[RadioSvit]

It seems it was more popularly used by Philco than in others, but I think that they used the other types on their higher end radios and/or abandoned the technique post war, or maybe it did not work well on beam power tubes? 41s (6K6) and 42s (6F6) are pentodes.

So . This scheme worked only on pentodes. A significant advantage of this scheme was the simplicity and economy of the additional cascade, but there were two significant disadvantages - underestimated maximum power (5 ... 6 watts instead of 8 ... 10 on two 6F6) and slightly greater harmonic distortion (up to 5% ... 8% instead of 2% ... 3%). That is why, in my opinion, such a scheme was used only in budget models and middle-class models.

But I will say that I do not hear significant distortions in Filko 37-640. Its sound is just very nice and despite the smaller speaker than the 9K is more powerful and clean.