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.
