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In my ongoing process of building up to a homebrewed general coverage receiver/transceiver, a first step might reasonably be a crystal set, where a tuned circuit and simple, or even improvised, rectifier extract enough power from the transmitted signal to power an earphone.

Crystal radios usually can only receive strong local stations. It occurred to me, however, that I'd gain a lot of sensitivity if I were to connect an RF amplifier stage between the antenna and the tuning RC circuit. This would also let me test the RF stage of my future two-tube receiver in a way I don't have the instruments to do otherwise.

In this case, an untuned RF amp will just amount to making a small antenna do the job of a very long wire, and then some -- but the output of even a very basic RF amp will be far more power than an ordinary germanium detector needs. Burning out the detector isn't desirable, obviously.

One way I can foresee to avoid destroying the detector diode is to use a different diode, with higher power tolerance -- after all, I don't need the very low forward voltage of a germanium part, since I'll have a signal measurable in volts, not microvolts (or millivolts after resonance boosts it a bit). Am I correct in thinking nearly any rectifier part with suitable voltage ratings could be used as the detector in this kind of circuit?

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  • $\begingroup$ Excellent question. However, wanting a receiver that works without a tuned circuit is a pretty tall order, as Jim nicely explained. :-) Not only does the tuned circuit prevent all the strong stations from being on top of one another in a jumble of interfering with each other (you would hear them all at the same time), it's like adding an active amplifier. $\endgroup$
    – Mike Waters ♦
    Aug 14, 2019 at 19:19
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    $\begingroup$ "RF amplifier stage between the antenna and the tuning tank circuit." -- not asking for a receiver with no tuner. Asking what detector to use to replace the delicate germanium number after the tuner. $\endgroup$
    – Zeiss Ikon
    Aug 14, 2019 at 19:24
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    $\begingroup$ The antenna and the tuned circuit are inseparable in a "crystal set"... there isn't really a "before" or an "after" the tank circuit. If you add an amplifier you have a 1-transistor tuned rf radio receiver, nothing wrong with that. And the 1N34s I find online say 50 V 50 mA, which is a lot of power, more than 100 mW. It would be unusual to find this power level in a receiver, even the LO would be more like 10 or 20 mW. $\endgroup$
    – tomnexus
    Aug 14, 2019 at 19:34
  • $\begingroup$ @tomnexus I agree. However, from a historical perspective, the first receivers did not have a tuned circuit. I am speaking from the perspective of the time before vacuum tubes, when spark was king. $\endgroup$
    – Mike Waters ♦
    Aug 14, 2019 at 19:42
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    $\begingroup$ @MikeWaters certainly. My many-part comment was really for the OP. $\endgroup$
    – tomnexus
    Aug 14, 2019 at 19:54

3 Answers 3

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A 1N3409 is a 39 volt Zener, completely unusable in a crystal radio (unless you are sitting at the base of the broadcast tower. ;-)

Historically, 1N34 germanium diodes have been used because of the low voltage drop and therefore high sensitivity. They are a dime a dozen, so you can afford to blow up a lot of them.

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  • $\begingroup$ There are Schottky diodes that are superior. Sorry I can't remember the P/N right now, but I'll try and look it up when I can. $\endgroup$
    – Mike Waters ♦
    Aug 14, 2019 at 19:28
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    $\begingroup$ Okay, remembered the part number wrong -- I meant the very low forward voltage germanium diode family commonly used for "Boy Scout" crystal sets. $\endgroup$
    – Zeiss Ikon
    Aug 15, 2019 at 11:31
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Many radios do include a preamp that amplifiers the RF signal coming from the antenna. However there are many issues to consider. A truly broad amplifier will not only amplifier the radio signal you want, it will also amplify noise and even other radio signals on other frequencies. Most preamps are therefore some what narrow in bandwidth and their tuning is part of the radio tuning. This is complicated and limits the usefulness of a preamp in most homebrew radios.

You are also correct that if the output of the preamp is too high it can damage the next stage of your radio. A limiter is often need which again can get complicated.

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  • $\begingroup$ Does not answer the question about what I can use for a detector. Untuned RF amps are common in vacuum tube designs, I'm working up to that, but need to know what detector will work with my RF amp, since I don't have a way to measured the RF amp ouput to even tell if it's working. $\endgroup$
    – Zeiss Ikon
    Aug 14, 2019 at 19:00
  • $\begingroup$ Hi Jim, IMHO your last paragraph would be better suited in a comment. I am deleting it. $\endgroup$
    – Mike Waters ♦
    Aug 14, 2019 at 19:08
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After some additional thought, I recalled I'd seen designs for single-tube "reflex" radios, where the tube acts as both an RF amp and, after detection of the audio modulation, as the AF amp to provide enough gain to drive a speaker.

This is actually the next step in the "build up from simple to complex" process I've set myself -- in my question, I was asking about this without the AF amplification (the audio signal would require an earphone, lacking enough power to drive a loudspeaker).

And when I searched for "single tube reflex receiver" I found this:

enter image description here

A single tube receiver with a pentode RF amp (differing from the tube I intend to use mainly in filament voltage), albeit in a tuned RF configuration, and a 1N34 germanium diode detector. Which answers my original question: yes, any diode with suitable ratings could be used, but the 1N34 family is "tough enough" to be used in a preamplified receiver and I needn't look further for a more suitable detector diode.

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