Checking Voltages

It's one thing to replace caps and check/replace resistors, in which I have been fortunate in repairing a few radios that only required that and exterior repair. However, I am weak when it comes to checking voltages. I know very little of the B+/B- theory. For instance, with tubes in their sockets and power on, one must check the sockets from underneath with a multimeter. Where does the B+/B- theory come in? The neutral line on the multimeter--do you just clip it to the chassis. My philco 42-323 is in good physical condition, however, all the tubes are good, and I only get a low hum from the speaker. I know there are more than one question here, however, I would appreciate all the help I can get. Thanks in advance!!

If the chassis is ground then that's how i would do it but i would use tube extenders so that i could test the voltages above the chassis with the tubes in their sockets.

Nick

Nick, what's the best place to buy these tube extenders? Thanks for the reply.

I don't know where to buy them.I make my own out of bases from dead tubes,tube sockets and film canisters.Their actually really easy to make.Just make shure that you connect the pins in the tube base to the same pins in the socket or else you WILL have a bad day.After you do this you solder a second set of color coded wires to the socket and run the other end out through holes in the film canister.You then solder small sewing pins onto the wires that come out of the canister.The pins are to make a strong point to connect your test leads.You finish it off by using hot glue to join the tube base and socket to the ends of the film canister.
It is essential that you use color coded wires to make the extender in order to prevent major problems.Here is my color codingf system for a 8 pin tube.1-Black,2-Blue,3-Red,4-Orange,5-Yellow,6-White,7-Grey,8-Brown.Use only one color for all of the wiring on any one pin.
Hope this is not too confusing.If yopu need any help with this please ask.

Nick

Make sure you use a isolation transformer befor doing any voltage measurments.Think safety first.You can make your own from tube baces ..B+ measured from ground ,tubes voltages should never go past max ratings on tube data sheet.You will see diffrent voltages due to aging of resistors or caps,+/- 20 % is good for voltages and resistance unless stated otherwise..Sprman : )

Thanks Nick and sprman. Nick, quite an innovation. I'll have to try it. sprman, yes, I have an isolation transformer and a variac. Started last summer with the Ole light bulb (poor man's variac), but got lucky and got a deal from ebay. Thanks, guys, for the feedback...and enjoy the holidays.