A few months ago I was thinking about buying some used Neo-Classic 250s. I called the Manley repair guy (in Arizona) to ask him about the serial number of the amps (when they were manufactured, any updates since then, etc.). I also asked him if there were any common problems that the 250s had. He said the most common problems with the 250s is tubes. He highly recommended using the factory recommended tubes. He has seen other tubes take out resisters and capacitors and it gets very costly.
If you're considering other tubes than the Tung Sol EL34, I would recommend calling Manley first.
If you're considering other tubes than the Tung Sol EL34, I would recommend calling Manley first.