Filament secondaries in parallel

[hummel]

Hello All,
First forgive me if this has been answered before. A few searches didn't find what I was looking for. OK...
I will be building a stereo PP amp using 4 EL34s. So, the total heater current is 6 amps. I have a mains transformer that seems perfect, 340-0-340 with two separate 6.3 V windings, one at 2A and one at 4A. Is it OK to simply parallel these two for 6A? Seems intuitive but, for me, transformers are still a bit mysterious. Or is the fact that they are of unequal current a problem? If it is, is there a way to balance the load, so to speak?
(BTW, for the input and splitter, I'll be using another transformer so their heaters do not add in to this.)
Thanks!
- Michael (aka Hummel)

 

[Sheldon]

I'd try it. The resistance of the heaters should maintain a reasonable balance. But to be sure, put a small value resistor in line with each winding and calculate the current for each under operating conditions to make sure. You can remove it or leave it in after you check. Of course, make sure the windings are paralleled in phase first.

Sheldon

 

[hummel]

Sheldon,
in fact, I have wired it up at about one-third, ~2A, load and nothing seemed amiss. You make a good suggestion. I'll try again, this time monitoring the current contributed from each winding. Even at this reduced load it should show the two windings contributing their appropriate share.
Thanks!
- Michael
(BTW, I think you meant the 'resistance of the windings' in your first sentence. The heaters are all in parallel, anyway.)

 

[Sheldon]

(BTW, I think you meant the 'resistance of the windings' in your first sentence. The heaters are all in parallel, anyway.)

No, I did mean the resistance (total) of the parallel heaters. A better way to say it is that both windings have to see the same voltage, and if the windings have the same number of turns and the wire size is properly matched to the current rating for each, the current under load should share pretty close to the expected ratio.

Sheldon

Edit: Actually, thinking about it a little more; If the unloaded voltage is close to equal on both windings, they should share fine under load.

 

[Elvee]

To check that both winding have exactly the same number of turns (which should be the case), connect only two ends in phase (the "dotted ones" for example), and measure the voltage between the two terminals left open: normally, you should read a few mV corresponding with inevitable flux leaks here and there; if you read more than ~100-200mV, there is probably 1 or ½ turn difference (unlikely). In this case, you always have the option of inserting one more turn (or half a turn) of plastic insulated wire to one of the windings (without dismantling anything of course)

 

[hummel]

Sheldon,
Yes, I see what you are saying. But, it is the winding resistance difference that will correctly apportion the current. You allude to that by mentioning the wire size which got me thinking (a rare occurrence but it happens sometimes) . I just measured the windings (Kelvin leads) and the 2A winding has a resistance of 109 mOhm and the 4A winding 57 mOhm (close enough to 2:1). So, indeed, the windings will 'self-apportion' the load. At least I think that is the case. Anyway, I'm gonna try it.
Thanks again!
- Michael

 

[Sheldon]

Yes, I see what you are saying. But, it is the winding resistance difference that will correctly apportion the current.

Yes, agreed.


Sheldon

 

[hummel]

Elvee,
thank you for the suggestion. I did measure the voltage difference and it was only 2.2 mV (all secondaries under no load). This gives me confidence that the transformer is up to the task.
Thanks again for the replies, the insight has helped me.
- Michael