schematicWhat do you think of my 80m band radio transmitter I designed? Originally I though about using an Armstrong oscillator, due to its simplicity and sine wavyness, but I heard that crystals are better, what oscillator circuit to use?

I decided to use tubes because I have around 40 of them laying around and I want to make the old guys at the ham radio club happy.


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    \$\begingroup\$ Well, to start with, you have no component values and no reference designators -- this is just the topology of a circuit, not a complete design -- and there's no way to talk meaningfully about it. Not to mention that you posted it sideways, making it extremely difficult on anyone who might want to help you. \$\endgroup\$ – Dave Tweed Feb 2 at 22:45
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    \$\begingroup\$ In any case, we discourage broad, open-ended design review questions here on EE.SE, because the answer(s) tend to become long strings of unrelated edits and/or comments. While this might help you with your immediate problems, it is of no value to the site overall. We DO allow design review questions in which you explain your choices and then focus on a few points about which you still have doubts. To get a better feel of what is or is not acceptable, search for "design review" on the meta site. \$\endgroup\$ – Dave Tweed Feb 2 at 22:45
  • \$\begingroup\$ @DaveTweed Sorry about that, im gonna upload a ltspice simulation with the values and flip it, i was uploading it off my phone so I couldn't flip it. \$\endgroup\$ – thegamebusterpl Feb 3 at 9:44

You will not make the old timers happy because the frequency drift will be by modern standards very bad .Your AM linear will splatter terribly if it is not precisely adjusted .If you must make good sounding Old school AM then plate modulation will be easier to set up and waste far less power .Your Valves will keep the old timers happy +1 .

  • \$\begingroup\$ I know plate modulation, but that would require an another power tube and - the worst of all - me having to wind a transformer. Im looking for and adequate xtal oscilator soon. \$\endgroup\$ – thegamebusterpl Feb 3 at 9:56
  • \$\begingroup\$ @ thegamebusterpl.The orthodox way is to have an audio power amp rated at half or better of the expected DC input power of the PA valve.The use of a modulation transformer was normal .You could just use a modulation choke which could be made from flourescent lamp ballast chokes in series .A 40 watt 50Hz ballast has about 1 Henry of inductance.Series Modulation will give good sound with no transformer but your PA HT volts should be doubled. \$\endgroup\$ – Autistic Feb 3 at 18:31

It takes a lot of work to make a tank-tuned oscillator stay on frequency; back in the day it would be a project in itself, in a separate case, so that you could use it on multiple rigs. I'd use a crystal.

Getting a pure tone out of the oscillator isn't terribly important as long as your signal is pure going out the antenna. If you're running a class B final then you'll need all sorts of harmonic filtering anyway.

If you really want to go old-school, go with a class C output stage and modulate its input power. But don't ask me for help -- that's getting above my pay grade!

  • \$\begingroup\$ It has an class C output stage already, on the grid of the output pentode there is only a half of the am wave, because of the diode. Its turend into a full am wave due to the presence of an resonant circut at the anode. What simple crystal oscillator circut do you reccommend? I dont know a lot about xtals, because ltspice is my main learning tool and you know how hard it can be to model actual crystals there - there is little literature about it and most people just recommend using a voltage source. \$\endgroup\$ – thegamebusterpl Feb 3 at 9:50
  • \$\begingroup\$ "It has an class C output stage ... because ..." With all due respect, no. The class of an amplifier stage depends on the construction of the amplifier stage, not what you feed it. It certainly should not be, because a modulating stage should only feed linear amplification stages, which class C is not. Because of the way you've arranged the cathode bias those tubes may be operating in class C, but that would be a mistake. \$\endgroup\$ – TimWescott Feb 3 at 17:00
  • \$\begingroup\$ "What simple crystal oscillator circut do you reccommend? ... because LTSpice ..." I recommend that (if you can) you get an old copy of the ARRL Handbook, or the RSGB Handbook -- anything from 1940 to 1976 should be good. Those books were intended for folks who were home-brewing tube radios, so there's lots of info (and some straight-up projects). As for the oscillator itself, a typical good crystal oscillator is a tuned-grid-tuned-plate oscillator with a tuned circuit on the plate and a crystal on the grid, using the parasitic capacitance of the tube for feedback. \$\endgroup\$ – TimWescott Feb 3 at 17:07

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