[Novice-Rigs] Transformers

[Brian Carling]

On 31 Dec 2001, at 16:08, KU4YP wrote:

> this seems to be a simple question, but this is the first tube
> prject i
> have tackled and i do not know how to measure the electrical value
> of transformers. 

OK Red, best way to do this is with a variac and a meter.

You use the variac in case you are on the wrong windings and using this as a step-up 
transformer.
Heaven knows if you apply 120V AC to a 5V filament winding you are going to low a fuse or 
circuit breaker, AND produce over 2000 volts out the primary for a few seconds!
Not to mention what would try to come out the high voltage scondary - maybe several times 
that. But it would not last long! Don't want you to blow up your nice transformers you 
found under the tables at the hamfests!

If you are SURE you know which winding is the primary (USUALLY pair of black wires on 
American xfmrs) you connect your AC source via the variac to these. (see below)

Set your VOM to AC volts and connect the test leads to the red wires first 
and see what high voltage you get there.
I usually apply about 3 or 4V to make sure the fuse won't blow, then gradually bring her 
up to about 30V on the primary. Watch your fingers and don't come in contact with the HV 
wires!! Use clip leads with the power off. As you bring the variac up to 30V, you should 
see some AC voltage appear on the red HV wires. Multiply this by 4 and you will know what 
your unloaded AC supply is for the B+ rectifier ("B Plus" means HT for all you 
non-Americans.)

If there is a red and yellow-striped wire, that is the center tap for the HV secondary.

Next, for the heaters, you go to the GREEN wires which will typically give you 6.3V AC or 
something close to it. Some transformers will also have a pair for 5V that supplied the 
tube type rectifier with its filament power. These are often YELLOW.

Remember colors vary from manufacturer to manufacturer but the normal standard was:

120V AC = Black and black OR black - white
High Voltage = Red-Red and maybe an addition of Red with a yellow stripe for CT
6.3V AC = Green-Green and maybe an addition of Green with a yellow stripe for CT
5V AC = Yellow-Yellow

Remember two things: Under no-load conditions, these windings will give you a higher 
voltage than normal. You might see 6.9V or even 7.2V AC for the heater / filament 
winding. Same with the B+ secondary. You will maybe see 1000 volts with no load, but as 
you apply power, this will drop due to the internal resistance of the xfmr windings.

I hope this helps. Keep your fingers and other body parts AWAY from the electricity and 
stay safe!