I will play with it a bit today. Tomorrow I will go get the resistors for screen and triode. What values do you reckon I should get ?
The bulb gives you an idea of what happens inside of a TX, but it is far from a good instrument. Is +/- the difference from a neon screwdriver and a voltimeter. You know that there is some voltage or RF power but no how many. In any case it's interesant because you have an idea of the circuit is live and if you can touch it or not.
Regarding the TX. Consider that you built a small transmiter basically from junk. Nowadays info abounds on internet. But when I started ham radio in my 16's, late of 1980 i'net didn't exist so I follow what said my ancestors in the hobbie. Most of them now are exinct and in fact were over 70 for those eras. More still, most of them started their travel during 40s and 50s were we lived heavy times.
Making gross figures, 300DCV @15DCmA means 4.5DCW. Taking that the efficiency of a class C amplifier is about 60%, your small TX gives about 3RF Watts. That's no few. Old walkie talkies in our infancy gave 0.1W at 27.125MHz.
Tubes, transistors and MOSFETs have similarities, but each one also have their own whims. Tubes in general are more resistant to errors than semiconfuctors. And although theybare becoming museum parts, are funny to play with
Regarding the TX. Consider that you built a small transmiter basically from junk. Nowadays info abounds on internet. But when I started ham radio in my 16's, late of 1980 i'net didn't exist so I follow what said my ancestors in the hobbie. Most of them now are exinct and in fact were over 70 for those eras. More still, most of them started their travel during 40s and 50s were we lived heavy times.
Making gross figures, 300DCV @15DCmA means 4.5DCW. Taking that the efficiency of a class C amplifier is about 60%, your small TX gives about 3RF Watts. That's no few. Old walkie talkies in our infancy gave 0.1W at 27.125MHz.
Tubes, transistors and MOSFETs have similarities, but each one also have their own whims. Tubes in general are more resistant to errors than semiconfuctors. And although theybare becoming museum parts, are funny to play with
Try halving the value for the triode plate. Say, 47 or 56Kohm. Use 1W to be safe. For 802 screen, anode current in a pentode is +/- a fixed multiple of screen current, beween 4 to 6 times. So write down DC screen and plate voltage and resistance. Thus, compute the current and use a new resistance who give, say, 1.8 times the actual screen current. In any case choose the immediate higher resistance value, and operate it keeping in mind that this will alterate once more the tank adjustements. And after ~2 minutes in operation, and in deep darkness observe the plate of the 802 pentode. If it gets red and you are sure it is not the light reflected from other source, thus the tube is overloaded. That's no good for him. Shut down the PSU and use a bigger valued resistance. In any case about 20mA plate plus screen is a good value.
Enjoy the new toy (wirh rhime).
Ok, sounds good.
Will do, tomorrow when I get the resistors.
What would be the approximate range for such device?
I know that there are many outside factors that can drasticly change the range.
Regarding new pentode's screen dropping resistor.
Is the approach to compute it correct?
This is what I came up with, but I'm thinking that it is not correct.
Is the approach to compute it correct?
This is what I came up with, but I'm thinking that it is not correct.
Well done. Looks good. You are starting to think like an Engineer. In any case, like your tube may have several hours of running, don't expect exact results.
Regarding the range. I am not sure if you are thinking in frequency range or the area of where the signal is radiated.
The frequency must be only a very limited to the crystal frequency plus or minus 4~5 KHz. Crystals are very stable in frequency and has low tempco (temperature coefficient) but doesn't move too much in frequency range.
The area of service must be few meters around. The power is reduced and you don't have an aerial and you can't radiate it because it's illegal. You can radiate non ionizating radiations (apologies for the redundancy) with a call sign (this means you must go to a radio club and take a couse where you learn the operation of the radiostation, technical and legal knowledge, rules etc.), but including you have one, there are bands of frequencies mundially accepted where you can do it. You mustn't radiate signals anywhere you want because you can produce interferences in many other services/systems: radio, tv, aeroplane and security, etc. Over it, a basic antenna like a half wave dipole needs 150/6.5MHz = 23mts long.
