox AC30 is one of the most iconic guitar amplifiers in history. Born in the late 1950s at the British company JMI (Jennings Musical Industries), it helped define what we now immediately recognize as the classic British sound. The original design, based on an output stage using four EL84 tubes, has gone through more than half a century of evolution and numerous modern versions that retain its general concept while adopting different circuits and construction techniques.

In this article I cover two repairs carried out on modern-production Vox AC30 amplifiers, differing both in version and construction. The first concerns an AC30CCH, the head version, which arrived at my workshop because of an intermittent fault that caused the mains fuse to blow apparently at random. Later on, we will look at another version of the AC30, this time in combo configuration with built-in speakers.

The unit shown in the photographs is therefore a Vox AC30CCH. While checking the tubes, I quickly found something suspicious in the GZ34 used as the rectifier. When testing it on the tube tester, occasional internal sparks could be triggered simply by lightly tapping the glass with a finger. It was therefore an intermittent fault, perfectly consistent with the behavior reported by the owner.

The faulty GZ34 was therefore the main suspect behind the random fuse failures. This type of fault is also a good example of how important tube quality and reliability can be: a rectifier operates under far from gentle conditions, and an internal defect can easily cause overcurrent conditions and apparently inexplicable malfunctions.

I therefore replaced the faulty GZ34 with a new Tung-Sol. Among currently manufactured tubes, it is a brand I am happy to use when I am looking for a reliable and well-made component. In this case, the cost of the new tube was significantly higher than that of the previous one, but for such an important part of the power supply I prefer not to make a choice based solely on price. A reliable rectifier is essential both for the proper operation of the amplifier and to prevent its failure from unnecessarily stressing other components.

Considering the age of the amplifier, approximately 16 years at the time of the repair, I took the opportunity to check the condition of the electrolytic capacitors as well. Four 22uF capacitors used in the main power supply filtering showed a rather high ESR (Equivalent Series Resistance) value.

These components operate under high voltages and, over the years and after countless thermal cycles, they can inevitably deteriorate. The way the amplifier is used can also contribute to stress on the power supply: for example, switching the standby off immediately after power-up subjects the circuit to different conditions compared with allowing a short warm-up period first. In this case, however, the measurements were quite clear, and I preferred to replace the capacitors before further deterioration could lead to more noticeable problems.

For the replacement I chose Mundorf M-Lytic capacitors. These are capacitors commonly associated with the Hi-Fi sector as well, but that does not mean they are out of place inside a guitar amplifier. In older tube equipment it was normal to find robust and generously rated power supply components, whereas in many modern products cost and physical size inevitably become important factors when selecting components.

Of course, the “vintage sound” of an amplifier should not be attributed to a single capacitor: many other aspects of the circuit, construction and components differ between a vintage AC30 and a modern one. Some modern versions also include circuit solutions and active components that simply did not exist in the original models. My goal in this case was therefore not to turn a modern AC30 into a vintage AC30, but to restore the amplifier using reliable components suitable for the job they are required to perform.

The Mundorf M-Lytic capacitors chosen as replacements are slightly larger than the original electrolytics. It was therefore necessary to install them with a slight overlap, making use of the available space inside the chassis. From an aesthetic point of view this may not be the most elegant arrangement possible, but electrically and mechanically the capacitors fitted without any particular difficulty and without interfering with the rest of the amplifier. In a repair of this kind, I consider it far more important to use a suitable component and install it correctly than to choose an inferior replacement simply because it has exactly the same dimensions as the original. Once the repair was completed, the amplifier was of course subjected to the usual functional tests.

Vox AC30 combo

The second Vox AC30 covered in this article is a combo version, with the speakers built into the cabinet. In this case, the fault reported by the customer was that the clean channel was not working, while the rest of the amplifier appeared to be operational.

