Design voltage for 38-116 oscillator grid ? - Printable Version
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Design voltage for 38-116 oscillator grid ? - dixierat - 12-17-2009
Oscillator grid voltage is minus 3.5 with tuning cap fully open and 10 millivolts positive fully closed. Seems both voltages should be more negative. Radio plays well overall but seems to have reduced sensitivity at low end of broadcast band. Would like to get comments on what these voltages should be.
Re: Design voltage for 38-116 oscillator grid ? - Ron Ramirez - 12-17-2009
I looked the 38-116 up...the original Philco service data does not give the oscillator grid voltage. And as I no longer own one, I'm afraid I can't help you.
Anyone else want to try?
Re: Design voltage for 38-116 oscillator grid ? - Radioroslyn - 01-20-2010
If you think it's a problem try few different osc tubes. Make sure all you or components around the osc are within tolerance. You may find that some will oscillate better than others. What sort of meter are you using the measure to grid voltage? I've found that digital meters may give an improper reading when used in an oscillator circuit. Best to use a VTVM (vacuum tube volt meter)or a fet volt meter. A vom will load the circuit so much that you won't get a good reading.
Food for thought
Terry
Re: Design voltage for 38-116 oscillator grid ? - dixierat - 01-20-2010
Thanks Terry for comments. All rdgs with DMM. Have FET and will xcheck.Problem was lo sensitivity on lo end.Osc plate and screen design are 225V and 90 V respectivly. My rdgs were200V on osc plate and 66V on screen.Osc rdgs were -2.0V with tuning cap open and +0.5V fully closed.I think a positive osc grid voltage is a no-no.Installed a NOS 6A8G and osc grid rdgs were -3.5V open and+0.1V closed.Replaced rect with NOS 5X4G andOsc plate and screen were 243V and104V respectively.Osc grid rdgs were-5.4V and-.29V open to closed.Very sensitive using 50 foot flat top in attic but still concerned that lo end may not be as good as "new".Have 17 pages of notes on work-problems; most fun I've had in years.Any thoughts on what is normal osc grid voltage when adjusting tuning cap from open to closed?
Curt
Re: Design voltage for 38-116 oscillator grid ? - Radioroslyn - 01-21-2010
Generally speaking positive voltage on the osc grid means it's not oscillating. The amount of negative voltage on the grid is determined by the amount of feedback in the oscillator circuit. If you wanted to check the osc look at the output with a scope at the mixer tube. I'm not quite sure what the 6N7 plays in the osc output without making my hair smoke. Certainly having the plate and screen voltages up where they need to be is very important. More gain= more output.
Terry
A little off topic - Radioroslyn - 01-21-2010
I was thinking as an aside if you think all this oscillator stuff is fun you might enjoy building a regenerative receiver. It will help you understand these relationships between gain, feedback, and frequency. There simple to build and are very sensitive. It can be as simple as a one tube set that can be powered by batteries. This is I build years ago http://www.flickr.com/photos/42774321@N08/3937747399/ and used in my ham station. For some ideas search Hitchhiker Radio.
Terry