RF signal has two components when radiated through an aerial. The first component is the close to the antenna and (if I remember properly), it decreases as the cube of the distance. This covers apx the first 3 or 4 wave long around the antenna. So, duplicating the distance, field strenght (S in radio jergon) decreases 8 times.
At remote points, signal strenght decreases as the square of distance. Being the behavior like k/(x^2) where k is a constant and x is distance, S doesn't dissapear completely never.
Regarding the range. I am not sure if you are thinking in frequency range or the area of where the signal is radiated.
The frequency must be only a very limited to the crystal frequency plus or minus 4~5 KHz. Crystals are very stable in frequency and has low tempco (temperature coefficient) but doesn't move too much in frequency range.
The area of service must be few meters around. The power is reduced and you don't have an aerial and you can't radiate it because it's illegal. You can radiate non ionizating radiations (apologies for the redundancy) with a call sign (this means you must go to a radio club and take a couse where you learn the operation of the radiostation, technical and legal knowledge, rules etc.), but including you have one, there are bands of frequencies mundially accepted where you can do it. You mustn't radiate signals anywhere you want because you can produce interferences in many other services/systems: radio, tv, aeroplane and security, etc. Over it, a basic antenna like a half wave dipole needs 150/6.5MHz = 23mts long.
RF signal has two components when radiated through an aerial. The first component is the close to the antenna and (if I remember properly), it decreases as the cube of the distance. This covers apx the first 3 or 4 wave long around the antenna. So, duplicating the distance, field strenght (S in radio jergon) decreases 8 times.
At remote points, signal strenght decreases as the square of distance. Being the behavior like k/(x^2) where k is a constant and x is distance, S doesn't dissapear completely never.
Looks like school did help me good this time (ohms law).
Nicely explained. Yes I was wondering about the area of service.
I was affraid that it woudn't be too big. Yesterday for example I wanted to see how speech sounds and without much thinking I played political speech of our politician... I'm politicaly neutral, but man, some people do hate that guy...
But I just wanted to hear how it sounds, not thinking about the content.
Then I started thinking, what if somebody receives this...
Ok... if I would get the call sign, I would probably invest into second hand tx / rx, but just for curiousity, to see if I'm thinking correct. If one would want to connect aerial to this tx, another coil would be have to be wound around the final tank coil and antenna would connect to one of the coil's ends, other end to gnd & aerial gnd?
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There are several ways to hook an antenna to a transmitter. It depends on antenna, the line connecting them and the transmitter itself. The most common method for tube TX is the pi tank. Other that I used but not too common nowadays is the link coupling. The line itself is of importance, as opposite to power or speakers wiring. The line at RF have a characteristic impedance [it is a non disipative resistance, sqrt (L/C) where L is the inductance / metre and C the capacitance / metre]. You surely listened about 75 ohms coax. This value isn't measurable at DC using a DMM. The aerials also have its own characteristic impedance. An optimum design is when the line, the antenna and the transmitter are impedance matched. So a 50R TX is matched to a 50R line and 50R aerial. Any of them that causes mismatch, causes standing waves that travel from aerial to tx and viceversa increasing line current, voltage and power looses. So it is an entire item. Surelly you can learn more and better in the internet, books and in the training during the process to own a call sign.
Particularly, there is an interesting page covering RF items called RF CAFE.
Particularly, there is an interesting page covering RF items called RF CAFE.
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Thank you for the explanation, will take a time and read those articles. I changed both resistors and there is a massive improvement in sound volume and also it sounds a bit better, it lights up a bulb in herz loop to full brightness now (I had made 1 and 3/4 turns at the end of the final tank and hooked bulb to that, so I have good reference point for comparision.
Had just replyed... Had to take time, as I start and everything I remember while typing... here comes the book hehe.