Old Dial Tone

This is an advanced project


This is a somewhat controversial topic. Old Dial Tone is a generic term and it depends if you grew up (most telephone collectors are OLD!) in Bell System Central Offices or in the Independent Telephone World. It also depends on the type of Central Office that served your home.

Old Old Dial Tone was generated by a vibrating reed. That varied widely. In the 50's and 60's the example of this was the WE 701 PABX, a single cabinet on a customers premises. Usually this had 4 CO Lines and an Intercom Line, with the dial tone. The vibrating reed had been replaced by a relay, connected like a buzzer.

Old Dial Tone, if you were in a Western Electric Step Office was a motor generator "thing". The generator was an alternator (the name of the generator that generates alternating current or tone). Two tones were generated, 600 Hz and 120 Hz, these tones were not precise, just close to 600 Hz and 120 Hz. The 600 Hz tone was modulated by the 120 Hz tone, this is NOT mixing of the tones.

Independent telephone companies used 600 Hz modulated by 133 Hz or other combinations. Gradually dial tone was generated by transistorized oscillators leading to even more variations. As Touch Tone came into existence Dial Tone had to be standardized as the old dial tones had harmonics that could cause false detection of digits. Precise Dial Tone is 350 Hz and 440 Hz mixed together.

If you want old dial tone it is not easy to generate, PC's do not modulate the two frequencies, but combine them.

The Grandstream ATA HT-502, used on VOIP Telephone lines or on C*Net, has the ability to change dial tone, but will not generate true old dial tone.

Dial tone generators are few and far between unless you want Precise Dial Tone. This leaves a build it yourself circuit and those can get somewhat complicated.  To be correct you need two sine wave oscillators with the 120 Hz one modulating the 600 Hz one, easier said then done. Search on Google for circuits, but there are not many.

I have a quick and dirty one here. This is simple to build and sounds ok.

If you want to use that as a one shot dial tone look here.

I have been working on a design that is truly modulated dial tone, but have only drawn up the circuit, not done actual construction. If you want to be the first to build it, give me at a call on the Telephone Collectors Network at 1-383-4335 Pacific Time 9A to 6P. Here is the circuit:


Bill Geurts has been helping me in this project.


Note: You will need to determine the capacitors to use with the transformers to get the frequency you want.