Tube Diode Question.....

[J Rindt]

When comparing a GZ34 to a 5U4 (hope I have the right nimbers...) the 5U4 draws a lot more heater current. Just kind of wondering why that is. Does the 5U4 cathode use material that requires more heat to "release" electrons.?
Thank You

[HotBluePlates]

The 5U4 draws about 58% more heater current than the GZ34/5AR4.

The 5U4 delivers ~21% more steady state current, and ~24% more transient peak current than the GZ34/5AR4.

I think those differences account for most of the higher heater demand. Additionally, we know the 5U4 has a higher internal voltage drop when using it as a rectifier, which implies the plates are further from the cathode. My guess then is that the increased distance plus the higher current ratings (deliverable to the load) require more cathode area and higher emission than is required of the GZ34. Therefore, more watts input from the heater to "make it go."

The 5U4 is also "directly heated" which means the 5v filament is also the cathode; that means the emissive characteristics may be different than the GZ34's indirectly heated cathode (in which the "heater" is separate from, and really is used to heat, the cathode).

[J Rindt]

You say "I think....."
Those sound like pretty intelligent "guesses" to me.
Thanks for the info, I appreciate it.

[PRR]

5U4 is a very old-old type re-released on an Octal base.

IMHO, US tube designers neglected rectifier optimization.