What would you guys pay for an NOS 5AP4 with a good filament? I found some, but haven't acquired them. Since this is a rare tube, there are no eBay listings to go by.

Post a no reserve auction, you'll find out.

Post a no reserve auction, you'll find out.

VK needs a "like" button.

Those tubes have been selling in the $500 range.

Do you know which are good brands for this particular tube, or are they all the same?

There is quite a variation in the usability of 5BP4s for television. In particular, the DuMont tube is terrible.

I haven't seen such variation with 5AP4s. They all look about the same.