PT current

[mrn1ngstr]

Ill start this by saying Im still learning the finer points of transformers and their sweet, sweet mysteries. How much over the necessary current can you go when selecting a PT? For instance, for an EL34 I need 1.5A for filament and about 115mA for plate/grid, 6AU6 300mA filament and 15mA plate/grid, 6AV6 300mA filament and 1.2mA plate- totaled I need at least 2.1A filament and 131.2mA plate/grid. How much over that can I provide? Right now, Im looking at a PT rated for 6.3V/3.5A and 720V/138mA.

[xtian]

The only drawbacks to too much available current are size, cost, and no sag.

Did I understand you?

[mrn1ngstr]

Im not at all concerned with cost on this one, so theres that. I have plenty of space so size is not really an issue either. As long as there aren't any adverse effects Im not aware of, Ill deal with no sag. Im mainly gonna be tinkering around with this. Had the original iron been good, Id have left it basically spec, with only the most minor of changes to get it guitar ready.

[Phil_S]

Current required by the circuit is really a demand side item. The transformer is the supply side. As such, the PT is really passive. It is capable of supplying the demand side up to it's limit. If the demand exceeds the rating of the PT, generally, too much heat is created, the internal insulation melts and the windings short. This makes them either fuse (ohms = 0) or go open (ohms = infinity; depends on mother nature), but the result is the same effect. The transformer wire insulation is a high temperature-rated varnish-like substance that coats the wire. For this reason, you should buy the transformer that will match the expected demand.

I suggest it can be impractical to buy a PT that is too much over the circuit requirements. They start to get big and heavy. This has consequences for the thickness and size of the chassis. If you get silly enough with it, you'll need to buy the transformer-roadie package because you won't want to carry it around. It can also make it tipsy due to the weight at one end.

The one you are looking at seems to be appropriate for what you want to do. A little extra can be a good thing, particularly if you want to tinker.

[mrn1ngstr]

I get all that dealing with the construction, weight and ways they break down, I was just unsure if there were other things I hadn't really anticipated with regards to this. One thing Ive learned from doing this stuff is to always find out how much is too much, if at all. Thanks for breaking it down though. The one Im looking at is slightly bigger than the original, but not enough to be an issue with regard to space and weight. The biggest change is, instead of something rated to run a single 6L6, Im getting something thatll provide juice up to a single KT77.

[MakerDP]

You want to shoot for 10% to 20% over on the current rating. That one you selected I would consider to be technically "correct" but I would try and go over just a little bit more on the plate windings. The filament rating looks great though.