I have a early Swan MKII that came with 3-400's but had the bias added for 3'500's with a jumper that can removed for using the 3-500's. I purchased it around 1980 and use it almost daily plus a few weekend contests a year. The original 3-400's still have close to full output. I purchased 4 3-400's that were new about 5 years ago for $150.00 (they were advertised as new and as far as I could tell were). 3 were good one was gassy. I rotate the 3 every year and the fourth made a cool lamp for the shack. No doubt the 3-400's are going to out live me!!!!!