The Pleiades one electron tube EC92 FM music radio transmitter

At your risk and at your responsibility.
Such circuits can create overtones or RF harmonics.
They can also create electromagnetic interference.
Any voltage or current can be dangerous.
Take all safety precautions.


See previous posts.


Some more information may be added here.


It is derived from this schematic found in radio morning star kindly sent by www.261.gr:


http://members.home.nl/radiomorningstar/Pagina%205_bestanden/Fm1.gif
from
http://members.home.nl/radiomorningstar/Pagina%205.htm


A varicap circuit for FM modulation had been added by consulting another schematic.


Varicap is connected as follows:


Modulation input connector in series with 100nF. This 100nF in series with 10KΩ.


At the other side of 10KΩ is connected the varicap dot pin. The other terminal of the varicap is connected to ground. Varicap is BB105 or BB103.


The other side of the 10KΩ resistor is also connected to a 3-6 pF capacitor whose other terminal is connected to EC92 anode.


220KΩ is connected in parallel with this small capacitor.
10KΩ is connected in parallel with varicap diode.


This project had been abandoned for many years but it is revived recently in Pleiades after rediscovering the following very simple trick to make the sound fantastic.


If CD player line out is directly connected to mod input, the sound is horrible with bass and mid heaviness as all radios have the specification of cutting treble so that hiss or FM noise is cut.


So the transmiter must boost highs at 75μS or 50μs depending on the country where the radio had been bought.


For treble boost the very simple Pleiades (R,L) high pass filter is used. It is just a 160KΩ resistor in parallel with a 390-470 pF capacitor.
From Physics we know that R times C or resistance times capacitance makes time.
Ie 1 Farad times 1 Ohm is 1 second.
If we multiply 160KΩ times 400pF we get approximately 75μS or microseconds.


So bass heaviness and mid heaviness is reduced and treble sparkles.


This RC filter is connected in series between CD player line out and modulation in.


But signal modulation is low if a CD Walkman line out is used.


So the CD Walkman output is stepped up by a signal transformer.


Very nice sound. Big like commercial radio stations.


So setup or signal path is:


Sony Walkman CD line out - Altec 15095 step up transformer from 600Ω to 15KΩ - Pleiades (R,L) high pass filter - Pleiades EC92 FM transmitter






It may be worth trying an RF choke instead of 10KΩ load resistor so that power supply can be reduced. And trying a battery electron tube such as 1U4? so that a portable electron tube FM transmiter of too quality can be made. A fuse must be used in series with any battery for fire hazard safety.