UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - 6-gang FM stereo tuner heads

regenfreak · 25th May 2022, 5:47 pm

The attached photos show that one of the weirdest oscillation waveform that I have come across from an oscillator! Before I replaced the EC81 by ECC85, the EC81 develops instability and crazy harmonics at normal plate voltage of 90-100V. I had to drop the plate voltage to 17V before the EC81 produces stable output. The valve was running at space charge voltage! I dont have an explanation for this. Maybe EC81 is a "hot rod", excitable valve or the oscillation coil couples inductively with the 5 other resonators before i installed the shields. I used silvered plated coils and the 6-gang variable capacitor are also silvered plated so the unloaded Q of the LC tank can be quite high.

At the moment, I get excellent stereo fidelity and sensitivity with the 6-gang tuner even it is not yet properly aligned.

25th May 2022, 5:47 pm	#48
regenfreak Heptode Join Date: Nov 2018 Location: London SW16, UK. Posts: 655	Re: 6-gang FM stereo tuner heads The attached photos show that one of the weirdest oscillation waveform that I have come across from an oscillator! Before I replaced the EC81 by ECC85, the EC81 develops instability and crazy harmonics at normal plate voltage of 90-100V. I had to drop the plate voltage to 17V before the EC81 produces stable output. The valve was running at space charge voltage! I dont have an explanation for this. Maybe EC81 is a "hot rod", excitable valve or the oscillation coil couples inductively with the 5 other resonators before i installed the shields. I used silvered plated coils and the 6-gang variable capacitor are also silvered plated so the unloaded Q of the LC tank can be quite high. At the moment, I get excellent stereo fidelity and sensitivity with the 6-gang tuner even it is not yet properly aligned. Attached Thumbnails Last edited by regenfreak; 25th May 2022 at 5:53 pm.