I built this Kegger inspired KT88 amp a while back. I hadn't put it into regular use as the PT seemed to get pretty hot. Well I finally got a meter with a temperature probe so we are going to nail this down once and for all.
The best I could gather from a search of the forums on edcor's web site they like there PTs to be limited to a 50° F temperature rise. If I did the calculation correctly that means I can go from the 24° C ambient temperature up to about 52° C.
Been running it for a couple of hours now. In the first half hour or so it stayed right at ambient. It is now at 42° C and is, I think, stabilised. Will see where it is at the end of the Strauss Horn Concerti 1&2.
The best I could gather from a search of the forums on edcor's web site they like there PTs to be limited to a 50° F temperature rise. If I did the calculation correctly that means I can go from the 24° C ambient temperature up to about 52° C.
Been running it for a couple of hours now. In the first half hour or so it stayed right at ambient. It is now at 42° C and is, I think, stabilised. Will see where it is at the end of the Strauss Horn Concerti 1&2.
What you really want to know is the temperature inside the windings, which you can get at via DC resistance measurement. For copper, 20% resistance increase equates to very nearly 50C temperature rise, which is about the limit for vintage iron. Modern insulation systems may do better, but you'd want to confirm that with the vendor.
That looks like a modern transformer. Still wouldn’t want to go over 40C temp rise, which is pretty darn hot.
How much voltage sag are you getting from no load to full load? 10% would be safe for EI, 20% would be in the gray area (which means measure the temp), and 30% would be what you get with a factor of two overload. A factor of two overload may take hours to stabilize the temp. A sag measurement gives a rough estimate in a couple of seconds. Of course all that goes out the window with a choke input filter, which will sag 30% at low currents (by design).
How much voltage sag are you getting from no load to full load? 10% would be safe for EI, 20% would be in the gray area (which means measure the temp), and 30% would be what you get with a factor of two overload. A factor of two overload may take hours to stabilize the temp. A sag measurement gives a rough estimate in a couple of seconds. Of course all that goes out the window with a choke input filter, which will sag 30% at low currents (by design).
Thanks guys. Yes it a edcor purchased brand new a few years ago when I built the amp. I will try the DC resistance test first chance I get. I don't know the voltage sag. I would need to set up a cradle and check with and without the rectifier tube installed. It will take a little longer to do that.
PS filtering is CLCLC.
PS filtering is CLCLC.
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50 deg *rise* in a 25 deg room implies they are good for 75 degrees.
That's hot. Too hot to hold.
Yes, hot, but about any winding wound in the last 40 years will stand 75 deg C for years.
As said, you really want the winding temperature. And you really do NOT want to cook them to the rated limit.
Your observation is 122 deg F on my meat-meter. I can hold that a long time. I would NOT worry about it.
Why no mention of the transformer's design specs and what you think is being drawn through it? I like your opening post about not using the amp much because you think the PT seemed to get pretty hot. Are you within it's power ratings, just to start with? Do you drink coffee? What temperature? Have you checked the temp of your hot water faucet? Can you hold your hand in that going full hot?
I can't remember the specs as it has been a few years. I lost a lot of my documentation in a computer crash. I thought it was spec'ed adequately however.
For what it is worth it is essentially the power amp part of this project...
been laying low - revisiting a KT88 partial FB amp
For what it is worth it is essentially the power amp part of this project...
been laying low - revisiting a KT88 partial FB amp
OK. I listened for almost 4 1/2 hours today tracking the temp rise and measuring primary resistance just before turning it on and within seconds of turning it off. The results follow but in short the temp never exceeded 50°C (just touched it occasionally) from 25°C ambient and resistance rise was between 15% and 18%. It seems I didn't over-spec it but I think we are pretty safe. I will probably still rig up a test cradle and check the voltage drop but I am not nearly as worried.
PT tests:
11:50AM:
Cold Primary Resistance = 1.6Ω
Ambient temperature 25°C
12:35PM:
39°C
1:23PM
47°C
2:49PM
49°C
4:11PM
49°C (occasional flicker to 50)
1.9Ω
PT tests:
11:50AM:
Cold Primary Resistance = 1.6Ω
Ambient temperature 25°C
12:35PM:
39°C
1:23PM
47°C
2:49PM
49°C
4:11PM
49°C (occasional flicker to 50)
1.9Ω
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