Tremendous RF problem -- phono stage is a radio

You might get lucky with changing tubes or cables, but it's unlikely and it's just a chance. The best thing is to shield the phono stage. There are a variety of ways of doing this. Most RF can be shielded by a wire cage that is tied to ground. There are other shielding materials such as "mu" metal that are also very effective. I had the same problem in one home and was successful with a piece of mu metal above and below the unit. The best shields enclose the unit entirely, but just like an antena that works best at a certain location, shielding can often be "incomplete" and still get the job done.

Rives...For many years I used shielded 300 ohm twinlead for FM signal. I am in a weak signal area, and noise from autos on the nearby road was a problem. It worked very well, but the surprising thing was that the shield was, by design, not tied to ground at either end. It was floating.

Another way (military practice) is to take a broom and clean RF off...:-)

Thank you all for your suggestions -- the "military" one is excellent. It turns out that I have some mu metal (the contents of one's garage are sometimes rather curious), and I can have it cut to fit the walls of a box that will cover the phono stage, not exactly beautiful, but the phono section is hiding in a cabinet anyway, so it won't matter. I suppose I could allso use an old wire cage that once housed a pet rabbit -- another long story. One wonders whether Faraday would be amused.

The various suggestions that have been made are worth a try. However, let me note that low impedance/high current circuits, which means a MC pickup and solid state amplification, are less likely to have your problem.