Old dog, new tricks...

[cowboybob]

I've been involved with electronics (for fun and as a career) since the late 1950s'. Honestly, I thought I had seen it all with old tube type receiver circuit designs.

For the last few years I've occasionally restored some old 1930's - 40' s radios. Mostly Philcos. Generally, a good cleaning, replacement of the electrolytics and a "tuning up" is all that's needed... . Circuit design was pretty much what I've come to expect, especially in the audio amp stage(s).

But my most recent acquisition's circuit design (regenerative, with its' own attendant issues, like the weather... ) caught me up short and made me laugh, with a WTF! thrown in:

That's right; that's the volume control at the antenna input... . Input impedance matching something of a bear, maybe?

Now, in all fairness, this was Philco's entry level receiver (the "Jr.") at that time. And the "Philco 80 Whistles" note is amusing as well:

 

[audioguru]

I made a transistorized AM regen radio to hear the airplanes and air traffic controllers at the airport. It worked pretty well and I managed not to get caught causing interference.

 

[dknguyen]

I made a transistorized AM regen radio to hear the airplanes and air traffic controllers at the airport. It worked pretty well and I managed not to get caught causing interference.

Would a receiver expected to cause interference?

 

[cowboybob]

Regenerative receivers can, under certain conditions (i.e., inadequate circuit shielding), produce unacceptable levels of RFI.

 

[KeepItSimpleStupid]

Totally wierd. Never saw that before.

Most of the tube radios around the house are broken. There's a Zenith, one that I don;t remember the brand, and a 1050's AM/SW Magnavox mahogany console that's gorgeous. The latter hums a bit. I did manage to snag the FM tuner option off of ebay. It's without tubes and tube shields.
It was a 78 RPM turntable only, but a multi-speed deck was added and an aux tube amplifier to make it stereo. I definitely want to rebuild it, but hands are too full right now. there should be a few more radios around the house too including a "portable" tube radio with the 22 1/2 - 67 V battery (I think).

The Magnavox looks similar to this https://people.ohio.edu/postr/bapix/Magnav1.htm , but the cabinet is much prettier. The left and right side have a brass diamond-shaped grate with grill cloth about 1/2" behind it. The entire right front panel is hinged. The FM section goes on top of the AM/SW section hanging upside down.

 

[JimB]

That's right; that's the volume control at the antenna input... . Input impedance matching something of a bear, maybe?

Just a simple RF Gain control, rather than the more conventional AF Gain control somewhere around the first IF amplifier. (late edit, that should be AF amplifier)

Impedance matching?
Don't over think these things, in normal use this receiver would have been connected to a random length of wire, the feed impedance of which would have varied considerably over the tuning range of the receiver.

Would a receiver expected to cause interference?

Regenerative receivers can, under certain conditions

We need to be careful of the terminology here.
There are "Regenerative receivers" and "Super Regenerative receivers".
The operation of the two types is somewhat different.

In the regenerative receiver, there is positive feedback which is adjustable.
To receive an AM signal, the regeneration (feedback) is advanced until the amplifier stage is not quite oscillating.
This will give the best sensitivity and tightest selectivity.
If the regeneration is advance too much, the stage will become an oscillator, and unless there is a buffer amplifier between the regenerative stage and the antenna, it will radiate quite nicely on the frequency to which it is tuned.

Making the regenerative stage oscillate is (was in the old days) a nice easy way of receiving CW (morse code) signals which are simply on/off keying of an unmodulated carrier.


A superregenerative receiver on the other hand is oscillating all the time, and being switched in/out of oscillation by a "Quench Oscillator".
Superregen receivers can be very sensitive for a simple circuit, and do radiate a lot of wideband noise.

JimB

 

[audioguru]

Would a receiver expected to cause interference?

Many regen and super regen receivers oscillate at an ultrasonic frequency that causes audio interference if the signal is FM multiplex stereo.