Here is a real nice quad of NOS, USA made G.E. 6550A output tubes. These have 3 getters and metal bases. They were tested on calibrated Hickok 800 and test very close at 10.500, 10,500, 10,500 and 10,500. Nominal for this tube is 6000.
It was called the 6550A, because of the introduction of a exotic 5-ply metal plate material manufactured by Texas Instruments. This metal, produced by cladding a copper and iron core with aluminum outer layers using explosives, made for a more reliable tube with greater bias stability. The clad metal was better a dissipating plate heat, and thus it lessened plate and grid emission and reduced hot spots on the plate. Some say that the GE tube had some improvements over the Tung Sol. Getting hard to find NOS 6550's.
price: 350.00 plus shipping. 
