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Author Topic: 1 tube QRP tx thoughts  (Read 8147 times)

Posts: 814

« on: February 05, 2011, 06:44:52 PM »

I was going to reply to a thread but itwas recommended to post it as new topic.
Re 1 tube glwbug transmitters.
First chirp. After some testing here it appears that chirp is often caused by the final circuit being slightly out of tune with the  crystal or low q circuit or tuned grid circuit depending on the design.
I have a 50L6 tx , single tube  rockbound  and went through it with it. I also have a 40 m 245 tubed TNT. Much of the trouble comes with tuning instructionsdip this peak that back of this  etce tc. I made a Rf loop (1 turn or so depending on power and a flashlight bulb.) I now use it exclusively for tune up and in the 50L6 unit I have biult one in with a long slot in the front panel so I can see it. I discovered (quite by accident  as most good discoveries are) when I tuned with the Rf loop bulb I had no chirp when done.
The Tx is built from the circuit that appeared in Electronics Illustrated many years ago (41 I think. ) A 50c5 can also be used . It uses a transformerless supply with a doubler and a large resistor in series with line ac to supply the 50 volt filiment. Since my rx is a regen with a 6sl7 I think if I built this again I would use the 6l6 or a 6v6 and then supply both tx and rx from the same filiment supply. I would also supply both B+ from a single supply. I rigged up a deal for a standby switch today from a household rocker style light switch. With a light touch of the hand I can disable the B+ in the rx when transmitting. Believe it or not it smoked a tube in it yesterday by leaving it on so enough of that. Things I like are the tx works good and has given no trouble. I also like it cost almost zip and what I didnt have in a junked ac dc radio I had on my parts box. I used a TV colourburst crystal at first but that is not the best spot to be. I got a bunch of old crystals this week from a friend and sorted through . I know have it on 3562 so if you hear me  come back.
I also took some old bakelite cases and opened them up and installed the newer metal case crystals inside so I only need one style plug. What I dont like about it is a single tube tx cannot be spotted easily because your receiver cannot handle the key down even on a dummy load and so it is difficult to get both on the same freq. I need maybe a low voltage B+ for spotting only. With a 2 tuber or equivalent  the osc would be running always and at low power so it could be used for spotting the receiver. I also dont like being rock bound. I added a VXO circuit so I can wiggle a bit but really it isnt real practical as an 80 meter crystal doesnt want to wiggle a long ways.  Right now it would allow me to get on freq with someone but I can move far enough away to avoid a QSo in progress. I have begun to see if I can make this Beam power tx run in TNT mode which gives a bit more leeway. My first try it worked but was very dirty . Next try will be tomorrow I think. . Aside from the beam power tube the circuit is not a great deal different than my TNT  with the 245 tube. I fabbed up the low Q coil and gridleak deally today on an old crystal case so I can plug it in. I have to establish a polarity with it so the cap /resistor combo winds up on the right section of the circuit. It is for me anyway a great thrill runnig these homebrew wooden radios using a regen to hear and the 50l6 to transmit. Next goal? I would like to try screen modulation. But that is for now a long ways off. 

Posts: 47

« Reply #1 on: February 05, 2011, 09:59:43 PM »

Interesting post with a lot of points.  It's been many years since I fooled with those one-tube CW transmitters it seemed everyone used to build in the older days.  I remember the degree of chirp was more related to the integrity of the power supply.  The more regulated the supply, the less the chirp.  In the case of a not-so-stable power supply, it helped chirp to drop the plate voltage on the tube a bit to reduce the sudden pull on the supply when the key connected.  The RF output would of course drop, but if you were shooting for true QRP you might not be able to run full plate voltage on tubes like the 6AQ5 anyway.

Posts: 814

« Reply #2 on: February 06, 2011, 05:53:40 AM »

Tis true. I am really shooting for FUN. So far it has been successful.  I guess even the 50l6 is outside true QRP.

Posts: 17482

« Reply #3 on: February 06, 2011, 07:21:32 AM »

Chirp due to power supply regulation was a particular problem with many VFOs of the era,
but with crystal control I think it was more related to the starting conditions of the
oscillator.  I know with my rig (electron-coupled Miller oscillator) tuning it for maximum
power output made it unreliable to start and prone to chirp. 

There may be some advantage to adding a feedback adjustment - this shouldn't be
too difficult to do with the Colpitts variants by making one of the divider capacitors

With the right ratio of screen voltage to plate voltage there is a point where the
tube operating conditions don't change as much with voltage.  This has been
used to make usable single-tube VFO transmitters with minimal chirp, but I'd
have to dig out my copy of Technical Topics to remember the details.

Posts: 814

« Reply #4 on: February 06, 2011, 01:08:00 PM »

Interesting. I read this after the great experiment. I got the tube to oscillate with my low Q and resistor cap pack. I tried it both ways to see if it was working as I thought. It only works the one way as per TNT standard  design. It was tunable but the low Q coil I made is just at the outside edge of my tank circuit freq range so I had to make the cap full open to it tuned for decent output. It appears to me it is still outside a bit more so I will take three turns off that coil and try again. It did light my Rf loop indicator  and my dummy load when tuned so it is working. I will get it all on frequency and coordinated first. Then I may have to get more info from you on the screen plate E ratio. I see in the Bruce Kelly QSo Party that crystals were verbotten. I have been unable though to find what the popular set up was. Thanks for that tip. Good to know.
Don VE3LYx

Posts: 814

« Reply #5 on: February 09, 2011, 05:00:51 AM »

I had the thrill of working another QSO (WIAEA) with my vintage ham station 1 tube tx and 1 tube rx. I made a wooden box with a standby switch that cuts the B- to my rx during transmit. It looks period correct. Saves my ears and my RX tube. I now need to make a low power spotter circuit for my tx so i can use it to spot the RX. I think severely reduced plate voltage may do it. Maybe 45 volts I am not sure if it will still oscillate . I think I will put a resitor in circuit and parallel a normaly closed push button switch (an interupter) so If it works I can just push the button for spotting power and release for full power.
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