As always, before concentrating exclusively on the reported symptom, I carried out a general inspection of the amplifier and its tubes. It immediately became apparent that one of the ECC83 tubes had an open heater and was therefore completely dead. A fault of this kind can naturally cause the complete loss of part of the preamplifier and explained why the channel was not working.

During the inspection, however, I also found a second problem, less obvious but equally important. Under one of the output tube sockets there was a typical cold solder joint, recognizable by the characteristic small ball of solder that had failed to bond properly to the terminal. The electrical contact was therefore unreliable, and this condition had eventually compromised the operation of one of the EL84 tubes in the output stage, which was by then worn out.

This type of fault is a good example of why it is important not to simply replace the tube that appears to be defective. If the cause of the problem is not identified, there is a risk of installing new components while leaving the fault that damaged them completely unresolved.

I therefore properly resoldered the defective joint, checking the socket connection and making sure there were no other intermittent contacts in the affected area. I then replaced the ECC83 with the open heater and installed a new set of EL84 tubes, restoring the output stage correctly without leaving tubes with excessively different characteristics operating together.

After the repair, I carried out the usual electrical and functional checks: both channels were once again working correctly and the output stage was operating normally. The amplifier was then left running and tested to make sure that the fault did not reappear and that operation remained stable even after reaching its normal operating temperature.

For the final test I also wanted to do something a little different from the usual instrument-based checks on the workbench. Since I am not a guitarist, I can easily check voltages, signals, power output, distortion and the electrical behavior of the amplifier, but when it comes to judging how it actually responds in the hands of someone who really knows how to play the instrument, it is certainly better to rely on someone more qualified than I am in that field.

I therefore took the Vox AC30 to Erede del Sound, a musical instrument and gear shop in Sassuolo, where it could be tested with a real guitar and I could also get practical and acoustic feedback on the result of the repair. The shop deals in guitars, amplifiers, effects and other musical equipment, both new and used, with options to buy, sell, trade and part-exchange gear.

For anyone in the area who would like to take a look at guitars, amplifiers or pedals, Erede del Sound is located at Via Refice 62 in Sassuolo (MO). In my case, it was above all the ideal place to do what I cannot fully simulate on the workbench: put the amplifier in the hands of someone who really knows how to play it and verify that, in addition to showing the right numbers on the test equipment, it also had the behavior and sonic response expected from an AC30. In the two videos below you can see and hear the amplifier during this final test.

In Slavic mythology, Zorya is the goddess of dawn. She is the one who opens the gates to the sun every morning, bringing light after darkness. I chose this name because it perfectly describes what a good microphone should do: not invent anything, not color the sound unnecessarily, but reveal what already exists, bringing it to light.

Every recording is born from an unrepeatable moment. A voice, an instrument, a nuance, an emotion. The role of a microphone is not to transform that moment into something else, but to capture it as completely as possible.

Zorya was born exactly from this philosophy. As many people know, I have never liked copying. In the audio world there are entire markets built around replicas of historic equipment. In the field of valve microphones the situation is even more evident: there are now hundreds of Telefunken U47 clones.

The curious thing is that none of them truly sounds like an original U47. Some modify the circuit, some use valves completely different from the historic ones, some use cheap television pentodes such as the EF80, adapted as best as possible for audio use. And then there are the output transformers, often made according to criteria that have more to do with cost reduction than with sound quality.

The truth is that copying is the easiest road. It is the road of those who need a reference because they are unable to create something of their own. I wanted to build a microphone that was only itself. A microphone that no one could constantly compare with an original from the past. A microphone with its own sonic identity. An SB-LAB microphone.

To start from solid foundations I chose a K47 capsule, one of the most appreciated and musical capsules ever developed, mounted inside a 60 mm style body that offers plenty of space for a mechanical and electronic construction without compromise.

The choice of valve was less obvious. Instead of pentodes such as the EF80 and EF86, used in many commercial projects, I chose the 5670.

