I've got several "curtain burners" (RCA T4-10, etc.)that I'd like to get working again by replacing their defective resistance line cords with AC capacitors.

Does the value of the capacitor have to be exactly what is calculated? If not, what is the tolerance with these?

I've also heard of lots of ways to determine the value of the caps but nothing about where I might get them. I need about 5 of around 7uF. I'd like to avoid the massive motor starter caps if I could.

This site explains it all.
The nicest thing is that at the very bottom of that page he provides a link (zip file) that you can download. It unpacks and creats an Excel spreadsheet file that does all the calculations for you.
I use it all the time.
GREAT!.

http://www.vintage-radio.com/repair-restore-information/valve_dropper-calcs.html

:This site explains it all.
:The nicest thing is that at the very bottom of that page he provides a link (zip file) that you can download. It unpacks and creats an Excel spreadsheet file that does all the calculations for you.
http://www.vintage-radio.com/download/vintage-radio-calculations.zip
:I use it all the time.
:GREAT!.
:
:http://www.vintage-radio.com/repair-restore-information/valve_dropper-calcs.html
:

Nothing wrong with using files of this sort as a shortcut but you will do yourself a favor if you learn to do these and other calculations yourself. Understanding what is really going on will serve you well in this hobby. The more you learn the more you will enjoy.

:This site explains it all.
:The nicest thing is that at the very bottom of that page he provides a link (zip file) that you can download. It unpacks and creats an Excel spreadsheet file that does all the calculations for you.
:I use it all the time.
:GREAT!.
:
:http://www.vintage-radio.com/repair-restore-information/valve_dropper-calcs.html
:

This subject comes up all the time. The link Peter has posted is a good one. However the question about how does the dial lamp work now? Lift the last tubes heater going to ground, now add in series the dial lamp from that point going to ground. Use a #44 lamp, and a 47 ohm 1 watt resistor shunting across the dial lamp. Also here is a link showing how a 300MA heater string with an 8.2 MFD AC capacitor inside the old ballast tube can. Scroll drown the page to see it.

http://www.oldradiosrus.com/build.html

:I've got several "curtain burners" (RCA T4-10, etc.)that I'd like to get working again by replacing their defective resistance line cords with AC capacitors.
:
:Does the value of the capacitor have to be exactly what is calculated? If not, what is the tolerance with these?
:
:I've also heard of lots of ways to determine the value of the caps but nothing about where I might get them. I need about 5 of around 7uF. I'd like to avoid the massive motor starter caps if I could.
:Hi, Robin, usually i use the small x2 type poly caps for this job they are designed to be connected straight across the line supply, they are available in many values , up to about 2uf. you can connect them in parallel to give you the right value. you will have to do some maths(Pythagoras!!) to find the correct value and you need to connect a surge limiting resistor in series(100 ohms or so). In europe these caps are usually rated for 250/275 volts AC and are available from most component suppliers i have converted several sets and they work fine. hope this is of help.. john