Sometime around 1965 Gates changed over to 807s in the BC-1T. I believe the reason was that 6BG6Gs were an early sweep tube that was used less and less in TVs, and the price was going up. Gates provided a retrofit circuit board with 807s, with detailed step-by-step instructions for the installation. The characteristics of the tubes are very similar (both 6000 micromhos transconductance, 0.9A filament, etc). I remember, because I was working as CE of a local daytimer that used a 1T, and did the conversion over night so that they were ready to go by next morning's sign-on. https://tubedata.wernull.com/sheets/141/g/GL807.pdf https://tubedata.wernull.com/sheets/127/6/6BG6G.pdf The 6BG6G is essentially a 6L6G with a plate cap on top. So is the 807, but (except for the very latest production before the tube was discontinued) it has extra shielding to make it more suitable for RF amplifiers along with a 5-pin base, but even so the 807 is still squirrelly. The performance of the two tube types in that transmitter was identical as far as I was able to tell.