The Almarro A318B is a Japanese integrated tube amplifier that stands well apart from conventional designs. Its most distinctive feature is the use of the massive Russian 6C33S output tubes, also known by the Western designation 6C33C-B, operating in single-ended Class A configuration.

This design philosophy differs considerably from the more common single-ended amplifiers built around tubes such as the 300B, 2A3 or 845. The 6C33S features extremely low plate resistance, very high operating current and remarkable power dissipation. I recently received an Almarro A318B for a complete inspection, tube evaluation and instrumental performance testing.

Inspection and Maintenance

The amplifier showed no evident faults and no circuit repairs were required. The work therefore focused on a complete inspection of the amplifier and the condition of the installed tubes.

The service included:

• general visual inspection;
• tube condition verification;
• power supply voltage checks;
• output tube bias adjustment and verification;
• instrumental power measurements;
• extended burn-in test at operating temperature.

The original 6C33S output tubes were replaced with genuine NOS Russian tubes manufactured during the 1980s, individually tested and matched before installation.

Why Use Genuine Russian NOS 6C33S Tubes

The 6C33S is well known for being particularly critical regarding plate current stability. Many modern production tubes exhibit wide parameter variations and significant bias drift during warm-up.

Under these conditions, accurate bias adjustment becomes difficult because the operating current may continue changing even after several hours. Without active compensation circuitry, modern production tubes often cannot provide a truly stable operating point.

Older Russian NOS production generally performs much better in this respect. In the amplifier serviced here, the bias settled after approximately five minutes of warm-up and remained stable throughout a four-hour continuous burn-in test.

Actual tube life always depends on operating hours, cooling conditions, bias adjustment and operating stress. However, genuine Russian NOS 6C33S tubes from the 1980s have consistently demonstrated significantly better reliability than many modern productions and, when operated within correct specifications, can realistically provide around twenty years of normal home use before replacement becomes necessary.

An Extremely Hot Running Amplifier

One of the most obvious characteristics of the Almarro A318B is the large amount of heat it generates during operation. The two 6C33S tubes operate at high current and dissipate considerable power even with no audio signal present. Additional heat is produced by the power supply and the remaining circuitry.

During testing, the chassis reached very high temperatures. Heat also spreads to the front panel to the point that even the volume control knob can become uncomfortably hot to touch.

This behavior is not necessarily indicative of any malfunction but is simply a consequence of the amplifier’s design philosophy. Adequate ventilation is therefore essential. Plenty of free space should be left above and around the chassis, avoiding enclosed cabinets, low shelves, heat-sensitive objects and locations accessible to children or pets.

The Claimed 18-Watt Output

The Almarro A318B is commonly advertised as delivering approximately 18 watts per channel. During the service I therefore decided to verify its actual output power using laboratory measurements.

The test was carried out by applying a sine wave to the input while driving a suitable resistive load. Output clipping was monitored with an oscilloscope as the signal level was gradually increased until waveform distortion became clearly visible.

The maximum undistorted output measured approximately 6 watts RMS per channel. Increasing the drive further produced peaks of about 11 watts RMS per channel, although at this point the output stage was already operating with severe distortion and such power cannot be considered suitable for high-fidelity reproduction.

RMS Watts and “Music Power”

The advertised 18-watt specification does not correspond to the undistorted RMS power measured during laboratory testing. It is possible that the published figure was obtained under different measurement conditions, at a much higher distortion level, or by using commercial ratings similar to the so-called “music power”. Without detailed documentation describing the manufacturer’s measurement procedure, it is impossible to determine how this figure was obtained.

RMS power represents the true continuous output power delivered by an amplifier into a specified load, calculated from the effective output voltage. The so-called “music power”, on the other hand, is not a genuine technical unit of measurement. In commercial practice it has often been obtained simply by doubling the RMS power rating, producing a larger number for advertising purposes without any real increase in amplifier performance. It is therefore an arbitrary marketing definition with no practical value in laboratory measurements and unsuitable for meaningful comparison between different amplifiers.

Applying this old commercial convention to the measured values would produce approximately:

• 12 “music watts” before clipping;
• 22 “music watts” at maximum saturation.

