Power Trans runs Cooler when Amp Driven Hard ?

[SoulFetish]

Not sure if I understand.
A mains transformer (or any transformer connected to a low impedance voltage source) must no be designed to idle near saturation (especially with a stacked core). Otherwise its behaviour (primary current, dissipation) would be uncontrollable. Tolerances in construction, core material and mains voltage fluctuations might lead to "full" saturation where input current is only limited by the primary DCR.

I agree, and I would imagine any such transformer would fail rather quickly if used in an audio amplifier. But I was able to find a couple of references I remembered reading about that mentioned applications where good regulation was only really needed under full load, and small core/high magnetizing current designs are used.

From Rod Elliot's site:
Transformers, Part 1

"For a given core size, a higher magnetising current is the result of using fewer turns on the primary, and that improves regulation because the wire can be larger. However, if the no-load current is too high, the transformer will overheat because the core saturates, due to the high primary current. A transformer that is never operated at no load can be designed to be far smaller than otherwise.

If we assume that a transformer for a particular application must provide good regulation and that it is only ever operated at full load, there is no reason to make the core as large as would otherwise be necessary. We can also use fewer turns and reduce resistive losses. Modern microwave oven transformers fall into this category - if they are operated with no load, the magnetising current can be so high that the transformer would overheat and fail, but when run normally (powering a magnetron), they are perfectly suited to the job. Most are also fan cooled, allowing them to be smaller still!

When a transformer is only operated at full load, magnetising current is no longer a major consideration, and the number of turns needed is based on the effective voltage across the winding at full load. A 1kW transformer might normally have a primary resistance of around 1.0 to 1.2 ohms, but if that can be reduced, copper loss is also reduced. At 1kW, the primary current is 4.35A, and that would reduce the voltage seen by the transformer by perhaps 5 to 6V RMS. Rather than designing the transformer for a nice low magnetising current at 230V, it can be designed for a somewhat higher magnetising current at 225V - magnetising current alone might be as much as 1 or 2A - perhaps more. "

and:
Transformers, Part 2

"It is very common that an MOT taken from an oven that is less than 15 years old will be wound such that the transformer is well into saturation at no load. In one unit I tested, the unloaded current was 1.2A (yes, 1.2A - not a misprint). The core started to saturate at only 150V, and by 240V was very heavily saturated. In its intended use, this will not cause a problem - remember that core flux decreases when the transformer is loaded, and a microwave oven also has a fan, and normally never runs for very long. The transformer is never operated unloaded unless the magnetron supply circuit is faulty or the magnetron itself is dead."

---
But ignoring this for a moment, I'm still curious about core losses (eddy currents, etc.) differ from copper losses in how the surface temperature is affected.
For instance, today I had a B-52 head on my bench that was only operated for observation on my scope and audio tests. It was only briefly under full signal conditions for a short time stress test new output tubes, and at nominal loading for a couple of minutes to run through some audio tests. When I flipped the chassis around so that it was "right-side-up", I notice that the PT was pretty warm to the touch. I was monitoring the mains current, and nothing seemed to indicate anything abnormal for this kind of amp. But I took note of this, because the design, schematic, and layout of the amp looked a LOT like an Egnater design, and I've seen several of his Chinese power transformers fail miserably.

[Helmholtz]

Yes, I mentioned the reduction of effective primary voltage (and consequently voltage-time area and magnetizing current) by the load current. It can be explained from the transformer equivalent circuit.
But a mains transformer for a variable load as a class AB/B amplifier cannot be designed to saturate at low/no load. Otherwise it couldn't even be tested at no load or stand-by without blowing the mains fuse.

[mozz]

Do you have a watt meter? Measure at idle. Measure at signal. Draw will go up with signal if a PP design. If wattage stays the same, there is no possible way anything is running hotter.

[HaroldBrooks]

It's not too big or invasive a job to fit such a fan on to the amp chassis, see http://www.blueglow.de/FenFan.html for how it might be done.

Nearly doubling a cathode bias resistor value is a big change; it will almost certainly result in a significant degree of bias shift at high signal levels - stick your volt meter across it and crank out a power chord.
A bit of bias shift, squish as Aiken calls it, is fine, but a lot will tend to thin the tone out.
Excessive idle dissipation in cathode bias is usually a result of the HT being too high; given your scenarios, I'd prefer to add 150 ohms into the HT supply side of the power tubes' current path, rather than their cathode return.

Today I removed the 350ohm cathode resistor and put in a 250ohm one in it's place. The original 200ohm factory resistor was actually 219ohms, so I decided upon the 250ohm as a good compromise to just knock of a 1/2 watt plate dissipation on both tubes.

I have to tell you, the sound is indeed better ! You guessed it for this amp and circuit. Low bass is better with the hotter original type bias, and the amp is just like it was when I first got it, a tiny bit louder and fuller sounding.

Funny how I was obsessing about the transformer being too warm, and neglected the change in response and "thinning" tonal changes from the much cooler bias. With this amp it was noticeable. Never underestimate the ability to trick yourself based on a single desire or criteria. We are all "Biased" along with the very amps we work on.

You will be happy to know I also installed a really small (but fairly powerful) 40mm 12vdc fan running from a small wall DC convertor and mounted in the amp. I might one day look the internal hookup from the filament voltage, but for the present this works stunningly well at cooling down the transformer, and is pretty easy to run as it's now installed as "part of the amp" with it's own small power supply so setup is not burdensome.

After two hours of playing and idling the highest power transformer temp was 122 deg Fahrenheit, and that's really "cool".

Thanks for your sober thoughts regarding this post. I got to eat my cake and keep it now on this one !

[HaroldBrooks]

I hope you don't run the amp enclosed in the white box (bedside locker) as shown in your pictures.
Nothing wrong with using a fan.
The PT is not necessarily the most temperature sensitive component in an amp.

Sorry for the extreme delay in response ! I don't run the larger push pull amp in question in that cabinet, but I have run a Fender princton and little Gretsh single ended 6V6 amps in that cabinet. They are cool running after a couple of hours respectively, thanks to a 4 inch DC fan I added in the back of the cabinet, and a slot IKEA built into the cabinet at the bottom, so fresh cool air is ever present in that little isolation cabinet and that makes for a better environment than if the amp was outside, particularly when the back of the amp is facing the exhaust fan. The fan is a fast one, and I'm not sure of the CFM flow, but has to be fairly high.

Thanks for the warning just the same. I have a couple of infrared thermometers and I've checked everything from the tubes, to the chassis, to the transformers, all run cooler because of the forced air exhaust setup.