Featured VT: 6LR8

I had a few of these come my way: the 6LR8. This is a triode/pentode originated for use as a combination plate coupled multivibrator and vertical deflection amp. For a vertical deflection type, the ratings come in pretty stiff: P_D= 14W for the pentode power final. I drew up some loadlines, and these do indeed look promising: The h₃ estimate is quite low, and it doesn't seem to be too far into Class AB₂. If there are any problems with higher order harmonics, and I wouldn't be expecting these anyway since vertical deflection duty values good linearity in the finals. Any residual pentode nastiness can easily be tamed by means of local NFB. A PP pair could easily do 30W into a quite reasonable 6K (P-2-P) load, and stock Edcors come quite close to that anyway. Conveniently, the same envelope includes a small signal, high-μ triode. Its characteristics look linear enough for audio amp duty, and you could use them to form an LTP driver, or use them as grid driver cathode followers. Though I'd prefer the latter, as there are better small signal audio triodes for front end/phase splitter duty. The V_HK ratings also high enough so that cathode follower duty isn't going to be a problem in that regard. The possible drawbacks are that Novar (large nine pin) base, and the rather stiff heater current requirement. Those pins are awfully thin to be pumping 1.5A through them. You also have to be careful when selecting sockets. There are all too many cheaply made Novar bases with the holes out of alignment. If you have to force the pins into the socket, that's just begging for a failure of the glass seals. Letting out the vacuum, especially when powered up, will certainly have some pyrotechnic displays, and possibly catastrophic failures elsewhere. The other good thing about the type is that it is also available in some odd heater voltages originally intended for operation as a series string across the AC mains. 21LR8 -- 21V/0.45A 31LR8 -- 31.5V/0.3A These types are definitely cheaper, and probably more available due to reduced demand. You could either wind your own heater xfmrs, or have custom jobs (Edcor) make them up for you. As with all these TV deflection pents, the screen grid ratings are low enough to make pseudotriode and UL operation impractical. Another definite audio "sleeper" that didn't have audio final use mentioned in the spec sheet.