An IF Strip
With the IF Filter sorted out I continued with the IF strip. Not using any common sense I decided that three EF184 pentodes would be up to the task. At this time I hadn't seen the Marantz 10B schematic with its six tube IF amplifier and three stage limiter driver. And a total of 18 tuned circuits, whereas I only employ six... But I digress. From the highly informative UKW Tuner Projekt page I learned that a pentode IF amplifier needs careful attention to prevent oscillation, mainly the decoupling of the screen grid to the anode decoupling capacitor. This way a bridge is created. This sort of works with the high transconductance EF184s. But because I built the amp in a weird L shape I still needed additional screening.
The ratio detector from the R316 was reused because I didn't want to purchase new transformers and I also didn't have them around in my junk box. Amazingly it can withstand the anode voltage across its coils. Once I got the whole shebang more or less stable I plonked it onto the chassis and hooked it up. To get audio out of the demodulator I added an opamp which also takes care of the 50 µs deemphasis and jacks up the audio level to match the reference tuner.
Immeditately it became clear that gain is lacking. I need about 30 dB more gain to achieve full limiting. The effect is that interstation tuning noise is very low but also that low level stations have low audio level. To fix this I estimate another two extra amplifier stages are needed. While listening I also noticed some hum in the demodulated audio. This was due to the local oscillator getting modulated by the supply voltage ripple. An extra electrolytic capacitor across the oscillator took care of that.
Performance of the tuner as it is now is fair. The selsctivity is not great but better than the typical tube radio from its era (1960). Limiting is an issue but also better than said radio. The gain of the front end is about 10 dB too low and if this can be fixed the limiting issue will be less troublesome.
Next steps: I'm putting this project on hold. The front end needs research and the IF strip must be rebuilt. A second IF filter could do no harm. A stereo decoder is still beyond the horizon. For now my craving to create a vacuum tube tuner has been sated. I have a Proof of Concept and I know what I need to do to improve it. But the performance of a handful of semiconductors is still far away.
22 December 2015