Below is the attached datasheet for the 13E1. To those on this forum, I do not know if it qualifies as a legitimate "boatanchor" tube. However, it has a 90W max plate dissipation, and it can be driven by "modest" voltages in the range of 350-450Vdc to produce about 16W in triode mode and 25W in UL mode. http://www.turneraudio.com.au/MonoblocSE25.htm
Turner comments on the 13E1 in triode, UL and tetrode modes:
"it has an Ra of only 300ohms, and it will generate a very sounding nice 16 watts into 1.5kohms."
"I found that the screen could be connected to a tap on the OPT primary at 66% of the turns and there was negligible increase in the complexity of THD spectra compared to triode, where the THD consisted almost entirely of 2H. 33% taps were too similar to tetrode. So I settled on the 66% UL tap for the 13E1, and got up to about 27 watts instead of only 16 watts with triode, but because Ra with 66% UL is 600 ohms, some global NFB was essential to keep the output resistance of the amp low for a good damping factor."
"In pure tetrode the tube is not very linear and generates a large array of harmonics even at a few watts. It is not suited to SE use in pure tetrode, ie, with a fixed screen voltage. But when the screen is connected to the anode to make the tube work like a triode the linearity is no worse than many other large genuine triodes"