This is my first attempt to build a FM tube tuner and is a work in progress. It uses a 6CW4 and a pair of ECC85. The 6CW4 is not really essential but it will make the receiver more selective;and the tiny nuvistor looks "cute". The tuner head is supposed to be mounted on a bigger chassis made out of aluminium.
At the start, I knew it would be significantly more challenging than building MW tube receivers. Since there is no book or web site telling me how to build an FM tuner from scratch, I had to take a deep dive, reading every textbook on FM receivers design theory and studying the schematics of many existing tuners. I don't think many people are interested in DIY FM tube receivers. It seems the majority of hobbyists are interested in homebrewing HAM CW and SSB circuits.
Initially, I was going to use a pair of ECC85 as RF, mixer, oscillator and AFC using a twin gang air variable capacitor. But I have a triple gang variable cap from Ebay of unknown origin. So I have settled with 6CW4 as ground cathode RF tuned amp, and ECC85 as the untuned ground grid RF amp.
I cannot use the three-point tracking solutions for MW receivers because FM's tuning capacitors have a much small Cmax/Cmin capacitance ratio, and a padder is often not used. I derived a simple compatibility equation to allow me to find the trimmer values for my triple gang air variable capacitor so that it tracks perfect perfectly across the whole FM band for a DIY set of RF and oscillator coils.
At the moment, I am still debugging the tuner head. At first, the oscillator is not oscillating when I wired it in Hartley configuration. So I re-wired it in self-excited configuration (no feedback coil; it is magic
) The oscillator is now oscillating exactly 10.7MHz below the RF. I have been running into problems with the plate currents for both ECC85 and 6CW4 are abnormally high; so a lot of head-scratching wondering if I got the values of cathode bias and plate resistors correct.
My biggest problem is to find the motivation to build the remaining IF amp, limiter, detector mounting on an aluminium chassis. It is a difficult project and it is very easy to get discouraged. Lately, I got distracted and I put this project aside for a month. I have been building shortwave and FM solid-state kit receivers. More worrying is that I start to get interested in Ham radio technology..I need to get myself a kick in the butt to finish this FM project.
P.S. I was toying with the idea of replacing the conventional ratio or discriminator with the 6BN6 gate beam tube as a quadrature detector. The tune 10.7mHz coil can be made form modifying an 10.7MHz IF.