Transformer VA

[Littlewyan]

If I have a 15VA Transformer with both HT and Heater secondaries, how can I work out the current limit for both windings? And if one winding is using less current does that mean the other can use more?

[tubeswell]

The VA rating is rated for what the max primary draw is. The total VA on all the secondaries combined can't exceed the VA on the primary winding. What VAC do you get on each secondary when the primary is hooked up (to the mains)?

[Littlewyan]

240VAC and 6.3VAC

[pdf64]

Those may be the rated voltages at full load; if so, what do they measure when open circuit / unloaded?
The regulation of small PTs is usually around 10%, so expect them to be that much higher.
Pete

[Littlewyan]

I don't know about under load but the 6.3V Secondary is running at 6VAC at the moment, yet its not running at full load. The supplier said that the 6.3V Secondary can take 1A and the 240V Secondary 30mA. However he said he did manage to get 1.1A out of the 6.3V Secondary as the transformer has a 15VA Core. I just want to see if I can run any of the secondarys even higher.

[tubeswell]

240VAC and 6.3VAC

so assuming the 6V winding is rated for a couple of 6V6s and a couple of 12A_7; i.e.; 1.5A, then 6.3 x 1.5 = 9.45VA

Leaving 5.55VA for the HT winding. 5.55/240 = 23mA

Hardly seems right does it. Are you sure it's a 15VA primary?

Edit: seeing you replied as I was typing, I see the heater winding is rated for 1A, which is kinda like 1 x 12A_7 and 1 x EL84 (and no lamp), or 1 x 12A_7 and 1 x 6V6 and a 300mA 6V lamp

[ToneMerc]

I don't know about under load but the 6.3V Secondary is running at 6VAC at the moment, yet its not running at full load. The supplier said that the 6.3V Secondary can take 1A and the 240V Secondary 30mA. However he said he did manage to get 1.1A out of the 6.3V Secondary as the transformer has a 15VA Core. I just want to see if I can run any of the secondarys even higher.

Usually KA is given for the HT winding as the filaments and rectifiers windings are usually known or stated.

15VA/480V= 31mA

TM

[jazbo8]

I don't know about under load but the 6.3V Secondary is running at 6VAC at the moment, yet its not running at full load.

Then the seller over-rated the transformer... unless your mains is too low?

[Leo_Gnardo]

Meant for a preamp at best. That'll support 2, maybe 3 ordinary preamp tubes. Ask it for more and you'll get smoke.

[Littlewyan]

It is only rated for Pre Amp Valves.

6.3VAC * 1A = 6.3VA

This leaves 8.7VA left for the other secondary.

8.7VA / 240VAC = 36mA

Mains isn't low, I get 240VAC out of my wall socket. Its not like the PT is being overloaded as at idle the 240VAC secondary runs at 340VDC and under load it goes down to 330VDC. When I get a chance I'll take the valves out and measure the heater secondary to see if it goes up to 6.3VAC.

Reason I'm asking about all this is I wanted to try an ECC99 for my Output Valve (currently using 12AU7). Unfortunately the ECC99 draws 800mA for it's Heater!!!!! So I think that's out of the question.