[regenfreak]

Just a quick update: I have come back to the FM valve tuner project after messing around with HF SSB receivers for a few months.

I built an oscillator test rig to test the performance and stability of the VHF valves:ECC85, 12DT8, 6BS8 and EC81 (Russian 6C11) as an oscillator. They all work fine on the test rig. The EC81/ 6C11 is an excellent UHF triode rated up to 750MHz. It gives completely flat oscillator output across the full range of frequencies when it was measured with a RF probe. Both the 6CW4 RF amp and ECC85 mixer/Rf AMP work well. In addition, the air gang tracks perfectly. However, I cannot isolate the root cause of ECC85 not oscillating well in the tuner, in spite of hours and hours of debugging. Therefore I started from a clean slate by building a minimalist 4-valve FM tuner with the line-up:

RF amp and mixer: 6N3P (USSR valve commonly used in FM and TV, the double triode has low noise and high transconductance.)

Oscillator: EC81 (6C11)

1st IF amp: 6BA6

2nd 1st amp/limiter: 6BA6

detector: ratio detector with diodes

The tuner works very well with good selectivity and sensitivity using an balanced ribbon indoor antenna. The IF transformers and ratio coils were stripped from a broken japanese FM radio. The dual air variable tuning gang were obtained from ebay. I think it was British made and was designed to track between 88MHz-100MHz. I was able to make it track perfectly from 88MHz to 108MHz after solving the compatibility mathematical conditions for the matched trimmers. Overall I am happy its performance as a mono tuner.


At the moment, I am working on my next project; a stereo tuner with the line-up:

Cascode RF AMP: EC88CC

Oscillator Ec81 (6C11)

mixer: 6CB6A

1st IF: 6BA6

2nd IF: 6BA6

1st limiter/3rd IF: 6AU6

2nd limiter: 6AU6

I will use a discriminator coil and broadband IF transformers to maximise the stereo audio fidelity. I omit AFC because it is not required in open chassis in which the temperature change is small. The 6C11 oscillator frequency is very stable.