833C versus 833A vacuum tube

[MRMILSTAR]

My VTTC currently uses an Amperex 833A tube. As most of you know, you have to be careful of the power level when running in CW mode (non-interrupter). When in CW mode you have to pay careful attention to the plate color. If it gets too hot it will melt. Because of this I can't operate my VTTC at full power in CW mode, only in interrupter (staccato) mode.

I have been considering replacing the 833A with a 833C. My understanding is that the graphite plate of the 833C will allow me to operate it at a higher power level in CW mode. I have never used a 833C. My understanding is that the graphite plate never changes color regardless of power level. My concern is that I will no longer have that visual feedback to know if I am over-powering the plate. I wouldn't want to accidentally destroy an expensive 833C.

Does anyone have any experience using a 833C in this application? Do I need to worry about the plate temperature?

[klugesmith]

Without experience here,
I would expect plate color to be roughly the same for blackened steel and graphite that are radiating the same power.  People with practical experience may know otherwise.

Any difference would come from the surfaces having different emissivity (with respect to blackbody).

Hypothetical blackbody surface, radiating a given amount of power, has an easily predictable temperature and color.
Other materials at the same power level have higher actual temperature, because their e < 1.
They can have color different from blackbody at that higher temperature, because e varies with wavelength.

Here's a picture from
https://rbellew.wordpress.com/2014/05/04/vacuum-tubes-magic-1940/

[MRMILSTAR]

I know that for the 833A the plate can operate safely at any color up to reddish-orange. One you start getting into reddish-yellow, its time to back off.  It may be that the 833C can safely operate at reddish-yellow and beyond because its graphite plate will not melt at those temperatures. I don't know.

This is only a problem in CW mode. I can operate at full power in interrupter mode. This is the mode of operation that I usually use.

[Uspring]

Yes, as klugesmith says, a shiny surface radiates much less power than a black one.
Consider a shiny surface placed next to a black one. They will stay at the same temperature (if neither one is heated), even though the black one will absorb much more of the heat radiation between them, than the shiny one. That can only happen, if also the black one radiates much more than the shiny one.
That might be another reason for the choice of a graphite anode aside from the melting point.

[MRMILSTAR]

UPDATE:

I've been wanting to try a 833C tube in my 833A VTTC for a while. The reason is that I can't run it very long at full power in continuous mode or else the 833A plate will melt. I have to use interrupter mode to operate at full power for an extended period of time.

I was interested in the 833C because the plate is made of graphite instead of metal as in the 833A. I finally saw a decent deal on a matched pair of new 833C tubes on E-Bay so I bought them even though I only needed one.

I replaced the 833A with a 833C and I can now run my VTTC in continuous mode at full power for a long time without the plate being destroyed. The graphite plate barely gets red. If I tried this with the 833A the plate would melt after a short time.

I am also happy to report that these 833C tubes appear to be very good quality even though they are made in China. Unfortunately vacuum tubes aren't made in the USA anymore. They are mostly made in Russia and China now.