Craig,
The 7199 isnt a better audio tube than a 6U8, 6AN8, 6BL8, or any other dual section tubes. The only extra feature of a 7199 is its spiral, hum canceling filament. But in reality, the hum canceling has little to no effect on actually reducing induced hum picked up by the filament. If you look at the operating characteristics of the 7199 and 6U8, youll see that theyre identical.
The majority of Scott amplifiers utilizing a split-load phase inverter design used the 6U8 tube, but this was before the 7199 came out. Later on after Scott introduced the 299C, the 7199 tube was released by RCA, and claimed to be superior to any other dual element tube. RCA stated in their tube manual that the 7199 was designed strictly for AF (audio frequency) applications. However, the tube really wasnt that much better. Many 7199s had high failure rates, and were more expensive. Scott used the 7199 for a short time in the 299C and 222C, and then switched back to the 6U8 tube. During this short period, no circuit changes were needed to accommodate the 7199.
There are more brands of the 6U8 tube available than the 7199. The 7199 was marketed by RCA, Sylvania, and Westinghouse. The 6U8 was made by dozens of manufacturers, some better than others. You have a greater variety to choose from if using 6U8 tubes.
The circuit modification is simple, and only requires that a few pins be switched around. No sound improvement or difference will be experienced. The whole idea of the mod is to save money. The 6U8 and 7199 typically last around 2-3 years, depending on how many hours the amplifier was used.
Ive searched around the internet and the cheapest price I found on the 7199 (all makes) was $25. Then I searched for the 6U8, and found them as low as $0.99. I used to get 6U8s at Radio Shack for $0.99, but they quit selling tubes last summer.
Bottom line: There is no great difference between a 6U8 or 7199. Both are good tubes when performing up to spec. At $25 a pop, the 7199 really isnt a good investment when you could use an identical tube for much cheaper.
Bill
wjkubacki@aol.com