Did I really build this old amp way back in 1973 ? Unfortunately the answer is YES. So 44 years later it still works but I'm sure it needs a "going over". So.... ordered from HARBACH a power supply and soft start combo kit, and a keying interface so I can retire the external Ameritron ARB-704 which currently keys the amp. So far...so good. Next step get a set of tubes. Now it's getting "dicey". Over the years I have followed the trials and tribulations of the various available 572B's always being "scared-off" by the quality of the made in CHINA "copies". The now 44 year old CETRONs in my amp still produce over 400 watts on most bands, but I want to re-tube the amp and hope for the best. I find DX Engineering has 572B's for $82~. DXE claims these tubes are so good that "matched sets are no longer needed", and these tubes can be mounted - to paraphrase - AT ANY ANGLE. I like that because the SB200 mounts the tubes HORIZONTALLY which was a BIG no-no over the years for non-CETRON brand 572Bs. I'd get free shipping for a pair and the warranty seems good too. Then I download the DXE spec sheet for their branded 572B and noticed the "Limit Rating for Anode Voltage" is ONLY 1250V. Last time I looked my SB200 put over 2200V on the tube anodes. The SB200 manual mentions "approximately 2400V". My hope is that the DXE spec sheet is wrong and the 1250V should actually be 2250V or more. I might ask them about this discrepancy as not that I expect their tubes will fail but if there is a failure are they going to deny warranty coverage because I "misused or abused" the tubes ? For what-it's-worth the original CETRON 572B was rated at 2750V Class B Audio service, and 2000V Class C Telegraphy. Maybe RF Parts or Ameriton is a better source for 572B's. I doubt it but we shall see.