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.