I am trying to learn schematics and this is probably a stupid question but I need to ask. I am looking at the schematic for a Philco 71. I was told if I wanted to understand schematics to try and trace the wiring on the radio and line it up with the schematic to get an understanding of how it all works. When I start looking at for example tube 44 and 36 on the schematic I see 6 points of contact going to each tube yet there are only 5 pins on each of these tubes.
I hope someone can provide a simple explanation because I’m trying to learn how to read these schematics but find it somewhat confusing.
I have attempted to google my query as well as looked at some of the literature available and no luck.
I hope you all have some patience. Thanks.
Hi Phil - This is where schematic reading gets more intuitive. The tubes you have are generally known as double ended, thus there is a tube electrode connected to the top cap. Most of the tubes you will be working with on these sets will have the cap wire connected to the control grid of the tube, or it is also called grid one. Be gentle with the cap as it was glued down at the factory and 75 year old glue doesn't always hold nowadays. Tubes without the cap are single-end and will have a S in the tube number, as in 6SQ7 versus it's capped counter part 6Q7. So in your diagram you can see the small black square on the left side of the tube where the grid connects indicating that the cap is the grid connection. You'll often read here that you could touch the grid cap with a finger to listen for a response.
Thank you very much guys. I feel rather foolish now, but I learned something today I hadn’t thought of.
There were no tubes in the chassis when I had it upside down trying to figure out where that 6th connection went too. I didn’t even think about the grid caps.
Appreciate the replies without embarrassing me!
Just keep in mind: while in the time of old the cap was usually the Grid cap, which is low voltage and safe to touch, some tubes (generaror tetrodes/pentodes, that are sometimes used in tube amps) have the plate caps. What is more, if a tube has a plate cap, the voltage there is usually high enough to put you on your fanny.
People who do not drink, do not smoke, do not eat red meat will one day feel really stupid lying there and dying from nothing.
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