Old Dial Tone
By Colin Chambers and William Geurts
Old Dial Tone is usually referred to as 600 Hz modulated by 120 Hz. There are some variations on this and it is not really precise. It is more “close” as the Central Office tone generators were motor generator sets driven from the 48 VDC Central Office battery voltage that could vary slightly and make small changes in the speed.. Compare this to Precise Dial Tone that is 350 Hz and 440 Hz. Old Dial tone is not compatible with DTMF or Touch Tone®.
Some old dial tones on PBXs and not Western Electric equipment used vibrators or vacuum tube circuits to generate dial tone. Not all of these were true 120/600 tones.
Old Dial Tone is NOT the two frequencies “mixed” together. If they were mixed it would easy to generate. A number of telephone collectors have attempted to build circuits and become discouraged. Colin designed the first trial circuit:
This “sounds like” some sort of old tone, but is not modulated and is just a single frequency. It was to be a starting point.
The circuit is a two transistor mulitvibrator, a very simple oscillator circuit. You can use a U1 (less the varistor on the back) receiver in place of the transformer to listen. The level of the tone in the receiver will be about right for dial tone when you hold it to your ear.
Here is my original “perf board” project:
This uses a 9 Volt Battery and if you hold the U1 to your ear the volume level is just right for dial tone as heard on an old phone. Put your mouse on the picture and click on it for an MP3 recording. Build it and listen, it is so much better.
For more volume, change one or two of the collector resistors from 10K to 2.2K. You cannot use the 10K res for the higher voltage supply if you do this.
Calculation of the exact frequency requires the use of some detailed formulas, so the values were based on best guess.
Keeping things simple a 120 Hz (or close) multivibrator was designed to “modulate” the 600 Hz oscillator. The depth of modulation was controlled by the 4.7 MFD capacitor, well this was the thought. Changing the value of the 4.7 MFD would control the “depth” of modulation.
The way this was to work was that one oscillator would amplitude modulate the other. Well this was the theory. In practice it did not work out. The collector resistors on the 120 Hz oscillator were lowed in value to provide more power to modulate the 600 Hz oscillator, but it still did not sound good. In theory, it should have worked.
That was the end of the simple design. However the simple 600 Hz oscillator does not sound that bad and could be usable in some applications.
Wikipedia article on Dial Tone:
Fake Answer Tone - A variation on the 600 Hz oscillator that can be directly connected to a phone line:
Wikipedia article explaining multivibrators:
Astable Multivibrator, general explanation:
Bistable Muitivibrator, general explanation:
Samples of Telephone Tones (including old dial tone):
Notes on the Parts and Circuit:
Transistors may be 2N4401 or just about any NPN or PNP (reverse battery)
Circuit is forgiving if you vary the values slightly
Collector resistors can be lowered (see Fake Answer Tone) for more output
Connect circuit in series with a 9 Volt Battery and a telephone set, do not use the transformer
Or a common emitter resistor, go off hook on telephone, you will get “Dial Tone” and
Limited talk power. Sort of a fake Central Office, works better with rotary dial phones
The transformer is about $3 and works well with line current thru it. It can be considered a
“mini” telephone repeat coil.
Cost to build this circuit should be under $12.00.
42-TU016 replaced by TY-145P from Jameco.com for $3