EH Scott Radio Enthusiasts

The Fine Things are Always Hand Made

My FM Philharmonic is getting noisy.  All tubes test good and non-microphonic.  I have a stash of replacement tubes and I wonder what is the best strategy for selecting "quiet" tubes.  The audio section seems to be OK so the noise may originate from RF section.  Specifically, I wonder where to start replacing, what tube maker or tube tester results should I use, and whether there is some kind of a rig I could make to test tubes for noise out of the radio. Thanks, Jiri

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Scott recommended Sylvania tubes in the late 30s. I have certainly used other brands with good results. 

Kent

Most of the cheaper tube testers have a provision for an earphone to check for microphony, I have an old RCP tester that has this facility,

really crude, let the tube warm up and give it a gentle flick of the finger, and you can hear how good or bad the tube is, not let me down yet.

or you could test by substitution.

You have a stash, so just use the substitution method.  Don't rule out the audio section so fast, I would check the 6K7 and 6F8 first, then work my way to the front end.  Don't rule out another component besides a tube.  These FM's aren't as bad as the earlier ones, but still getting them really quiet is a job. 

Hi some of the tube testers let you test the tube for noise.TV-7tube testers will let you test the tube for noise.Try replacing each tube 1 at a time to see if they noise goes away.Try that with ALL the tubes ..My philharmonic 30 AM i have found RCA and Sylvania tubes work best.Many ways for noise to get in the set i have found some LED lights can add noise ,so to can a Plasma tv and of all things the house power meter out side.Angelo

I agree with Angelo on this...first make sure no external interference is introducing noise to your set, through either RF or AC noise. I do this by flicking off circuit breakers in our house, one at a time, while the set is playing. I've located noisy dimmer switches and a faulty outdoor lighting transformer this way.

You can also try finding a noisy tube simply by giving it a gentle flick, while it's operating in the set. I have located a few this way. I don't recommend this on the 20's and early 30's tubes with long, unsupported filaments, though...

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