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I am evaluating a new device on my system and I am curious.
It runs very hot, almost too hot to keep your hand on for very long.
It is a SS device about the size of to playing card decks side by side.
The device functions very well. I am not ready to divulge what it is.
I contacted the manufacturer about this and he claims it is normal and
they have tested this for many hours and it is designed to be left on 24/7.
They claim that the case is the heat sink.
what do you think?

If the case is a heat sink, a warm case means it is doing it’s job as a heat sink.

For Pass' DIY class A designs, he advises that the heat sinks are too hot if you can't keep your hand on them for 5s....

Roscoe

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I can explain it to you, but I can’t understand it for you.

I use an IR thermometer (like the ones for cooking) to check case and heatsink temperatures.

If you cannot touch the case/heatsink very long, it is probably in the 120F+ temperature range.

Depending on the devices used, typical operating component temperatures in solid state devices go into the 65C(150F)+ range like computer CPU's. Really depends on the device and components used. As an example, the new GaN transistors can handle 200C for output devices.

It also depends on the thermal impedance heatsink to case and case to junction. It is always a tradeoff between operating temperature and heat sink size. The hotter the heat sink runs, the more heat transfer per square CM of sink area. For cooler sinks, the radiating and convective areas have to get larger often offsetting the desire for smaller products. If you don't have to handle it and it works well I would not worry about it. The biggest problem will be if the case heat warms up electrolytic caps too much which will shorten their life.

Try as I must, I cannot keep my hand on one of my GM70 tubes very long (do I smell something cooking?)

Seriously, echoing other advice if you can touch it, it is OK. I do get a bit worried about power transformers running uncomfortably hot to touch. The transformer for the Dynaco ST35 is a hot running transformer with a bad history of failure. But a solid state heatsink, no problem.

the warmest part of the cover measures 119 degrees F

In general, hotter temp = shorter life.

Keep it clear of obstructions, not shelved over other heat producing gear, clear on all sides.

Keep ambient temperature under control as well.

Putting a small, quiet fan in place to force cool it should extend it's life and give you peace of mind.

Good idea Stuart
It uses a 7.5vdc 5amp PS so I could run a small PC fan mounted to blow across the top,

At what point is too hot becomes too hot for safety sake?

I remember there was a vendor at CAF who had a large tube amplifier sitting on floor that was too hot to touch in order to relocate.

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I reject your music, and substitute my own.