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Jose' -

I haven't had my coffee yet, so bear with me...

To deal with the 866jr's you'll need a separate filament transformer. You'd need to pre-heat the 866jr's for 2-4 minutes before applying B+. Yup, really 2 minutes in warm weather, and 4 minutes when temps drop below 60 degrees F. Any less and they will flash over. A digital delay on the primary of your B+ transformer works nicely. I can suggest sources for these.

I'd probably leave the hybrid bridge alone, and just use a single damper diode before the first choke. This should not be powered until the B+ is provided. It gives a nice slow turn-on. Use a beefy type, like a 6CG3. Bypass the diode with a high value resistor - like 100K, and make sure this resistor is rated for your B+ voltage. This resistor seems to be necessary to establish some sort of "ground reference" At higher voltages it may also be a good idea to clip unused solder tabs from the bottom of the damper diode socket to avoid arcing.

If you can deal with plate caps, 816's or 866A's are MUCH more forgiving and easier to locate.

Then again, you're dealing with a much lower B+ than I use (825v) - 866's are really overkill. You should have less trouble with flashing over.

Pre-heat the 866jr's for at least 30 minutes on filament only before their initial use to redistribute the Hg. Also repeat this if you move them from a "vertical" position.

Make sure they aren't too close to your transformer - you'll get some "wierdness" from the magnetic field.

Lastly, my experience is that a large number of 866jr's will test fine, but flash over at much lower B+ than their rating. This is especially true of older globe or ceramic based types.

Well, now you know why Ham's were smart enough to ditch 866's as soon as reliable substitutes became available!

- Gary



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  • Ping! - Gary Kaufman 12/30/0405:15:22 12/30/04 (1)


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