Driver tube flashing! ST-35

My driver tube on my ST-35 is flashing (only one channel)when turning on!
The sound is still good as usual!
Any idea? Time to replace the driver tubes?

I would appreciate your inputs.

Thanks

Bob's suggestion is a good one. I always have spare tubes around and also a set of "junkers" for testing and burn-in purposes. its a pain because you have to buy the extra tubes and store them but when a tube dies you can instantly replace it instead of waiting for one to be shipped.

I thought there was a post about the Mullard flash as being normal.

Thank you, Bob, for your suggestion!
I did swap the driver tubes and the same tube is now flashing on the other board.
So it is certainly the tube is 'on the way out'.

Thank you.

But...is it a Mullard? MANY Mullards "flash" brightly at turn on and do so for their whole lives. There may be nothing wrong with the tube.

I don't know who made the 12DW7s in my dad's old EICO ST-40 but they "flashed" like that the whole time he used it. Still might, but it's been in storage for more than thirty years and I would not want to just plug it in and turn it on!