On Friday, 12 October 2012 01:44:25 UTC+11, Alejandro Lieber wrote:
Patrick Turner wrote: On Wednesday, 10 October 2012 18:48:54 UTC+11, John L Stewart wrote: 'Alejandro Lieber[_3_ Wrote: ;963267']I am thinking in building an AM transmitter for 7.1 Mhz (40 meters) using a EL34 in class C as final. The idea is to modulate it by varying the negative polarity of the suppressor grid (g3). Suppose I load the EL34 plate with 500 volts DC and a current of 150 ma with 0 volts suppressor, does anyone have any idea what negative voltage in g3 is needed to reduce the anode current by half, needed for screen AM modulation ?. Alejandro Lieber LU1FCR Rosario Argentina Real-Time F2-Layer Critical Frequency Map: http://1fcr.com.ar --- news://freenews.netfront.net/ - complaints: --- As the G3 voltage is biased negative the G2 current will increase in proportion to the decrease in anode current. That can result in a dangerous increase in screen dissapation. To prevent that, the screen needs to be supplied thru a limiting resistance. Refer to the attachment lifted from FE Terman. The EL34 will easily handle your 500 volt supply, it is rated to 800 volts on the plate. But the socket is something needs to be of good insulation. As Patrick T has said, an 807 would be a better choice. Many amatuer transmitters were built using TV horizontal output tubes which are still common. That gets around the socket insulation problem. And it better isolates the RF input circuits from the output. Less likely to have parasitic oscillations. Cheers, John +-------------------------------------------------------------------+ |Filename: Suppressor Modulation B2.jpg | |Download: http://www.audiobanter.com/attachment.php?attachmentid=305| +-------------------------------------------------------------------+ -- John L Stewart Amateurs I have known used to use 807 mainly for all their RF amps and AF modulators because they were cheap from Army disposal stores and had better insulation for anode connections. But one ham said that a pair of EL36/6CM5 ( TV line output )could make 300Watts in class C. Other octal line outputs with anode top caps were 6DQ6 and 6CD6. Then there was the 6146 which many hams used. My first ham transmitter (1969) used only two valves: a EL36 from an old TV set and a 6V6. The EL36 was a crystal controlled (3.640 Mhz) oscillator with 10 watts output, plate modulated by the 6V6 with a reactor. The microphone was an old carbon telephone one with the DC current taken from the cathode of the 6V6, and a 3.2 ohm to 10..000 ohm small transformer as voltage amplifier. No power transformer, 300 V DC directly from the 220 V AC line. I made hundreds of contacts up to 700 km. Much more fun than what I get now with my commercial Single Side Band rig. ........ Patrick Turner. -- Alejandro Lieber LU1FCR Rosario Argentina Real-Time F2-Layer Critical Frequency Map: http://1fcr.com.ar --- news://freenews.netfront.net/ - complaints: ---
I remember lots of ppl were into ham radio. But during these last years, whenever I have tuned in to someone, he was usually older than me, maybe 80, and spending the whole time talking about his latest hospital operation, droning on and on and on, and ppl were expected to wait until he'd finished, then someone else would give a big long lecture about his medical conditions. It seemed to me ham radio wasn't about really connecting to other men for a useful reason, but was about being a technical nerd and narcissist. I can tell you this mode of human existance suits many blokes who are very ill at ease when having any real deep and meaningful personal interactions with anyone else. Maybe WW2 ****ed them up pretty bad. Besides, ham radio offers a chance to escape the missus and kids while you spent 20 years in suburbia raising a family after 1 month's bliss with a young missus, then 30 years waiting to die. The ham radio shed was no place for the missus or kids, or maybe even grand kids, so often very full of cigarette smoke from the chain smoking ham, who ignored his health conditions, and expected doctors to do miracles. I think I found out early the basic trouble with ham radio is that its like listening to Castro giving a 3 hour lecture on wonderful socialism on Cuban radio, and sure, nothing wrong with socialism yet soon after it is implemented all human progress is stunted and most humans die of boredom and frustration. Mind you, listening to Romney is alarming, worse than Castro. Just as well these silly old dinosaurs didn't get into ham radio, they'd be able to drone on for hours about how they ****ed everything up, or how they were about to **** everything up, while never ever admitting how they were ****ing things up right now.
But yeah, 6CM5 plus a 6V6 and telephone mic probably could sound good and go far. Just like you need a horn speaker with a 9 watt AF amp, you need a good antenna for 9 watts of RF. I helped a few hams erect huge rotating beam antennas above their houses. Some had elements that overhung the house. Neighbours were always appalled seeing these things going up on the house next door, and they complained about TVI and RFI even when the ham wasn't transmitting.
As I mentioned in a previous post, my friend in 1962 had an 800Watt transmitter with 2 x 813 outputs. He operated as a ham for many years, a rare young man although very much a young nerd - but who had handsome features so he was a hit with the girls, and once he quite casually stole a GF from me. But having a motorcycle before I could get one was a huge boost to getting pussy. Of course I soon realised she was just another air-head deadbeat tart, and very hard to get along with like most other shielas at the time who were so often wild and bored and unable to settle down, or able to cook or even do a good BJ. Anyway, my friend had this weatherboard shed with flat corrugated iron roof. The iron and boards and door and one window were then probably 50 years old and all been taken from a small cottage that had been demolished nearby maybe in 1958. But this iron had spent its life under gum trees and settling leaves had taken their toll on the zinc. My friend's old man was a bank manager and extremely tight with money, so getting new iron was out of the question. So once the old iron was very worn and disturbed and re-used in yet another damn flat roof under overhanging trees it quickly gathered leaves and leaked a lot, and there wasn't any silicone in them days and the friend constantly battled leaks and dampness in the shed. Of course the friend wasn't too law abiding, and operated as a pirate ham by addopting the call sign of someone local who'd died about 5 years before, sufficiently long ago for most ppl to have completely forgotten he existed. But one day my friend said somebody was returning his CQ, and said "Ah John, good to hear you are back on air again, someone said you had died..." and my friend had to be diplomatic and make up a story then and there, and then say dinner was ready and the missus was calling out, so "over and out..."
My friend didn't want to learn morse code, an essential requirement to get a ham radio license. This regulation stopped hundreds of people who would have liked to become a ham radio enthusiast. Governments acted as Giant Kill Joy departments in 1960.
My friend's ex-army radio set transmissions attracted the attention of the PMG, Post Master General or post office department which also regulated and controlled all radio and TV transmissions. My friend worked as a telephone linesman in the PMG after leaving school earlier than me, and he knew about how other sections worked, and about their radio detection van which drove around to find illegal radio transmissions. Well, one day while transmitter was on, he watched the van drive right past his house. He was using the shed roof iron as antenna, about 3 metres x 4 metres, flat. Electronically, it was very poor, but with such high power the RF still went far. I recall the antenna lead from Xmitter just went out the door with large aligator clip to roofing iron, and a light globe was used to see if it was tuned right. Later he spoke to blokes in the van, and they said they'd "detected a very powerful signal in Warrangi St, but we couldn't get a proper fix on it and we couldn't see any antenna.." My friend knew his set overloaded their gear. He could hardly keep a straight face.
Patrick Turner.