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Re: 833A's plate color



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/