It´s been kind of quiet on the diy front lately as I have been trying to put some isolation into the TAO of Tubes workshop before winter sets in. Not much progression in the insulation either unfortunately as I have had to work. (A necessity for buying more transformers) But the Urge took hold of me and a litle side project developed for my 13EM7 amp.
I remember magic eyes, or cats eyes as we called em´, from the Tandberg and Radionette radios of my youth. They have a magic quality VU meters and displays lack. After surfing about I decided to build a VU meter based on the Russian version of EM80, the 6N1P tube. This tube is nine pin miniature (B9A socket). Quite Cheap on ebay. It has a triode preamp, and provides two fan shaped shadows on a vertical fluorescent surface. By adjusting the gain, full light can indicate the point at which the amplifier starts clipping.
A schematic Gaby posted at Sound Garage Tales proved to be suitable. It needs between 200 and 250V anode voltage, which fit the 13EM7´s 215v. It was just a matter of hooking onto the last capacitor in the power supply filter. For a signal it needs about -2 V. Several hundred Kohms input resistance means the amps output strength remains the same. I took this of the anode on the power triode section of the 13EM7 tube, in practice this means pin 2. The heater voltage is the only problem. The 13EM7 uses 12,7 V which means I need two magic eye tubes in series, or a resistor and make some heat and loose 6 volts.
The schematic is credited to Michael at the 18Watt site
I built it exactly as described. The capacitors were bipolar 0.1uF, and I had a 500K trim pot instead of the 220k specified. I still had enough precision though. This is a simple circuit with high fun factor. I heartily recommend it!
|Grid1 voltage,V||minus 2|