Magic Eye VU Meter…. What…???
Ok.. so we got the idea of what these classic beauties are and perhaps many of us would have seen and experienced them as well…
The intent behind this article however is to share the way, we at insigniaHiFi incorporated the VU meter in our Valve amplifiers. In good days of Tube radios, they used to have a so called indicator valves installed which used to glow (Fluorescence) when the circuit hits resonance on channel match. Since these tubes offered some characteristics (listed below) which were very suitable for their use as VU meter, we explored and experimented and adopted them for the same. Before going directly to them, lets have a look at how they look, work and how many types are there and of course just a look at them will make you realize why so much fuss about it and why an article…
- A magic eye tube is an electronic vacuum tube that provides visual indication, usually in the form of green light, on an area called the target inside the tube. The target is partially illuminated with the exception of the shadow area, which varies in size and shape depending on the signal applied to the tube.
- A magic eye tube or tuning indicator, in technical literature called an electron-ray indicator tube, is a vacuum tube which gives a visual indication of the amplitude of an electronic signal, such as an audio output, radio-frequency signal strength, or other functions. The magic eye (also called a cat’s eye, or tuning eye in North America) is a specific type of such a tube with a circular display similar to the EM34 illustrated. Its first broad application was as a tuning indicator in radio receivers, to give an indication of the relative strength of the received radio signal, to show when a radio station was properly tuned in.
- The magic eye tube was the first in a line of development of cathode ray type tuning indicators developed as a cheaper alternative to the needle movement meters. It was not until the 1960s that needle meters were made economically enough in Japan to displace indicator tubes.Tuning indicator tubes were used in vacuum tube receivers from around 1936 to 1980 before vacuum tubes were replaced by transistors in radios. An earlier tuning aid which the magic eye replaced was the “tuneon” neon lamp.
Some Images from the Web
There are over 100 different types with varied applicability & installation positing (Horizontal/Vertical). A swedish site (sm5cbw.se) lists aboput 70- different types of magic eye/Tuning Indicator tubes.
In November 2006 AudioXpress magazine published an article explaing the capabilities of Magic Eye tubes in precise DB level measurements.
Following PDF explains the technic.
Our Choice of Magic Eye for the Stereo Integrated Amplifier
We decided to go for a VU meter based on Magic eye which could be mounted sided by side with the other valves & appear integrated with the rest of the system. In order to have such arrangement it was imperative that the tube should have the meter (indication) on the front face so that it would be visible from the front rather than top and also given this arrangement, it would facilitate easy replacements.
We chose to go for Russian 6E1 tubes for this which has following suitable characteristics.