The 5670, also known through its equivalents 2C51 and 396A, is an American double triode developed for professional and military applications requiring extremely high reliability. Introduced in 1952, it was used in VHF equipment, avionics systems and devices designed to operate up to 80,000 feet of altitude. It is a valve built to be rugged, quiet, stable and with low microphonics. These characteristics make it ideal for a valve microphone.

In the Hi-Fi world the 5670 has been known for decades as an excellent valve for high-performance preamplifier stages. The better-known 6N1 is a reinterpretation of the 5670, but a far less refined one.

What convinced me was a personal memory. Many years ago Giovanni Mariani, founder of GRAAF and one of the Italian designers I have respected most in my professional life, told me that he had used the 5670 in very high-level microphone preamplifiers. For me, that was an important confirmation. If a valve is able to excel in a professional microphone stage, then it can become the heart of a great microphone. And that is exactly what it became in Zorya.

Once the electronics had been defined, there remained the component that more than any other influences the character of a valve microphone: the output transformer. Here too, I decided to follow a completely personal path. For the core I chose Supermalloy, a material known for its extremely high magnetic permeability and for its performance in very low signal level applications.

The available cores were UI type. The simplest solution would have been to use two separate bobbins, one for the primary and one for the secondary. It is a common choice in many valve transformers, but it is not a choice I share. When two windings communicate mainly through the magnetic core, the higher frequencies and microdetail inevitably suffer. Contrary to many audiophile legends, no ferromagnetic material is truly efficient at directly transferring high frequencies. The finest information passes mainly through the correct coupling between the windings. For this reason I designed and made dedicated single-slot bobbins, printed specifically for this project.

The transformer was developed with a carefully planned winding structure, keeping primary and secondary as close to each other as possible. The result is a completely proprietary transformer, characterized by about 300 Henry of primary inductance and an output impedance of about 200 ohm. This is a sufficiently low value to drive professional inputs of 600 ohm and above without problems, even through very long cables.

During development another aspect emerged that I consider fundamental. Many valve microphones, if not all of them, provide relatively low output levels. This forces the use of high gain in the following preamplifier. At that point the final result depends not only on the microphone, but also heavily on the preamplifier itself. In practice, you spend a lot to obtain a quality microphone and then entrust a large part of the sonic personality to another device.

With Zorya I chose a different approach. The microphone already provides a high output level. This improves the signal-to-noise ratio, makes the use of very long cables less critical and, above all, drastically reduces the need for subsequent preamplification.

In tests carried out with an Avalon valve preamplifier it was necessary to engage the -20dB attenuator, because Zorya’s output level is significantly higher than what is normally expected from a valve microphone. The result is that what you hear is much closer to the native sound of the microphone. To the reality captured by the microphone.

To complete the project I also developed all the support electronics from scratch. Both the internal PCB of the microphone and the dedicated power supply PCB were designed from zero. The filament supply uses an inductor-filtered regulator, while for the anode supply I chose a 6X4 rectifier followed by a classic CLC filter. A traditional, reliable solution, perfectly consistent with the philosophy of the project.

From the power supply panel it is possible to select two polar patterns:

• Cardioid
• Omnidirectional

A second adjustment is also available, called:

• Fat
• Slim

This function allows the microphone response to be quickly adapted to the sound source and to the type of voice being recorded.

Zorya’s performance did not remain confined to the laboratory. The microphone was used for several months at The Walrus Studio in Spilamberto, Modena, by Roberto Bettini, who was able to work with both the prototype and the final version. His experience was extremely positive.

I used the SB-Lab Zorya valve microphone in the studio in its prototype form for several months, and recently tested the final version.

In my opinion, the Zorya microphone is characterized by high sensitivity. It is able to capture all the nuances of both simple and complex sounds, and has a powerful output that allows excellent signal-to-noise ratios to be achieved, even when recording relatively weak sounds.

When recording, due to its high sensitivity even at very low frequencies, it is advisable to use a high-pass filter in the preamplification stage set around 40 Hz.

