The 3-400Z is the forerunner of the 3-500Z and can be used as a substitute keeping in mind the lower max plate dissipation. I have used the two tubes in a single 3-400Z home built PA swapping the tubes every 18 months or so to maintain vacuum.

Although well used both tubes still provide a useful few hundred watts output. To avoid an unhappy buyer I would like the tubes to be tested by the buyer prior to purchase, so here’s the plan. I live in Victoria and will be at the Merville Hamfest on 27th April. If you live between Victoria and Merville I can come by your QTH sometime that weekend and you can test the tubes in your amp. If acceptable to you we can then negotiate a price. You give me cash and keep the tubes. Sound reasonable?