5F6A Bassman with 8417s

[David Root]

I just roughed out on paper a 5F6A circuit with 8417 power tubes. I'm curious about 8417s as they have the reputation of fine tone, very dynamic, not bass shy.

They also have to be used carefully in a guitar amp circuit as they have potential screen issues and there were quality control issues with them too.

I know the 8417 was designed very late, in 1965, and to my knowledge there are no current manufactured versions.

I am going to rework a previous early amp of mine that didn't work out too great, as parts of the circuit lend themselves to this design, and I don't have to buy a cabinet or a PT.

The PT is a One-Electron BFT-1B, a great hulking 15 pounder rated 277VA plate supply, which has a bifilar wound secondary with 385-0-385 and 260-0-260 windings. www.one-electron.com . I'll run the 8417 plates off the higher voltage winding and dedicate the lower voltage winding to the 8417 screens, thru a hefty choke. Will use the existing two 5AR4s in parallel for plate rectification and a separate RC bridge rectifier for the screen supply.

Any of you have experience/knowledge about 8417s, especially regarding choice of OT primary impedance, I would appreciate hearing about it.

It appears that at the quiescent conditions I will be looking at, about 470V or so plate and 300 to 325V or so screen, my calculations from the Sylvania plate curve indicate 3k0 to 3k5 in pentode mode. I know I could also operate in UL mode with about an 80V Zener on each of the screens.