Whatever Happened To The Vacuum Transistor Based Audio Amplifier?

With a working prototype demonstrated by KR Enterprises in the Netherlands back in March 1997, whatever happened to the Vacuum Transistor based audio amplifier? 

By: Ringo Bones 

Back in March 1997, Riccardo Kron of KR Enterprises accompanied by his wife, Eunice, visited the Netherlands to demonstrate a power amp said to use a totally new – in the audio world that is – output device called the “Vacuum Transistor” The amplifier had only just been finished and this was also the first time that Mr. Kron had taken it on a trip abroad. The demonstration took place in the listening room of a Dutch hi-fi magazine Audio & Technick with only four people present. One of them was Dutch hi-fi journalist Peter van Willenswaard and also includes the representative of Dutch distributor Analog Audio Productions who was also hearing the amp for the first time. 

The new Vacuum Transistor device is said to be a descendent of a device developed by the Russian military for use as a servo motor driver in Russian cruise missiles. It is well known that the Russian military continued using vacuum tube electronic devices in their defense equipment because, unlike solid-state devices, vacuum tubes can recover from the high-density electromagnetic pulses that follow nuclear explosions. Even though General Electric, RCA and United Electronics had been producing ruggedized military spec reinforced anode tubes for the US Air Force since 1947, one wonders if the RCA’s Nuvistor and General Electric’s Compactron also recover from high-density electromagnetic pulses – i.e. EMPs - that follow nuclear explosions. 

Back then, Mr. Kron refused to give clear answers to any technical questions about the device; this despite his assertion that the device had been patented and the attendees’ objection that a patent means protection. Nor was Mr. Kron prepared to show any measurements or give precise operating voltages or currents. Such an approach makes life sore for your typical journalist and for a while Mr. van Willenswaard thought he’d drop the subject. But when Willenswaard heard the amplifier, he thought that it was pretty thrilling. 

Then Mr. Kron told the attendees that the Vacuum Transistor was contained in a very thick glass tube, indicating that some tubes were 100 to 200 millimeters long – surrounded by a black metal body that serves as a heatsink. The Vacuum Transistor is capable of very high peak currents at 11 amperes. No other vacuum tube device of this size at the time comes anywhere close to this value, though solid-state power devices attain such a figure easily. This could be the reason why the name “Vacuum Transistor” was chosen. 

The amplification process takes place in the vacuum inside the glass tube and the attendees’ suggestion that it uses a heater was not met with direct denial. As for the bias and operating voltages, Mr. Kron disclosed only that the device’s input had to be driven with several hundreds of volts. Though this is probably a peak-to-peak figure, even so, it would imply a very low-mu device and a tough job for the driver stage. Maybe a military spec Sovtek 6922 preamp tube, with its higher voltage rating and higher anode dissipation in comparison to its “civilian” ECC88 and 6DJ8 preamp tubes have enough grunt to drive the KR Enterprises’ Vacuum Transistor device to full power. Optimum operating temperature is said to be 60 degrees Celsius, which Willenswaard later confirmed by touching a running Vacuum Transistor amplifier. The device can easily be replaced if necessary. 

Because of the enormous current capability, the amplifier is equipped with active protection circuitry that permanently monitors the current drawn from the power supply; the absence of such circuitry would mean that, in case of malfunction, the output device would fry the output transformer and / or power supply. The amplifier as a whole is without overall feedback and is capable of delivering several tens of watts into the load connected.  

There was about an hour’s time to audition the amp, which had been warming up during Mr. Kron’s introduction of the amp’s working principles. For comparison purposes there were a 6C33 based amp that Willenswaard was reasonably familiar with and a VV32B amp from KR Enterprises that Mr. Kron had brought along. Neither of these fully attained the level of sound quality of the better 300B amps of Willenswaard’s experience. 

But the Vacuum Transistor amp blew both away and by a wide margin. The sound of the Vacuum Transistor was big but not heavy, powerful and dynamic; with lots of low-level information; voices hung completely detached in space. Most important of all, the sound was incredibly real and to an extent Willenswaard never heard before. But he had only one criticism: The highs were a bit on the cool / analytical side. Caveats aside, today’s top vacuum tube manufacturers had never made a competing product aimed to challenge the KR Enterprises’ Vacuum Transistor audio power amplifier.