I was thinking about a level indicator for balancing a multi-amp system and got to thinking that a full up VU meter seemed like overkill. Why not just use a magic eye indicator.
The idea is to take a two or more element bar graph type like the 6AL7 with one element (say the left bar graph) connected to a variable voltage controlled by a front panel pot and the right bar graph fed an amplified and rectified microphone signal.
One doesn't need to know actual dB variation just match two signals. So you play a midrange warble tone the level of which is displayed on the right bar graph. Use the front panel control to set the left hand display to the same level. Now use warble tones at other frequencies of interest and adjust the amplifier gain controls so that the right hand display matches the left hand display.
Of course if one isn't concerned with the actual dB measurement one could just use any old junk box meter movement in the same way with no more complexity or expense but the eye tube would look cool.
The idea is to take a two or more element bar graph type like the 6AL7 with one element (say the left bar graph) connected to a variable voltage controlled by a front panel pot and the right bar graph fed an amplified and rectified microphone signal.
One doesn't need to know actual dB variation just match two signals. So you play a midrange warble tone the level of which is displayed on the right bar graph. Use the front panel control to set the left hand display to the same level. Now use warble tones at other frequencies of interest and adjust the amplifier gain controls so that the right hand display matches the left hand display.
Of course if one isn't concerned with the actual dB measurement one could just use any old junk box meter movement in the same way with no more complexity or expense but the eye tube would look cool.