Using the filtering options provided, it can adapt to different types of sound sources. Among all possible applications, I recommend it for recording vocals, where it can excel with thin voices such as female voices, giving them good body and presence, and with all acoustic instruments, especially those with high registers.

The post-production treatment of the recorded tracks is straightforward and the expected results are achieved, especially on vocal tracks, without the need to use complex sound processing.

Roberto Bettini

The Walrus Studio
Spilamberto, Modena

Many words can be used to describe the sound of a microphone, but in the end the only truly reliable judge remains listening. For this reason I decided to make available some recordings made with Zorya in real contexts and without artifices intended to mask its character.

• The first recording is dedicated to an acoustic guitar. In this case the signal coming from the microphone was recorded without any treatment. No equalizer, no compressor, no plugin, no software processing. What you will hear is simply the sound produced by the guitar and captured by Zorya.

• The second recording is a sung vocal test. Here too, the goal was not to create a finished production, but to allow the microphone’s behavior to be heard on one of the most critical and important sources of all. The voice is often the definitive test bench for any studio microphone.

• The third example comes from a complete recording performed by a Vasco Rossi cover band. Here Zorya is used within a real musical production, allowing its behavior to be evaluated in the context of a complete mix and not only on an isolated source.

For those who wish to carry out a more accurate listening session using a Hi-Fi system or reference headphones, the original files are also available for download in lossless FLAC format. Inside the ZIP archive you will find the three complete recordings used on this page.

>>> Download Zorya Microphone Demo Files <<<

The invitation is simple: listen, compare and let your ear be the judge. In audio, technical specifications are important, but listening always has the last word. But there is an aspect I consider even more important. Zorya is not just a prototype or an exercise in style.

A complete album has already been recorded with this microphone at The Walrus Studio in Spilamberto, Modena, and was later also released on vinyl. It is therefore not a project that remained on paper or confined to the laboratory, but an instrument that has already faced real professional recording sessions, contributing to the creation of a record intended for the market. For those who wish to hear a real example taken from this work, one of the album tracks is available on YouTube: https://youtu.be/dclP4Dq6hoY

Other professional productions and several bands have subsequently used Zorya during their recording sessions, confirming its versatility and its ability to adapt to very different musical contexts.

Zorya under the lens of the instruments

Real recordings remain the best method for evaluating a microphone, but I believe it is also correct to show some instrumental data collected during the development of Zorya. The following graphs are not intended to replace listening, but they allow a better understanding of the design choices that led to the final sonic result.

The first graph shows the bandwidth of the entire audio stage, including valve and output transformer. This test makes it possible to verify the frequency extension and linearity of the circuit, highlighting how the proprietary transformer developed for Zorya is able to correctly transfer both the lowest frequencies and the finest information present in the mid-high range.

The second graph shows the harmonic distortion spectrum. More than the absolute value of distortion, which is particularly low for a valve microphone, it is interesting to observe the distribution of the residual harmonics, present at particularly low levels. The harmonic structure contributes significantly to the perceived timbre of the microphone and represents one of the elements that differentiates a valve design from a completely solid-state implementation.

The third graph shows the acoustic response of the microphone in Flat configuration, without the intervention of the filters and equalizations selectable from the power supply. This measurement makes it possible to observe the natural behavior of the capsule and electronics, providing an objective picture of Zorya’s original voice before any adaptation to different sound sources.

As always, I invite you to interpret these data as a tool for further insight and not as a point of arrival. A microphone is not heard with the eyes, and no graph can ever replace what happens when a voice or an instrument actually starts to sound in front of the capsule.

Zorya represents my idea of a valve microphone. Not a copy of a classic. Not a reinterpretation of someone else’s project. A microphone designed, built and developed entirely in SB-LAB, starting from a blank sheet and arriving at a product capable of finding its own voice. If you would like to listen to it, try it in your studio or receive information about purchasing it, you can contact me directly. I will be happy to tell you about Zorya and, above all, to let you hear it.