Autotransformer unloaded power consumption

[Richardgr]

I have to find a way to power a Japanese amplifier. It can run on 110VAC or 117VAC.

In my stash I have an old autotransformer. It is marked "0 110 127 150" across the output taps. I have tested it unloaded with 220VAC in and measured "110 130 150" on the putputs, so far so good. It had actually been adapted to supply 220V from 110V. On the side is written ...
Prim 127
Sek 220V 1.37A
KVA 0.3

However it hums a bit, and I used a power measuring socket on the variac and 12W are being consumed by both variac and autotransformer.

Does it seem reasonable to lose that power in the autotransformer?

The goal is to feed the 117VAC input from the 110VAC tap, assuming my voltage is 235VAC here.

[Silicon]

Does the auto transformer have a 235V tap?

If not, you should not exceed 150V AC on the autotransformer.

Transformers that are run at a voltage greater than their design voltage will saturate and overheat.

[joebog1]

Its 300 watts so should handle that no problems. Auto transformers are designed to run a load so I think the consumption is fine.
I "read " that this will be used with the valve hi-fi amp in another thread. Should be perfect.
REMEMBER though, it has ZERO isolation.

Joe

[Richardgr]

The tap is 220VAC, which was the mains voltage back in the 50's. As Joe mentioned it will be used a fairly light task against its specification.

Regarding the isolation, I bought the transformer on an auction site, and the Primary and Secondary label made me assume it was going to be isolating, so that was a bit disappointing when I flipped the lid!

The amp it will be used with does have an isolating transformer, so the plan is to copy a trick I saw in a Knight Kit amplifier, and have a 5 wire lead to the autotransformer. The autotransformer has an earthed supply, then the amplifier is used to switch that, then it gets L N E back at 117VAC on 3 of the cables. I could use a 3 phase plug and socket but not so keen on having a 3 phase plug on the amp in case someone thinks it is a 3 phase appliance. I suppose I can solve that.

Cheers, Richard

[joebog1]

There is an old rule used by old engineers.

When you work on an amplifier, put the mains plug in your pocket.

I don't think its necessary to go with the five wire route. I would simply switch off the power point when you have finished listening to your amplifier, BUT switch the amplifier off first.

Joe

[Richardgr]

Good point! I always think before I do something, mainly on the DIY side, 'How would this sound when I have to explain it in A&E?'
That would have saved me the tip of one finger!

Advantage of the 5 wire solution is I can keep the SPST switch in the amp since it does not matter too much if only one mains supply to the autotransformer is switched.

You mentioned 6AR5 tubes are sought after? Is it just because they are scarce, or is it their quality?

[-ADBeta-]

In relation to the original question it sounds like your transformer has a limit of 150v, but a design input voltage of 127v?

Either way, autotransformers are designed a lot more..... loosely... than normal transformers, so your humming could be a bad connection on the yoke/sweep contact, it could be a cheap or damaged core, coarse windings etc.
They also tend to draw a bit of idle current. 12W seems high but again that could be a bad contact, a burnt out indicator drawing power, and general losses from it being a bit old or damaged.

[joebog1]

6AR5's are basically a 6AQ5 but might have different pinning. Some people, especially early AKAI users want them for their tape decks. I have never used them as they are a bit piddly except in a radio situation where you just need to hear what's going on.

As far as the humming goes, that case it's mounted in looks a bit shabby. Maybe it just needs the bolts tightening. either in the transformer itself, or the bolts holding the transformer to the case.
In any case, I always make a "gasket " from thin cardboard to mechanically isolate transformers from steel chassis or cases.

Joe