Can a power transformer be TOO BIG for a build?

[bastardbus]

Interesting thread and I recently got educated on this myself when I converted an old PA amp from 6L6's to 6V6's...the lighter load made the B+ and filament supply voltages go too high so I changed the cathode resistor back to a 6L6 value and installed new 5881's this evening and the voltages look much better....so a good lesson for me and I'd now say a PT can definitely be too big, before this did not realize that changing the secondary loads from the design values would make such a large difference.

Bill, there is two different factors in PT "size" one is the voltage output and other is current.

In your case you had a PT designed to run 6L6s which need voltages in the 400-450 area and the PT designed for 6V6s run at 300-350 area. EL-34s run even higher and EL84s run at lower voltages. In reality if using vintage iron you really should stick with the output tubes the PT was originally designed for.

What I am talking about is current. That means my PT though designed for a 6V6 voltage it has about a dozen extra pre amp and radio tubes in the unit as well. That equals more current needed and a bigger PT. My question was regarding using a PT with a much higher current supply then I need.

[billc]

I understand what the ratings are about, my point was that running the transformer at a lighter than design load and PULLING LESS CURRENT will in some cases cause the B+ and filament voltages to run high in itself and that is what I am seeing...B+ is about 30V lower running 6L6's and filament voltage is about 0.4 volts lower lower...so my transformer is in fact too big for a 6V6 build because it is designed to deliver more current on the B+ and 6V filament windings.

Your original post asked if a PT could be too big and the answer is YES.
Secondary voltages are NOT independent of current demand and transformers in general are not perfectly regulated, see link below....it discusses step down transformers but the principle is the same.

http://ecmweb.com/basics/basics-transfo ... regulation

also...

http://www.valvewizard.co.uk/Transformers.pdf

[bastardbus]

Well, you really just repeated what I said LOL

As far as my original post, the question was aimed at using a larger 6V6 based PT in a smaller 6V6 circuit. I would not even try to use something like a 6L6 based PT for 6V6s even with a similar current rating. The voltages will be too high despite higher current ratings.

Yes, I agree that running a large current PT in a smaller circuit might increase voltages but from what I have heard it is a very minor increase and well within tolerances.

[Regan]

I think Bill was just complementing your interesting thread and giving a recent insight he had into an amp that has a voltage range that would run both 6v6's and 6l6's fine.
You do know 6v6's aren't strictly 300-350v, right? they have been run well over 400 volts on a lot of vintage amps,, some which can accept both 6v6 and 6l6 tubes.
Same goes for 6l6's, some amps barely run 6l6's over 300v instead of the near 400v or more that is the norm.
Its when your in these grey voltage areas that you have to watch out- its not going to double voltage, but it can jack up significantly enough to push you over the cusp.
Thats why I suggested going for a larger output tube like a kt88 for those single ended ax84's,,,, they can easily run on the voltage, you have the current capacity and the tube would last for a long time cause it wouldn't be strained.

Regan