These numbers, however, provide no useful technical information. Proper amplifier evaluation requires RMS power measured into a specified load together with the corresponding distortion level.

How Does the Almarro A318B Sound?

Regardless of the published power rating, the Almarro A318B remains an interesting amplifier with a distinctive sonic personality. Its single-ended topology delivers a rich, natural and engaging midrange. Vocals are presented with excellent presence, while the treble remains smooth and generally non-fatiguing. Thanks to the exceptionally low internal resistance of the 6C33S, bass performance is also firmer and better controlled than that of many traditional zero-feedback single-ended amplifiers.

Its actual undistorted output of approximately 6 watts RMS nevertheless requires reasonably efficient loudspeakers, especially for higher listening levels or larger listening rooms.

Maintaining the Almarro A318B

The Almarro A318B requires particular care when replacing the output tubes and adjusting the bias. Simply installing new 6C33S tubes without proper measurements may result in incorrect idle current, thermal instability, premature tube wear and unnecessary stress on both the output transformers and the power supply.

Bias should always be checked only after adequate warm-up, ensuring that the current remains stable during operation. SB-LAB provides complete servicing for the Almarro A318B and other amplifiers using 6C33S tubes, including voltage checks, bias adjustment, tube selection, laboratory measurements, output power verification and extended burn-in testing.

A properly performed inspection helps preserve the amplifier’s original performance, improve long-term reliability and prevent damage caused by unstable tubes or incorrect bias adjustment. If you own an Almarro A318B or any other amplifier equipped with 6C33S tubes and would like a professional inspection or complete service, you can contact me through the Contact page. I will be pleased to evaluate your amplifier and recommend the most appropriate solution.

The Shindo Montrachet is probably one of the best-known amplifiers ever produced by Shindo Laboratory. Built around a push-pull EL34 output stage, it perfectly embodies Ken Shindo’s design philosophy: the absence of global negative feedback, extensive use of vintage components, and a sonic approach aimed more at character and tonal coloration than at laboratory-grade measurements.

As with much of Shindo’s production, the Montrachet was built using a large number of surplus components and design solutions that would be considered rather unusual today. This undoubtedly contributes to the amplifier’s charm and unique sonic personality, but it also introduces a number of peculiarities that are worth understanding when servicing these amplifiers.

The unit that arrived at SB-LAB exhibited a behavior that its owner had known for many years: a background hum that tended to appear under certain operating conditions and had always been regarded as one of the amplifier’s peculiar characteristics. Later, without any particular warning signs, the amplifier suddenly stopped working and became completely silent.

As always, the first step was an internal inspection of the amplifier. During the preliminary checks it immediately became apparent that a 500mA slow-blow fuse had been installed in series with the high-voltage supply. The correct value was not documented anywhere, and the only indication available consisted of a handwritten note apparently added at a later date.

Considering the presence of more than 330uF of total capacitance following the CLC filter, the 500mA rating appeared rather conservative. After verifying the absence of short circuits and carefully inspecting the entire power supply, I observed that the 500mA fuse tended to fail after two or three consecutive power cycles. For this reason it was replaced with an 800mA slow-blow fuse, a solution that proved completely stable throughout all subsequent testing.

Inspection of the capacitors did not reveal any significant issues. Only a pair of electrolytic capacitors used as cathode bypass capacitors for the EL34 output tubes showed characteristics that were no longer optimal. Since these components were not directly related to the main fault, they were replaced as part of routine preventive maintenance.

All tubes were then tested using a curve tracer. The EL34s, EF86s and ECL82 were still performing perfectly. The only anomaly involved an EL84 that exhibited an unusual heater short circuit. The tube was replaced with a tested and fully functional equivalent.

Since no official schematics were available, it was necessary to proceed by tracing the circuit directly. Fortunately, the overall architecture is reasonably easy to understand. The unbalanced input signal is applied to a pair of transformers used as phase splitters, generating the balanced signal required by the following stages. Two completely independent volume controls follow, one for each channel.

