The ECC32 / CV181: Most Musical Preamplifier Vacuum Tube?

Even though it has a similar pin layout and Octal-base as the 6SN7 family of tubes, is the ECC32 / CV181 the most musical sounding of preamplifier vacuum tubes?

By: Ringo Bones

Well-known as the output tube driver of the famed 50-watt Williamson Amplifier, the ECC32 or CV181 preamplifier or small-signal vacuum tube has such a gorgeous sound quality that’s, until now, still can’t be replicated by solid-state devices in the high fidelity audio world. As a dual triode in the 6SN7 family of tubes, it is not intended as a direct electrical replacement despite possessing a similar pin layout and Octal-base, but for situations where it does work, the ECC32 / CV181 – especially the NOS Mullard type – is one of the best sounding replacements you can use for any 6SN7 type preamplifier vacuum tube. Audiophiles admire it for really bringing music to life.

On average, the ECC32 / CV181 has a heater current that’s 350 milliamps higher than a 6SN7 tube – i.e. 600-mA versus 900-mA. The ECC32 has a slightly higher mu of 32 versus the 6SN7’s 20 and the ECC32 has a maximum plate voltage rating of 300-VDC while the 6SN7 has a higher maximum plate voltage rating of 450-VDC. On average, the plate resistance of the ECC32 at 14,000-ohms is twice that of the 6SN7’s 7,000-ohms, so it is not advisable to use high-capacitance interconnects / cables with ECC32 / CV181 vacuum tube equipped preamplifiers or it will result in a dark, murky sound. So the use of a Monster Cable M850i is out of the question.

Despite of the similar pin configuration and Octal-base, the ECC32 / CV181 preamplifier vacuum tube is not an exact electrical equivalent to a 6SN7 and should not be used to substitute a 6SN7 in preamplifier designs where the plate voltage could reach over 300 volts DC. The ECC32 may not work well in some applications where the negative grid bias is too high. Also, the ECC32 requires 950-milliamperes of heater filament current versus the 600-milliamperes required for the 6SN7 so the power supply transformer would have to be upgraded.

Despite these caveats, most knowledgeable DIY audio enthusiasts use the ECC32 / CV181 in place of the 6SN7 because the ECC32 vacuum tube has a gloriously rich midrange, smooth and open top end with very good bass articulation that, even now, still can’t be replicated using the latest ultra-wide bandwidth solid-state devices like FETs and bipolar transistors. Playback of recorded music just sounds more real with the ECC32 in a side-by-side comparison with the 6SN7 in its intended application. The 1950s era Mullard NOS ECC32 / CV181 types have a very well balanced sound from top to bottom. Great midband detail with zero harshness and never grainy – again something FETs and bipolar transistors still can’t do. The ECC32 / CV181 also have very good bass resolution but its bass is not extremely deep in comparison with the 6SN7 because of its higher plate resistance.

Despite of the vacuum tubes wide availability due to its use in pre solid state era missile and rocket telemetry gear – as in the US Department of Defense was rumored to have a 200-year supply of it during the Ronald Reagan administration despite of the overall vacuum tube scarcity in electronic stores frequented by civilians during the 1980s – the ECC32 / CV181 is only available in NOS (new old stock) from that dates from the 1940s and 1950s. Modern vacuum tube manufacturers from Mainland China and Russia and companies like Electro-Harmonix are, at present, not manufacturing their own versions of the ECC32 / CV181 preamplifier vacuum tubes.

The 6550 Beam Power Tetrode: The Black Sheep Vacuum Tube?

Despite its impressive looks and ubiquity in the hi-fi and electric guitar world, is the 6550 beam power tetrode the “black sheep” of vacuum tubes in terms of tonality?

By: Ringo Bones

Ever since 1990s era vacuum tube high fidelity audio amplifier manufacturers began designing their amps that make the output tubes visible from the outside, the 6550 beam power tetrode vacuum tube gained fame to the curious and uninitiated due to its impressive size (they are bigger vacuum tubes but they became a lot scarcer when the 1990s came). But long time vacuum tube hi-fi enthusiasts have doubts about its tonality when compared side-by-side with its more “gorgeous-sounding” counterparts – i.e. the EL34 and KT66 just to name a few – and does this mean that the 6550 is the “black sheep” of the power output vacuum tubes? But first, here’s a story on how this now “iconic and venerable” tube came to as we now know it today.

The 6550 high power beam tetrode vacuum tube was originally developed by Tung-Sol in 1955 originally for use as a servo amplifier. Given the vacuum tube’s “high-tech” origins, its probably the reason why U.S. Navy researcher named Donald Leslie used the 6550 as a cost effective power output tube for the power amplifier section of his now venerable Leslie Model 145 Organ Speaker to make it more affordable for a lot of people after he developed it back in the 1940s. The late, great guitar god Jimi Hendrix used the Leslie Model 145 Organ Speaker when he recorded the studio version of his now classic Little Wing. Thanks to Donald Leslie, the 6550 beam power tetrode gained a second life as an audio amplifier output tube instead of just a part of a servo motor driver controlling those lifts that send planes from the bottom to the flight deck of aircraft carriers – or controlling the flaps of the XB-70 Valkyrie as it flies at three times the speed of sound at altitudes of 70,000 feet. 

Based on the 6L6 beam power tetrode, the 6550 does appear to be a bigger-sized version of the 6L6 only that the 6550 is designed to have more power and stability than its predecessor. The 6550 operates with a plate voltage of 600-VDC and a screen voltage 400-VDC and a plate dissipation of 35-watts. The KT88, KT90 and 6550 although not identical are often interchangeable, dependent on external circuit parameters of course. The 6550’s glass envelope was originally wider in the middle than at the top and bottom, but a straight-sided design was later introduced by General Electric and Philips. 

Despite its widespread availability via U.S. Department of Defense contractors, the 6550 has been remanufactured for quite awhile now by Electro-Harmonix. Used in the 1978 Marshall JTM 50-watt combo amp, its tonality resembles that of a well-designed solid-state guitar amp. In a Dynaco Stereo 70 type amplifier that accepts EL34, KT77, KT90 vacuum tubes – the 6550 vacuum tubes has a lively, articulate sound with a silvery overtone reminiscent of 1990s era low feedback solid-state hi-fi amps – i.e. those 3,000 US dollar French solid-state integrated amps that are raved by hi-fi audio gear reviewers to be sounding like a powerful push-pull triode connected amplifier. Hence the “black sheep” label for this “servo amplifier vacuum tube”.