Cheap Seats: 3-500 tubes can be had for $169 from ESRC1 Three will run CCS duty on FSK or AM. http://www.esrcvacuumtubes.com/vacuumtubes_tubelist_tubes_3-500-3v4.html These are Chinese. "Warranty: Tubes are guaranteed for 90 days, except for breakage, open filaments, or when not operated within manufacturer’s specified ratings." These are the cheap grade of Chinese tubes. The RF Parts one for $375-400 are the better grade. It was my experience that unlike USA manufacturers did, China sells what are almost 'seconds' for cheaper prices. The first rate ones are more. I have been running a pair of ESRC1 Chinese tubes in a class AB2 modulator for years. Sometimes with 3500VDC and 800mA on voice peaks. In spare time I am working on the transmitter. It's an old homebrew, and is unstable at high power and modulation - so I will run a tone through the modulator (orange plates, 3000-3300V @ 500-600mA) while tuning the Class C stage. I mean to say, a 600-700W carrier, and 100% or sometimes a little more positive modulation and this goes on for 5-10 minutes at a time.. The $169 tubes have held up just fine. They were $135 when I bought them maybe 15 years ago. I do not offer the above as an advertisement, but as a statement of my experience and today's cost. and to say don't be afraid of the Chinese tubes at cheap prices. Try them, and you got 90 days, run them CCS for a few hours on and off, (if your amp has the nuts and bolts to do that) and you'll know quick if you need the warranty. So what tubes? OF COURSE!! The glorious Chinese Buffet of 3-500C tubes, and any amp that will take them. Two will run SSB and CW all day and night. Two not enough? AM cariers and FSK? CCS? Three will give 1500W dissipation, lots of margin on the tubes, and so cheap compared to amps with expensive tubes. If one wants three tubes but can not build from scratch, obtain two inexpensive type of identical linear amps that use 3-500Zs in pairs (SB-220 anyone?). Move the power supplies into one chassis and add a third tube to the other chassis. Not technically complicated to do the job, or a cooling upgrade the amp needs anyway. Good seats: The 3CX3000 is the only tube I would spend 'big' money on. 275W grid, 4KW anode, 1500W out at 100W drive. $799 rebuilt at RFParts. Amazingly cheap Watts per dollar. It should last a lifetime in ham radio use. I have been running one in a SSB-rated amp for several years. 100W exciter is perfect. The amp would to AM/FSK but the power supply is at its KVA limit for 'legal limit'. OK by me but I like overkill. So there's a beastly amp being put together that is CCS all the way with extremely generous ratings on all components. OK so space and power comsumption are of no consequence here. Only durability. Overbuild everything. Just don't spend lots of crazy money on little tubes when normal money will get a big tube. Happiness.