This arrangement may appear unusual from a modern perspective, but it fits perfectly within the philosophy of the amplifier. In a circuit characterized by relatively wide component tolerances and the absence of global negative feedback, achieving perfect channel matching is not always straightforward, if not impossible. The presence of separate controls allows the user to directly adjust the final balance according to personal preference and the music being reproduced.

Following the volume controls, a pair of EF86 tubes per channel directly drives the EL34 output stage. The four EL34s operate with automatic bias through separate cathode resistors.

Particularly interesting is the management of the auxiliary power supplies. An EL84 is used as a dropping element to generate the screen-grid voltage for the EL34s, while an ECL82 operates as a true voltage regulator complete with reference and feedback circuitry dedicated to supplying the EF86 stages.

It is worth noting that, in this specific application, the quality of the EL84 and ECL82 has an extremely limited influence on the overall sonic performance of the amplifier. For this reason there is no particular need to seek expensive NOS examples. It is sufficient that these tubes operate correctly and meet the electrical requirements of the circuit.

Regarding the hum issue, the investigation eventually focused on the rectifier bridge. As a first step, the diodes were replaced in order to rule out possible faults. Subsequent measurements revealed a significant improvement after introducing a damping resistor immediately after the bridge and before the first filter capacitor.

This is a solution that often proves beneficial in solid-state power supplies, particularly when large filter capacitances are involved. In this specific case, the final result was achieved using a 3.3ohm resistor together with 1nF capacitors connected across the bridge diodes.

The modification completely eliminated the noise generated by the power supply, although several characteristics remain that are part of the amplifier’s very identity. A certain level of background noise is inevitably associated with the extensive use of carbon composition resistors. Furthermore, the chassis is not connected to protective earth, which makes the amplifier somewhat sensitive to contact with the front-panel controls. During testing it was possible to observe that the position of the volume controls significantly affects the behavior of the circuit, with a greater tendency for noise pickup at the extremes of their rotation and generally more stable operation around their middle positions.

Examining the circuit, it is plausible that this behavior is related to the high value of the potentiometers connected directly to the EF86 control grids and to the absence of a ground reference for the control bodies themselves. Once the repair had been completed, the usual laboratory measurements were performed. These tests required additional checks and numerous confirmations to rule out any remaining faults, but all components proved fully functional and all tubes operated within correct parameters.

The technical data published by the manufacturer are extremely limited, and the various versions of the Montrachet differ considerably from one another. For this specific version, the only reference available indicated an output power of approximately 20W, a figure that measurements effectively confirmed. This version does not employ ultralinear operation. The screen grids of the four EL34s are connected together and supplied by the circuit built around the EL84, a configuration that causes the output tubes to operate as pure pentodes. The amplifier is therefore capable of reaching approximately 20W at maximum distortion.

The genuinely clean output power available before clearly audible distortion appears is approximately 6W RMS. The measured damping factor is around 0.5. From a frequency-response standpoint, this is a classic zero-feedback design.

The low-frequency roll-off does not appear to be an oversight but rather a deliberate design choice intended to achieve a specific tonal balance while avoiding an excessively dominant bass response. This characteristic may originate from the output transformers or from the relatively modest EL34 cathode bypass capacitors, rated at 22uF. The measured response is approximately 30Hz-30kHz at -3dB and roughly 80Hz-15kHz at -1dB. Despite this, during laboratory testing the Montrachet proved fully capable of driving the acoustic suspension loudspeakers normally used for evaluation.

The measured harmonic distortion at 1W is approximately 1.4%.

Naturally, some may point out that there are amplifiers capable of delivering substantially better measured performance. Zero-feedback amplifiers such as this one were never designed with the goal of maximizing objective accuracy. Instead, the particular coloration generated by the circuit forms an integral part of their sonic identity. For this reason, measurements should always be interpreted within the context of the design philosophy that guided the creation of the amplifier.

In the absence of the original schematics, the amplifier could only be evaluated through direct circuit tracing and extensive laboratory testing. At the conclusion of the repair, no anomalies were found that would suggest the amplifier was operating differently from its original design intent. All components were found to be functional and the measured performance, while far removed from what would be expected of a more modern design, appears entirely consistent with the design philosophy and distinctive sonic approach that characterize this model.