Power transformer draws current with no load

[Axtman]

I am getting back to working on a GK 200MK amp. If you remember from my earlier posts this amp's power transformer is always on even when the amp is switched "off".

The transformer was humming and I solved that by tightening the bolts holding the core leaves together. What I noticed is,without a load, the transformer draws about 7 watts or enough to dimly light my light bulb limiiter.

I thought unloaded transformers drew very little current. Should I be concerned?

[TomCarlos]

A few quick tests....


(1) You disconnected the amp from the bulb limiter and tested the limiter to make sure it has no internal short of it's own? Bad switch, stray wires, its own short to group?

(2) You opened the amp, disconnected wires going to the power switch and retested (taping off the hot lead)? If the bulb limiter is still lighting, test the wiring in your power cord for shorts.

(3) You removed the main fuse and see if the bulb is still lit?

Can we eliminate those two things from being the source of the problem?

[Helmholtz]

A no-load excitation/magnetizing current around 100mA is normal.

[xtian]

It is normal for PTs to draw 8-16w at idle, unloaded. OTOH, the Mackie SR1530z I just had on the bench, rated at 500wRMS, was only drawing 25 watts while pumping out some tunes! Our vintage amps are pretty inefficient.

[Helmholtz]

It is normal for PTs to draw 8-16w at idle, unloaded.

Is that real power or apparent (VA) power calculated from Irms times Vrms?

A typical PT may have around 10% total losses at full rated power. Large toroidals only have maybe 5% losses.
If well designed 50% are copper and the other 50% are core losses.

So a 200VA PT may have up to 10W core loss (real power) at idle.

[Axtman]

Just to be clear, I removed the transformer completely from the amp. There was nothing connected to the taps. I then connected up the primaries to the dim bulb limiter. I used on of those 'Kill-A-Watt" power consumption meters. It tests for power, voltage, current, cycles, etc. I have found it to be pretty accurate.

[Helmholtz]

I then connected up the primaries to the dim bulb limiter. I used on of those 'Kill-A-Watt" power consumption meters..

So you measured the sum of power delivered to lamp and PT.

What is the PT mains current without bulb limiter?

[xtian]

Yes, my experience is also with a Kill-a-Watt device showing me watts consumed from mains, and the figures I mention (~8-12 watts) are observed with no bulb limiter, just the PTs, connected only to mains.

[Tom Phillips]

I agree the other's who said that there is not necessarily anything wrong with your PT based on your measurements. Think of it like this ... The transformer is ON even though the secondaries are not powering anything The transformer may not appear to be doing anything but it is heating the room a little.

Following are some actual measurements I made of the magnetization power of a good properly functioning transformer with all the secondaries open circuit and nothing but the measurement equipment connected to the primary (i.e. no light bulb limiter.)
Fender P/N 022723 in a 1966 Vibolux Reverb: 137mA, 5.8W, 16.2VA.
This is provided just to illustrate the point since this PT is from a 35 to 40 W tube amp and your GK 200MK is a 100W solid State amp.


Note that there will also be differences in readings from various samples of the same model transformer and differences in the model to model design quality due to the price/performance target.

[The Dude]

If there was a problem with the transformer, it is highly unlikely secondary voltages would be correct.