Heater in Series w/ Cathodes of Power Tubes?

[lowell]

Anyone know how or why this could be? I have a Bell and Howell 179 model w/ no power transformer (yes it has an isolation transformer) that has the first preamp tube's (pentode) heater in series w/ the power tubes' cathodes.

[jrfrond]

You didn't say what tube#, but it is probably a directly-heated cathode design. In guitar amps, you typically see this in rectifier tubes.

[lowell]

aaahhhh ok, and they probably put it there so as to not need a dc supply for that tube's directly heated cathode, which would cost more and require more space.

[Enzo]

They use the current of the power tubes to light the preamp tube heaters. Unusual but not unheard of. if you are going to have say 24v on the power tube cathode, why not light two 12AX7s with it, for example. Oh, assuming the currents work out, of course.

[octal]

Silvertone/Danelectro did this on several models of guitar amps. (Especially the "split chassis" amps with a single 12AX7 mounted on the top chassis.)

[lowell]

mmmm... cool. thanks!