Probably been mentioned, but 12 BA6 can be used in PTO and BFO while waiting for a replacement ballast tube. Jumper pins 2 and 7 on the 3TF7. You have to replace both of course as the heaters are series from the 25 V supply . I only noticed this on a PTO being checked out. Stability /performance not checked . Cheers Pete V K3IZ