EL34 15Watt Transmitter.

[Simon123]

Hy!

I was checking some tube transmitter schematic and found this simple one:


Now i wonder if my thinking is correct:
U=350V DC
Prms=15W
Ppeak=30W
P=(U^2)/RL*2
RL=(U^2)/P*2
RL=350^2/15W*2
RL=4000 ohms

Now what i dont understand is:
The tank coil and output coil have turns ratio of 3:1, which means impedance ratio of 9:1
And that monopole antenna has impedance of 36 ohms.
Now RL=36*9=324, when the load for 15Wrms should be 4k.
I assume that the extra impedance we get from that variable capacitor at the bottom of antenna.
Am i correct, if not please correct me, so i don't explode everything lol.

Also i am wondering(if my thinking is correct), can i use a dipole (75 ohms) and about 6-7 turns, giving me impedance ratio of 50:1?

Thanks, Simon.

[vk6zgo]

Hy!

I was checking some tube transmitter schematic and found this simple one:


Now i wonder if my thinking is correct:
U=350V DC
Prms=15W
Ppeak=30W
P=(U^2)/RL*2
RL=(U^2)/P*2
RL=350^2/15W*2
RL=4000 ohms

Now what i dont understand is:
The tank coil and output coil have turns ratio of 3:1, which means impedance ratio of 9:1
And that monopole antenna has impedance of 36 ohms.
Now RL=36*9=324, when the load for 15Wrms should be 4k.
I assume that the extra impedance we get from that variable capacitor at the bottom of antenna.
Am i correct, if not please correct me, so i don't explode everything lol.

Also i am wondering(if my thinking is correct), can i use a dipole (75 ohms) and about 6-7 turns, giving me impedance ratio of 50:1?

Thanks, Simon.

Please don't waste  your time with this thing!

Simple self-excited single device transmitters are usually crap, a circuit where the device is a big, clunky device in a "G" series envelope intended as an audio power output tube is crap "in spades"!

The internal connections in the EL34 are long enough to exceed the inductance of L1, so the circuit will be tuned to a lower frequency than the desired 100MHz.

As well, the stray capacitance of the anode circuit will be larger than the value of C2 , again lowering the resonant point of the LC circuit.
(Back in the day, we used a "rule of thumb" for stray capacitance in video amplifiers using 7 pin & 9 pin"miniature" tubes of approx 20pf--this big octal tube will have a higher value than that).

Even if you get the thing to oscillate, it will be horribly unstable, & the deviation you will get from that varicap is unlikely to produce a reasonable output from a domestic FM receiver.

I don't think your simple impedance calculation is correct, but it's too long since I've messed with this stuff.

If you can find a "Radiotron Designers Manual", an RCA, transmitting tube
Manual, an old ARRL Handbook, or RSGB Manual, you may find a proper mathematical description of a tube in these operating conditions.

[rsjsouza]

My experience agrees with vk6zgo; the transmitter's frequency will be way too low for an efficient transmitter - if anything, you will receive the nth harmonic in your FM radio while polluting the entire spectrum from a few tens of MHz and up. Also, if you sneeze close to the circuit, it will vary and oscillate quite erratically. 

I once did a similar transmitter using a ECC81 (6AV6) as the exciter and a EL90 (6AQ5) as the main RF oscillator (a much smaller tube with a 7-pin subminiature base) and it polluted absolutely everything in its surroundings (TVs, AM and FM radios, you name it). 

I then used a book from Van Valkenburg (Basic Electronics) as a better reference to learn how to build something better. I still have it in hardcover form.

My tip: sell the EL34 at a high price tag (audiophiles have been interested in these).

[Simon123]

Thanks for the tips(and nice book titles). I wasn't intending to use EL34 anyway (I don't even have it).

Simon.

[4L1G]

Thanks for the tips(and nice book titles). I wasn't intending to use EL34 anyway (I don't even have it).

Simon.

Тут подходят лампы 6П6С, 6П3С, 6П9, 6П15П и даже Г-807, если хорошо настроить, то никаких гармоник не будет.
Here lamps 6P6S, 6P3S, 6P9, 6P15P and even G-807 are suitable, if you tune well, then there will be no harmonics.

[CaptDon]

I have found that when the plate operating load line is projected to be around 4k ohms the tank Xl and Xc are around 1/10th at perhaps 400 ohms both at resonance and the tank circuit is said to have a 'Q' of 10. I haven't built any low band amplifiers for about 30 years and was much better informed back then. An EL34 at 100Mhz? I doubt that works very well!! And not being crystal controlled (you can still F.M. a crystal with a varactor) it probably drifts from one end of the band to the other with enough spurious emissions to get you into some big trouble in the aircraft and emergency bands. B.T.W., in a PI tank circuit the 'loading' capacitor typically has an Xc variable from half the line impedance to twice the line impedance. I prefer PI instead of link coupling in my tank circuits.
 

[Gyro]

Thanks for the tips(and nice book titles). I wasn't intending to use EL34 anyway (I don't even have it).

Simon.

Тут подходят лампы 6П6С, 6П3С, 6П9, 6П15П и даже Г-807, если хорошо настроить, то никаких гармоник не будет.
Here lamps 6P6S, 6P3S, 6P9, 6P15P and even G-807 are suitable, if you tune well, then there will be no harmonics.

Maybe the GU50 too, if you have a 12V heater supply.

[4L1G]

  Да, как мне достать 12 вольт.

[Gyro]

Ha, yes, there are other bigger rails to worry about - just distinguishing from the more common 6.3V offerings, if experimenting with several types.

[4L1G]

Это я делал много раз, и отлично получается.
I've done this many times and it works great.
If the transformer has only 6 volts and suddenly you need another 12.6 volts. then without replacing it, you can get this voltage by the following train:
In addition, the relay for various control devices can be supplied with constant voltage.
-Если трансформатор имеет только б.З вольта и вдруг понадобится еще 12.6 в. то без его замена можно получить данное напряжение следующим обозом:
Кроме этого постоянным напряжением можно питать реле для разных устройств управления.