[morzh]

The R(theta-j)A is 62.5C/W.
So you temp rise vs the Ambient is 126C.
At 25C ambient (and this is when your radio starts) this is already 151C which is 1C above the 150C limit.
And if the ambient rises just 10 degrees (which should be easy with tubes around, plus the transistor itself, being very hot, helps bring it up), well, that is then 161C.

So, yes, get thee a heatsink that lowers the The R(theta-j)A to half of its no-sink value, and you just might be OK.

Here's your calcs.

Say, again, your TIP50 has R(Theta-J)C (to case) as 3.125C/W.
Let's use a heatsink with Thermal resistance "R(theta)SA (sink-ambient)" of 24C/W.
https://www.digikey.com/en/products/deta...0228387720

You resulting R(theta-j)A is therefore reduced to R(theta-j)C+R(theta)SAZ= 24+3.125= 27.125 C/W

At 2.016 watts your temp rise over Ambient is 54.7 C. So you could potentially have your ambient air at 90C and still not exceed the limit. And if you have space for a bit better heatsink, this will bring it down even more.
Like this one, with resistance of just 12C/W.

https://www.digikey.com/en/products/deta...0228387720