Hi David, unfortunately the GU74b tubes are not readily available here in the U.K. A good friend of mine used to service and repair the Acom amplifiers for the initial importer to the U.K. He used to have a supply of tested tubes but they have all been sold. I have recently purchased a tube (NOS) from the Ukraine and they don't come with any warranty. I think the last Svetlana tubes were produced in about 1993 so wherever I buy them from in Europe they will have been on the shelf for at least 28yrs! I'm a victim of my own good fortune in that the original tube lasted for around 15yrs hi! I could kick myself for not purchasing a spare tube(s) at the time I bought the amplifier. Hindsight is a wonderful thing! Again my thanks to everyone who has contributed to the thread. Keep safe and well. 73 de Ian GW4RDW.