Re: 833A's plate color

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Steve Conner <steve@xxxxxxxxxxxx>

Hi all,

The reason for the confusion, IMO, is that different tubes are 
designed to run with widely different amounts of colour.

I enjoy playing with tube amps for electric guitars, and the little 
power tubes in these- EL34s, KT88s, 6L6s, 5881s etc- are not supposed 
to show any colour on the plate at all. The received wisdom is that 
if they show colour something is badly wrong with your amp. My own 
experiments confirm that when the tubes are driven to their full 
rated power into the proper load resistance, they don't show any 
colour. The plate does begin to glow if you connect a mismatched load.

On the other hand, suitably designed transmitting tubes can run with 
the plate bright reddish-orange. There is that old ham story about 
tuning your transmitter by adjusting it until the plates glow just 
the right colour. It all depends on how the tube is designed in terms 
of heat loading on the glass and seals, provision for thermal 
expansion of the plate, cooling of the grids and screen grids, and 
what temperature it was baked to for degassing at the factory.

Another thing that might be relevant: I heard that graphite plates 
store more gas, so shouldn't be run as hot as metal ones.

Steve Conner
http://www.scopeboy.com/