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.
Very few SE transmitters were made but I remember buying a very much modded ex Army tranciever known as the "22 set" which had just one 807 as its RF output tube, and might have have used the audio 6V6 output for a modulator. But its now 50 years (1962) since I had that set which perished because the roof leaked very badly in the shack where I'd had it. Roof was made of "Malthoid", a horrible tar impregnated cardboard meant for sarking under roof tiles, and not meant for the roof itself, but in 1946, people used all sorts of stuff because better roofing wasn't available, and everyone was so penny pinching, including my dad, even though he was a vet. They wouldn't let him fight, so he was well off during WW2, but the shack was temporary because he assisted a number of Euro migrants for 10 years after the war - long story, and shack with mud walls is still there, but timbers are a bit wrecked after the 10 years of wet and rot.
I lost all interest in nerdy electronics by around 1964. I repaired the roof in about 1976, using some new "Fibro" sheeting in the form of diamond pattern roof times over the rafters and pine lining boards. Shack was real hot in summer, cold in winter. I cleaned out all the rusty old rubbish. I'm a bit rusty on RF stuff now, but I did at one stage have a separate receiver to listen to a friend on one F and using headphones, while using another F to transmit on, so it was like a telephone. I recall wrecking at least one old vintage radios to make a transmitter using one 6V6 and its old radio OPT to modulate the cathode of a 6V6 used as an RF amp which used a spare RF SW coil and one tuning gang, which oscilator signal taken from the existing mixer oscilator. In 1962 I guess my mate and I were ready for WW3 and could set up emergency communications if we had access to 240Vac, which of course wouldn't have been available if Russia had let off a 1 Megatonne bomb a kilometre above Sydney. WW3 was in all our minds because it wasn't long after the Cuban missile crisis. Then JFK got shot, and we thought anything was possible........
But i don't recall anyone using suppressor grid modulation. Real men used plate modulation because then the modulator power adds to the power made by the RF amp. My friend bought a "gigantic" ex army set which sat on a pair of empty 44 gallon drums, with 6 pull-out rack units and 807 modulation tubes plus 2 x 813 RF outputs working in 2,500Vdc. But he too secumbed to the distractions of the world, and the shed his father built also rotted to peices while he spent years living as a hippy up north. That set collapsed through the rotting floor of the shed, laid on its side in mud for 15 years, then stashed under the main house. His parents eventually died, and I heard he's got the set working again, after settling down at age 58, without completely secumbing to the ganga. I now prefer bicycles to motorcycles, and oh how life has changed in 50 years, but I do have a collection of radios and transmitters I was given after one ham I knew died about 7 years ago. If ever I move up from AF to RF, I have a pile of nice old communications stuff to repair/restore/get working etc.
But In 1994, I did make myself an RF gene with a 6BX6 used as RF amp and with control grid 1 modulation. It uses a fairly large value Rk bypassed for RF, but not for AF, thus Ia change is linearized and the modulation was fairly linear up to 80%. I later added an AF detector which then fed the AF detected back to a differential amp with 2 triodes used for audio amp so that there is NFB, and this made 95% modulation possible with maybe only 1.5% THD to the audio envelope, for all AF between say 20Hz and 3kHz, enough to test many old radios.
I even added a saw tooth generator to vary voltage across 10 x 68V 5 watt zener diodes used as varicaps for the oscilator. Used properly, the zeners make good varicap diodes capable of the high voltages found in grid circuits of tubed oscillators. I can get +/- 40kHz at 455kHz centre F.
Patrick Turner.