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Old 7th Jun 2008, 10:47 am   #14
Sideband
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Join Date: Oct 2003
Location: Croydon, Surrey, UK.
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Default Re: Another (possibly silly) valve question

I suppose it's still open to debate. Most AM/FM sets use something like an ECH81 as the AM mixer and most manufacturers disconnect the oscillator anode via the wavechange switch when using FM. However most sets work happily on AM even if they have been switched to FM for years.

It's interesting to note however that my Baird 301 has a high value resistor switched in to feed the anode of the oscillator section of the ECH81 when in FM mode. This indicates that some manufacturers where concerned about cathode poisoning.

When I first started learning about radio some 40 odd years ago, I was always told not to run a valve for very long periods without HT connected. Just how long 'very long' periods were, I don't know. I suppose now that valves are more difficult to obtain, it would be prudent to fit a high value resistor to the 'unused' anode so that at least it passes a small current.

Rich.
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