EL34 15Watt Transmitter. - Page 1
Forum section: electronics / beginners

author: Simon123
author_rank: Regular Contributor
timestamp: July 22, 2018, 07:20:13 am
content: Hy!I was checking some tube transmitter schematic and found this simple one:Now i wonder if my thinking is correct:U=350V DCPrms=15WPpeak=30WP=(U^2)/RL*2RL=(U^2)/P*2RL=350^2/15W*2RL=4000 ohmsNow what i dont understand is:The tank coil and output coil have turns ratio of 3:1, which means impedance ratio of 9:1And 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.

author: vk6zgo
author_rank: Super Contributor
timestamp: July 23, 2018, 02:47:59 pm
content: Quote from: Simon123 on July 22, 2018, 07:20:13 amPlease 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 tubeManual, anoldARRL Handbook, or RSGB Manual, you may find a proper mathematical description of a tube in these operating conditions.

author: rsjsouza
author_rank: Super Contributor
timestamp: July 23, 2018, 06:12:34 pm
content: 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).

author: Simon123
author_rank: Regular Contributor
timestamp: July 23, 2018, 10:02:54 pm
content: Thanks for the tips(and nice book titles). I wasn't intending to use EL34 anyway (I don't even have it).Simon.

author: 4L1G
author_rank: Newbie
timestamp: December 23, 2021, 01:24:32 pm
content: Quote from: Simon123 on July 23, 2018, 10:02:54 pmТут 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.

author: CaptDon
author_rank: Super Contributor
timestamp: December 23, 2021, 03:46:21 pm
content: 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.

author: Gyro
author_rank: Super Contributor
timestamp: December 23, 2021, 05:55:19 pm
content: Quote from: 4L1G on December 23, 2021, 01:24:32 pmMaybe the GU50 too, if you have a 12V heater supply.

author: 4L1G
author_rank: Newbie
timestamp: December 23, 2021, 06:32:44 pm
content: Да, как мне достать 12 вольт.

author: Gyro
author_rank: Super Contributor
timestamp: December 23, 2021, 06:38:46 pm
content: Ha, yes, there are other bigger rails to worry about - just distinguishing from the more common 6.3V offerings, if experimenting with several types.

author: 4L1G
author_rank: Newbie
timestamp: December 23, 2021, 06:57:48 pm
content: 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.