rf/audio screen voltage ratings

[woodyc]

Why is it that some rf tetrodes/pentodes/beam tubes permit way higher audio screen voltages and some don't? Just for example...

http://w5jgv.com/downloads/RCA%20813.pdf (400V rf/1100V audio)
http://www.sphere.bc.ca/test/tubes14/4-250a.pdf (600V rf /600V audio)

[J M Fahey]

Are you trying to upgrade your Champ ?
Not much use in Guitar amplification

[Steve Conner]

Where a lower screen voltage is shown for RF service, I guess it is to allow headroom for high-level modulation. The "Plate modulated" characteristics for the 813 have a footnote saying that the screen voltage is to be modulated in proportion with the plate voltage, so at 100% modulation it could jump up to 800V.

This holds true for all tetrodes and pentodes, not just the 813.

Another possibility is that the tube internals suffer more losses at high frequency, so the tube has to be derated by turning the screen voltage down. At VHF the current flowing in the inter-electrode capacitances can be tens of amps, and in a tube of old-fashioned construction like the 813, it all has to go through little skinny lead-in wires. They can easily be heated red-hot, causing all sorts of damage.

[woodyc]

Those are good points Steve.

I googled up a really interesting reference from eimac... http://www.clarisonus.com/Archives/T.../VTRatings.pdf. Its strictly about triodes but this is the most detail I've ever seen on the subject of ratings.

Since they rate the grid in consideration of the plate being at its limit at the same time (and they even refer to the voltage rating as "academic") my interpretation is that you could probably greatly exceed the screen ratings in some situations.