Adding loop antenna to pre-war radio

[Tamošius]

Greetings to the forum! A quick little about me, an a self taught electronics hobbyist going on some 15 years now. Started with vintage receivers, then test equipment and then post war tube radios. Not sure why, but have dove into pre-war octal tube sets and my first one is a Stromberg Carlson 130 in very nice shape, cosmetically. I have it up and running, had a bad output 6F6 tube with a burned cathode resistor and one of the candohm replacement resistors somehow went open. I noticed it is very deaf and after some learning figgered out it is only set up for a long wire antenna. So, would like to fabricate a loop antenna and not exactly sure of how to shoehorn it in.

I attached a full schematic, a partial schematic of the area involved and my proposed modification. I like to draw out to schematics to help learn the circuit hence the clean schematic pics. Incidentally, I discovered a couple years ago in my Grandpas old desk some schematic drawings that he hand drew sometime in the 1930's. He was also a hobbyist and we must be in tune with that way of learning.

Anyway, the inductance of the loop is somewhere in the 150uH range according to my calculations ((Upper Freq. of Receiver + IF Freq.) squared divided by Upper Freq. of Receiver squared x Local Oscillator Inductance in uH = Antenna Coil Inductance in uH). The oscillator inductance I think is ~90uH but that could be off as not sure if it measured correctly in circuit tho I did disconnect one lead. Assuming I can get the loop inductance figgered out, questions are:

#1 Does a 100pF cap need to be added the signal grid of the 6K7?
#2 Does C16 (.002uF) cap stay in circuit?
#3 Do I have the AVC spliced in correct?

I also will add a trimmer cap to tune the antenna. I also ordered some 180/46-44 Special Blend Litz Wire for the fabrication.

So, there it is! Look forward to some input and chatting.

Thanks,
Tom

[old_coot88]

Well first off, the radio should not be deaf, even with just a few feet of wire for an antenna. You need to troubleshoot first and get it un-deaf.: Nothing would be gained by trying to rig a loop as shown. But since you've ordered wire for it, it might be handy for some project down the road, like a crystal set maybe.
By 'very deaf', do you mean you can hear stations coming in faintly? Or is it stone dead? Having an RF stage is a premium feature, making it an extra hot performer.

[Alex KL-1]

Welcome to VK!

The set can have eventually some bad tube, or some misaligment?

Yes, some people even put ferrite antennas for less RFI. Same process; basically you use the same inductance from the corresponding coil (is good to measute also with a inductance meter). OR a little difference for compensate the shielded wire for it (needs to have very low capacitance if one needs to use shied cable to couple the antenna).

https://atrad-audio.co.nz/turneraudi...gineering.html

This guy are very serious about AM radios, is worth reading.

[Alex KL-1]

Originally Posted by Tamošius

Greetings to the forum! A quick little about me, an a self taught electronics hobbyist going on some 15 years now. Started with vintage receivers, then test equipment and then post war tube radios. Not sure why, but have dove into pre-war octal tube sets and my first one is a Stromberg Carlson 130 in very nice shape, cosmetically. I have it up and running, had a bad output 6F6 tube with a burned cathode resistor and one of the candohm replacement resistors somehow went open. I noticed it is very deaf and after some learning figgered out it is only set up for a long wire antenna. So, would like to fabricate a loop antenna and not exactly sure of how to shoehorn it in.

I attached a full schematic, a partial schematic of the area involved and my proposed modification. I like to draw out to schematics to help learn the circuit hence the clean schematic pics. Incidentally, I discovered a couple years ago in my Grandpas old desk some schematic drawings that he hand drew sometime in the 1930's. He was also a hobbyist and we must be in tune with that way of learning.

Anyway, the inductance of the loop is somewhere in the 150uH range according to my calculations ((Upper Freq. of Receiver + IF Freq.) squared divided by Upper Freq. of Receiver squared x Local Oscillator Inductance in uH = Antenna Coil Inductance in uH). The oscillator inductance I think is ~90uH but that could be off as not sure if it measured correctly in circuit tho I did disconnect one lead. Assuming I can get the loop inductance figgered out, questions are:

#1 Does a 100pF cap need to be added the signal grid of the 6K7?
#2 Does C16 (.002uF) cap stay in circuit?
#3 Do I have the AVC spliced in correct?

I also will add a trimmer cap to tune the antenna. I also ordered some 180/46-44 Special Blend Litz Wire for the fabrication.

So, there it is! Look forward to some input and chatting.

Thanks,
Tom

You can maintain C16, and not need to add anything; only you need to retouch the correspondent trimmer. The radio have a transformer, so no need to insulate the antenna with capacitors.

[old_coot88]

He still needs to troubleshoot and fix he 'deaf' problem before anything else.

[Tamošius]

Thanks for the tips. I did figger out that I had the sensitivity knob on the back turned the wrong way . The only station it pulls in with a few feet of antenna is one local AM station about 10 miles away. From what I have read, that is all that one should expect? Next closet stations are at least 50 miles away. I don't care to tether the radio to a long wire antenna so gonna experiment with a loop and ferrite options. In response to Alex, shouldn't one go around the existing antenna transformer with a loop antenna?
Tom

[Electronic M]

You should get several stations 60 miles away with a few feet of antenna during the day and more at night. May need alignment, might have a resistor that's drifted too high and is reducing gian, a weak tube or something else, but whatever it is should be found and fixed.

Many radios that need a long wire I'll hook one terminal of a loop from a scrapped 40s radio to the antenna terminal of the radio.... It's not as good as a long wire but it grabs the Milwaukee locals and the 50kW Chicago clear channel stations.

[Tamošius]

Ok, I will look into any gain issues. There is for sure some distortion on the output. Will swap some tubes and do some checking. Thanks!

[old_coot88]

Tom echoed exactly what I was fixin to say about hearing those stations on a few feet of wire. And it should hear the normal AM hash between stations. With the deluxe feature of an RF stage, that radio should be extra hot for DXing. Looking forward to seeing it come alive.

[Alex KL-1]

After all testings and with a better performance, you can experiment with different antennas if you decide for it, based on noise etc (ferrite antennas are less prone to some noise).
Not forget to remove the corresponding coil to add the antenna (the antenna will replace it). Carefully, for saving it if you can want to revert to original.

[Alex KL-1]

Originally Posted by Alex KL-1

.........
Not forget to remove the corresponding coil .......

Oops, I mean only cut the connections (and maintain it in place).