The cheapest tube upgrade you can make that will make an immediate difference is the 5U4G. Unfortunately (or fortunately for your wallet), the selection of good 5U4G is rather narrow. The best is probably the EML mesh plate which is also the most expensive. The second best to my ears are the 1940s Ken Rad CKR and Raytheon CRP which can occasionally be had dirt cheap at $30-50. There is currently a CKR posted on eBay in that range. If you're not biting on that, I probably will. wink.gif They punch way above their weight with great detailed sound. The third best is a mix of 1940s military tubes: JAN RCA CRC, National Union CNU and Sylvania CHS. The key to NOS 5U4G is the double pole structure in the middle which is normally characteristic of an early to mid-40s tube, sometimes referred to as VT-244. Not all JAN VT-244 tubes were made in the 40s. I would look to pay at most $50 for these tubes in prime condition. Nice smooth, layered sound that's only a step behind the above tubes in terms of refinement. Outside of the 5U4G you have the dirt cheap 5U4GB and the crazy rich U52. A good condition Raytheon or Japan-made 5U4GB would be an upgrade over stock tube, while the U52 excels in detail and soundstage like no other rectifier I've heard but at the expense of a bit of lowend.