Current rating for heater wiring in power transformer

[slidincharlie (Carlo P)]

I need to have three power transformers rewound.
What is the current rating requested for the heater secondary? What do I have to request to the manufacturer?
More generally, which method allows me to know this current rating?
Thanks,
Carlo

[guitician]

The gauge of the wire is the current rating. The lower the gauge the more current the wire can provide for a given core size.

[slidincharlie (Carlo P)]

The gauge of the wire is the current rating. The lower the gauge the more current the wire can provide for a given core size.

Do you mean that the manufacturer/rewinder should measure the wire gauge to determine the necessary current rating?
What if I have - for instance - the iron without coils? What should I tell the manufacturer to obtain the correct ampere rating for the 6.3V heaters, the 5V heaters and the high tension heaters?

[guitician]

That depends on the application. If they wind transformers for what you are building, they should know the design specs.

[52 Bill]

Carlo:
If you are having transformers rewound for specific amp circuit, you need to make a list of the tubes in the circuit, and then look up the heater currents of the tubes in a tube manual.

Add up all of these current ratings and add a little for safety margin and tell the rewinder that amperage rating. He should then be able to size the right number of turns and the right wire diameter to get the voltage and current rating that your circuit needs.

[tubeswell]

Hiya Carlo

If the lamp draws current from the heater winding, then don't forget to add the lamp's current draw to the sum total of the filament current draws.

When I get my PTs made up, I usually specify a 3A rating for the 5V winding - allows me to run 5Y3/5AR4 type tubes or 5U4 type tubes.

I usually add about 1A to the heater winding, over and above the some total of the filaments and the lamp, so if I needed a PT for a total of 3.2A on the filament winding, I would specify 4A etc.

Also don't forget the draw on your HT winding - the higher the current draw on your HT winding, the bigger the output tubes you will be able to sub in. It needs to be the total of what all the plates and screens will draw at idle, times two (some people say 2.5), for a factor of safety. Actually on 2nd thoughts some PTs are quite low current rated on the HT, to give some sag. (5E3 at 70mA is an example that springs to mind)

And lastly but not leastly, if the amp is fixed bias, get them to make a separate 50V bias winding as well. (if nothing else you can always use it to elevate heaters if necessary)

[slidincharlie (Carlo P)]

Thank you guys.
The trannies I want to have rebuilt are for the following amps:
- a 1482 Silvertone amp (6X4, 2x6V6, 2x12AX7, 6AU6), cathode biased;
- a sf ('77) Fender Deluxe Reverb amp (5U4, 2x6V6, 6x12AX7), fixed bias (well, you know this model ).
Is the bias winding a totally separate winding? Since only one bias tap wire comes out of the tranny, I wonder: where is the other end? (sorry for the dumb question: I'm here to learn) .

[tubeswell]

The bias can either be taken from a tap on the HT winding (usually at the 50V point, and the HT CT is the ground reference point) or it can have its own separate 50V winding, in which case you just ground one end of the bias winding and use the other end for your supply.

[52 Bill]

According to my RCA tube manual the heater currents for the Silvertone amp tubes are as follows:

6X4- 0.6 amp
6V6GT- 0.45 amp X2= 0.9 amp
12AX7- 0.3 amp X2= 0.6 amp
6AU6- 0.3 amp

0.6 + 0.9 + 0.6 + 0.3= 2.4 amps total.
A safe figure would probably be 6.3v @ 3 amps.

Now you need to do the same for the high tension current ratings.

[slidincharlie (Carlo P)]

Thank you Bill!

[slidincharlie (Carlo P)]

Tomorrow I'll bring my trannies to the man who will rewind them.
I'll use all your precious advice.
Thank you guys!