28-02-2021, 02:23 PM
Hi Amie
Very impressive work. Ed Dinning has introduced me to this forum and asked me to check your thread.
I am a lazy so i used existing IFs from scrap AM radios becuase they are relatively abandon.
Currently I am building a valve FM superhet tuner with a triple gang air capacitor found in ebay and have encountered lots of difficulties getting hold of FM IF transformers.
If you want to build an AM superhet without searching in the dark, study existing proven designs; either using either European heptode or American pentagrid convertors. You dont need to re-invent the wheel. It does not matter if it is Hartley or Armstrong oscillator; the circuit is very simple. I am not familar with heptode supehet design but i know the design of All American Five very well and built one last year:
https://www.vintage-radio.net/forum/show...p?t=166316
I did not have as sweep generator at the time when I built this AM supehet last year. I simply measured the inductance and use LC resonance formula to pre-set the If transformers before installation.
Typical All AMerican Five schematics are shown here:
http://www.wa2ise.com/
Many european battery sets use 1R5 battery convertor tube running B+ arround 90V that can be supplied with a series of 9V battery. I owned and restored 6 All American Five AM radios (RCA, Zenith, Emerson etc) so I know their design inside out; fundamentally they are all the same!!
The hardest part of building the superhet is the tracking problem. You either cheat by using an existing matched oscillator coil, padders and twin gang capacitors taken from a scrap radio or do things the hard way like me, writing a three-point-tracking program to find the matched solutions for the antenna, oscillator coil and twin gang tuning capacitor. I spent a couple of months testing the different three tracking methods from the classic radio textbooks (Langford Smith, Tehman and Henry) with my homebuilt supehet rig.
The second hardest part of building a supehet is mastering the art of good RF wiring. The most common problem is the oscillation of IF amp stage which can be cured by good wiring, ground and RF by-pass caps.
A 10:1 wont load down sweep of 455/470khz with an isolation resistor. Sweep AM if is realtively easy but it is difficult to sweep FM IF transformers.
The static sweep of FM transformers usually does not produce accurate dyanmics behaviours after they are installed into the radio. At 10.7MHz, the probe loading is significant even an isolator resistor of 100k to 200k is used. In addition, the Miller capacitance of the tubes will affect the tuning of the IF transformers.
Here is the thread that i was mucking around with stagger tuning of FM IFs;
https://www.vintage-radio.net/forum/show...?p=1317462
Currently I am figuring out how to sweep IF transformers using the NanoVNA. This requires matching the input and output impedance of IF transformers to VNA 50 ohms port. The resonance impedance of IF transformes are very high and vary sharply near resoance. it is very diffciult to design narrow band reactive matching pads for the VNA frequency sweep. Maybe I will try balun transformers.
Good luck.
Very impressive work. Ed Dinning has introduced me to this forum and asked me to check your thread.
I am a lazy so i used existing IFs from scrap AM radios becuase they are relatively abandon.
Currently I am building a valve FM superhet tuner with a triple gang air capacitor found in ebay and have encountered lots of difficulties getting hold of FM IF transformers.
If you want to build an AM superhet without searching in the dark, study existing proven designs; either using either European heptode or American pentagrid convertors. You dont need to re-invent the wheel. It does not matter if it is Hartley or Armstrong oscillator; the circuit is very simple. I am not familar with heptode supehet design but i know the design of All American Five very well and built one last year:
https://www.vintage-radio.net/forum/show...p?t=166316
I did not have as sweep generator at the time when I built this AM supehet last year. I simply measured the inductance and use LC resonance formula to pre-set the If transformers before installation.
Typical All AMerican Five schematics are shown here:
http://www.wa2ise.com/
Many european battery sets use 1R5 battery convertor tube running B+ arround 90V that can be supplied with a series of 9V battery. I owned and restored 6 All American Five AM radios (RCA, Zenith, Emerson etc) so I know their design inside out; fundamentally they are all the same!!
The hardest part of building the superhet is the tracking problem. You either cheat by using an existing matched oscillator coil, padders and twin gang capacitors taken from a scrap radio or do things the hard way like me, writing a three-point-tracking program to find the matched solutions for the antenna, oscillator coil and twin gang tuning capacitor. I spent a couple of months testing the different three tracking methods from the classic radio textbooks (Langford Smith, Tehman and Henry) with my homebuilt supehet rig.
The second hardest part of building a supehet is mastering the art of good RF wiring. The most common problem is the oscillation of IF amp stage which can be cured by good wiring, ground and RF by-pass caps.
A 10:1 wont load down sweep of 455/470khz with an isolation resistor. Sweep AM if is realtively easy but it is difficult to sweep FM IF transformers.
The static sweep of FM transformers usually does not produce accurate dyanmics behaviours after they are installed into the radio. At 10.7MHz, the probe loading is significant even an isolator resistor of 100k to 200k is used. In addition, the Miller capacitance of the tubes will affect the tuning of the IF transformers.
Here is the thread that i was mucking around with stagger tuning of FM IFs;
https://www.vintage-radio.net/forum/show...?p=1317462
Currently I am figuring out how to sweep IF transformers using the NanoVNA. This requires matching the input and output impedance of IF transformers to VNA 50 ohms port. The resonance impedance of IF transformes are very high and vary sharply near resoance. it is very diffciult to design narrow band reactive matching pads for the VNA frequency sweep. Maybe I will try balun transformers.
Good luck.
