Hot power transformer

[jKeenan]

Greetings All,

I completed my St-35 clone using Ned's PA774 power transformer.

I notice that this transformer is hot to the touch while operating.

It is a hot August day in Florida and I don't need the extra heat.

Does the Hammond 270HX power transformer run cooler?

Regards, Jim

[TerrySmith]

Nope, the Hammond power transformers get plenty hot too! :angry4: They are rated for 115v on the primary. The PA774 is tiny compared to the 270HX, so I imagine it will get really hot.

I have often thought of punching vent holes in the bells for extra cooling. Any opinions on this? :-k

[EWBrown]

I don't think punching holes in the end bells will help that much, if at all.
A better choice would be a slow quiet fan blowing air across the transformer. I've used the PA774, and the Hammond 272FX and 272HX, and all run very warm to hot during extended operation.

Perhaps putting a thermal conductive pad or even heat sink compound ("grease") between the bottom of the power trannie core and the chassis might help draw off some of the heat, but I don't think this is really necessary. If you are using a steel chassis and are getting some magnetically-induced mechanical "hum", then the pad below the trannie should reduce or eliminate this.

/ed B in NH

[Guest]

I notice that this transformer is hot to the touch while operating.
It is a hot August day in Florida and I don't need the extra heat.

The original Dynaco PA774 ran a lot hotter...there was almost 50% more
DC resistance on the secondary!

One thing I would suggest is to check the voltage across the cathode
resistor, do a little ohms law calculation and see how much current those
EL84's are actually pulling. If it's close to or more than what the
transformer's rated for (180 ma), I'd think about either raising the
cathode bias resistor value or getting a set of EL84's that don't run as hot.

If any transformer is run to where there's enough load for it to
regulate well (you don't want it run nearly unloaded), it'll
get warm, at least.

Some old TV power transformers actually had heatsinks
on the endbells...no room for that on an ST35, though..

[Shannon Parks]

If any transformer is run to where there's enough load for it to regulate well (you don't want it run nearly unloaded), it'll
get warm, at least.

There's a good power transformer document on the 'net somewhere - I wish I had archived it. It isn't by us tube DIYers but by a manufacturer, and, IIRC, a 50 to 60 degree F rise in temperature over ambient was normal and expected. I find that 135 degrees fahrenheit is very hot to the touch - maybe I can touch it for 2-3 seconds. This is the temp range I've tested the original PA774's and the Hammond PT used in the Ike, using a calibrated infrared meter. I've heard of some power transformers - like the Mark III transformers - getting over 170 degrees F after running all day. They'll still outlive your DVD player by a factor of ten, though.

As for heating up the house, that's the price of good music. :thumbup:

Shannon

[Guest]

I taped a thermocouple to the surface of the PA774 laminations. My thermocouple thermometer indicated the temperature stabilized @ 133 degrees F after six hours operating time and an ambient temperature of 70 degrees F. It is not as hot as I perceived.

With the info I received from y'all, I wil save the price of a new power transformer. Thanks!

Shannon, This project worked the 1st time with no smoke. Sounds great!

[erichayes]

Hi All,

Interesting (I hope) sidebar:

My winder started out making transformers for battery chargers and arc welders, and all he understood was 100% duty cycle. When I sent the specs for the 1773 power transformer to him, he assumed that it would be running at 100%, and wound it accordingly.

What I received was a PT roughly the same size as the PT for a Stereo 70. Quiescent temp runs around 25°C: full power sine wave (15+15 WRMS @ 0.3%THD) gets it up to a stable 65°C.

I've since explained to him the difference between continuous and dynamic duty cycles, but he still winds my trannies very conservatively. I've given up arguing with him, and the amplifiers reflect the battleship design he's given me in the way they sound. 6BQ5s rule!

[Ty_Bower]

The original Dynaco PA774 ran a lot hotter...there was almost 50% more DC resistance on the secondary!

Does anyone know the actual DC resistance of the secondary on a PA774? I've searched in vain...

I'm curious because I've got a real PA774 in front of me right now... I'm shocked to see about 170 ohms across the secondary winding. That's at least a whopping quarter ohm per volt, if you believe the 330-0-330 rating of this part. If you believe the chart, this transformer should be good for about 100 mA or so into a cap filter. Dynaco had it overloaded by at least a good fifty percent. No wonder it ran so hot!

Still scratching head...

[Blackburn Audio]

I measured the DC resistance of one of our dynaclones and it is 142ohms across the HV winding. The 774's do run hot but I wouldn't worry about it to much, I've had one sitting in the front window of our shop running 8-10 hours a day cooking in the sun for about 6 months now with no problems. We are working to upgrade them though to see if we can get them running cooler, its difficult though with the size restrictions we have to keep them fitting in the original ST35 Chassis's. Hopefully within a month or so we will have an upgraded version that should run cooler.

-Matt at Triodeelectronics.com

[EWBrown]

I have a couple of the early old original tar-dipped PA774s, and those do run very hot, and they are butt-ugly to boot... My SWAG that the goop was used to eliminate any mechanical AC hum / buzzing from loose lams or windings inside the core, but then they must have improved the tranny design and subsequently got rid of the "tar baby" approach. Good thing :o

Using the CL-90 takes a few VAC off the primary, so the iron gets a little bit of a break in the thermal department, and should run a few degrees less hot.

/ed B in NH

[thspeakerdude88]

I ordered the new PA774. I read that they had been redesigned with 120v primaries ( instead of 115) to help keep them cooler. Are they any better than a hammond?


Also how many different versions of the PA774 are there? I have heard of so far ( original obviously) DIY transformers, the ones from trioeel, and at least one other copy. The one I got is from triode.