Preemphasis for an EC92 electron tube FM radio transmitter

At your risk, and at your responsibility.


Note:
160KΩ x 470pF is equal to 75μs.
75 microseconds is the FM broadcasting specification preemphasis so that there is nice treble, no bass or mid heaviness, ie flat frequency response from transmiter to (deemphasised) FM radios.
So similar combinations of resistor in parallel with capacitor and the total,in series between music source and modulated oscillator are tried on these experiments.


An IM-81/UP, indicator standing wave chassis with its transformers, chokes were used as power supply and housing for an EC92 oscillator. A VR150 electro tube for stabilasation of voltage. Paper in oil capacitors for filtering. 6X5 rake train tube for rectifier.


Plate supply is about 75V.


Radios have deemphasis (reduction of treble) do premphasis (boost of treble must be used at transmission).


A (variable) resistor in parallel with a capacitor (heathkit capacitor box) was used in series with the Sony CD Walkman line out.


Just by listening and comparing the sound to nearby FM radio stations values of
R=140KΩ and
C=1.4nF
were found suitable as a first approach.


On a Jostykit HF65 transistor FM oscillator disabling the input transistor and feeding directly to the oscillator transistor values of
R=160KΩ
C=390pF
and a series 24nF capacitor were chosen to feed the base.
Frequency response had been measured to be flat.


Then this filter was connected to the EC92 transmiter oscillator. It sounded very nice.


Sound is not as loud and bug as the nearby commercial radio stations. Possible cause under modulation by signal lost at the series resistor.


So it was decided to connect instead of an active device an expensive Altec voltage input transformer. Impedance step up ratio from 600Ω to 15ΚΩ. An Altec Lancing 15095 purple color octal base line transformer.


Sound became big and as loud as the commercial radio stations.


Wish I knew this when I was a fourteenager.


Signal path:


To Have and not to Hold - Madonna CD - Sony Walkman line out - Altec 15095 wired as 1:4 - (170KΩ,1nF) preemphasis (treble boost) filter - one active device exciter or FM oscillator


Later addition:


Monitoring is with a 70s vintage JVC 9425W FM radio connected to the Pleiades linoleum Auratone 5C monitoring speaker with a Tesla 5in fullrange speaker


While Vicky had been listening to a Jobim CD thought this setup system she adjusted the Heathkit capacitor box to 470pF for a more bright sound and more depth as she described it. This is effectively the already existing 390pF fixed capacitor in parallel with 470pF.


Vicky commented that the sound is very nice, big and clean with the Jobim CD compared to the Madonna CD. A reason may be that the transformer ratio is large so that the Madonna's CD (which is louder anyway) over modulates the transmiter. Transormers of lower turn ratio may be tried too.