• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Focusing radiated cathode heat back on itself

This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
An idea I had some time ago that would perhaps work best with tubes that had cylindrical cathodes and anodes was for the inside surface of the anode to have a mirrored chrome-like finish. The idea is that normally it intercepts the majority of the radiated heat from the cathode and instead of absorbing it for no good purpose it would now reflect this heat back to the cathode surface thereby reducing the power needed to keep the cathode at working temp. Seeing the anode is usually perfectly concentric with the cathode, I expect this reflecting/focusing effect might be quite effective.

AX tech editor
Joined 2002
Paid Member
reflecting radiation


I have never heard this idea, and in view of the fact that the best minds in the business have been directed towards tube technology over the last century or so, that makes me suspicious.

I do know that the surface finish of the anode element is normally optimised to radiate heat into free space to improve max allowable anode dissipation, of course.

But, maybe you just are the first one to think about it.

Cheers, Jan Didden
AX tech editor
Joined 2002
Paid Member
reflected radiation

Just thought of something rereading my previous post -

The effect you mention is to some extend already taking place: the anode radiates not only outwards, but also inwards toward the cathode. If the anode dissipates more energy than the filament power, an effective transfer of heat from anode to cathode may already be taking place in normal operations.

Jan Didden
Certainly the outside of the anode should be dull black to radiate into free space, but like you suggest, if the inside of the anode is black then perhaps it does radiate more anode heat to the cathode than it would reflect cathode heat back to the cathode if it were shiny. Maybe some tube industry mover and shaker will read this and make one to see. :cool:

I read somewhere( likely here ) of running tubes like 6C33 and 6336 over their anode dissipation limits and reducing heater power to maintain proper cathode temperature. Those tubes have anode *VERY* close to the big cathode...:) No way to mirror-finish their inside surface though.
the thought takes me back to when I was younger and I had the idea of using an electromagnet from the outside of a triode valve to remove 2nd electrons.( all it did was to concentrate the primary electrons on to one spot on the anode} but you never know until its been tried.

after all there may be a small group of Russians positioning electromagnets around the outside of some tube somewhere in a secret room and nodding to each other as they are getting some quite good results.
We here are not the first to think of this. You can read about it in the RCA tube design book. (Not the Radiotron Design Handbook, but the book that talks about how to design tubes, available on Pete Millett's site.) There is a chapter about anode materials that mentions reflective surfaces on the inside of the plate. If I'm not mistaken, it advises against it in power tubes since the grids would also be heated in the process.

Let me put it this way, it would be nice for 6550 heaters to take less power, but would you really want the result to be that you have to use a 25K grid leak instead of the already burdensome 50K grid leak in order to keep the tube from running away. 1.6A of heater current doesn't seem so bad to me.
New Sensor KT88/6550s seem to have a reflective surface on the inside of the anode(at least it looks like it from the outside from the parts of the anode that bend outward near the seam). I will have to verify this in a tube autopsy as I have some that I fried in an experiment. That may be why they didn't take to extreme grid current. Of course, they still have the radiating fins on top but the reflective surface on the anode can't help the grids stay cool, if indeed it is reflective.
I think the real question is how much effect would this have. It might work measurably, but only in tiny amount. The cathodes have to be at something ungodly like 1500 degrees if I recall. The cathode is wrapped very close around the heater to achieve this. What percentage increase would we really see reflected back from the (relatively) remote plate structure? Not much, I'd wager.

If we were heating an entire battleship full of tubes, then saving a tiny fraction per tube might add up to something. But in a typical audio amp, I am thinking saving 2ma of heater current won;t be a major concern.

How would a polished reflective surface on the plate affect electron flow?
Joined 2006
Paid Member
I thinck the main advantage would be that we could increase the cathodes surface area in respect to the anode and hence would get higher current capabilities (given the same heating power and anode radiated power). Or, we could get a decrease of necessary heating power wich would then allow for more anode dissipation.
If we dont want either of those we just have to settle for lower anode temperature and those horrible things like longer life and
higher reliability.
From a manufacturers view the whole idea is bad offcourse,
higher manufacturing costs and less turnover.
I find it very unlikely that nowdays any technical improvements with unfavorable busnis returns stand a chance.
The times are gone when technicans made the decisions...
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.