Why do tube FM receivers hiss little?

[BG6RLC]

  This is a question that I've puzzled for a long time.

It's not unusual to see a transistor or tube  AM receiver do not hiss without connecting a antenna into the RX jack. I though hiss was the nature of FM. Any FM radio sold in the market nowadays, including ham radio, can generate lousy hissy sound when it receives an empty frequency (or just by getting rid of the antenna). However until recently I watch some videos about tube FM receiver which indicate that they seems quite silent when no outside signal come into the receivers.

I'm not professional in electronics so I think it is OK to just know the most basic reasons. Thanks!

[KB4QAA]

You are mixing and confusing two different situations:  1.  Without an antenna connected, and 2. With an antenna connected.

Most decently designed communications and
Short Wave radios built in the last 80 years will have very little internal noise when no antenna is connected, regardless of the mode e.g. AM, FM, SSB.

There will be a very obvious increase in noise, or 'static" when an antenna is connected.  The source of that noise comes largely from natural sources like the earths' atmosphere, the sun, galactic rays from other galaxies, etc.

Most modern consumer grade AM/FM radios have internal antennas, and therefore you are always hearing the natural static.  You would need to open them up and disconnect the ferrite loop or extendable antenna in order to compare the internal circuitry noise.   b.

[AA4HA]

Every electronic device that operates at a temperature above absolute zero generates a certain amount of noise.

You can take the best receiver in the world and put it in a Faraday cage (screen room) and turn it on. Outside signal levels will be way below the ability of the receiver to receive but you will still hear a hiss. That is the thermal noise of either the tubes or the semiconductor junctions.

It is the reason on why radio-astronomy telescopes often keep their receivers and amplifiers cryogenically cooled. It significantly lessens the effects of thermal noise that competes with very weak signal levels.

[KAPT4560]

 The internal thermal noise is also called 'shot' or 'poisson' noise.
 https://en.wikipedia.org/wiki/Shot_noise
 It is basically electron movement. High-velocity electrons hitting a plate makes noise.
 Older FM circuits were crude and rudimentary for the most part compared to today. Newer designs and features for FM AGC, traps, limiters, discriminators and muting have improved FM radios vastly.
 It is rare to hear multi-path flutter or VHF images on a newer FM radio.  Back in the '60's and '70's that was considered normal, even on a quality car radio.