Go to this site:http://www.ifwtech.co.uk/g3sek/boards/triode/triode-1.htm
If you have trouble with the URL put "triode board" in google. Then on the menu select download the manual. It is an education on amplifier and power supply protection techniques. It explains the metering, safety issues, surge protection.
You need the diodes Tom recommends. I have first hand experience using them and still managed to destroy some components in my amp when a "believed to be good" tube shorted grid to plate. My diode worked for awhile (a second or two ?) as the plate supply capacitor dumped amps into the amplifier, and then a small wire connecting the board the diode was mounted on to ground opened up. Somewhere in the docs I think you will see that the diode should be mounted directly to chassis, and that's why Toms suggestion to use two of them (one in the amp and one in the ps is a good one). It won't upset your metering if you do like it says in the triode board discussion. With advice from G3SEK, W8JI, and W1QJ you can't go wrong.
73, keep going,