Probably a very stupid question.......

[kc2ifr]

This is one of those questions that has no merit but its like asking why the sky is blue.......
I use 833's to modumalate my 4X1. At idle when transmitting the toobs show a little color about the size of a dime in the middle of the plate. I wont explain the structure of the 833, I assume u guys already know that.
As I modulate the size of the color in the middle of the plate grows to about the size of a silver dollar.
The question is........why is the color circular in the middle of a square plate? Other toobs such as the 4X1 show color in the whole plate...although maybe a little brighter at the center. I just dont understand whats happening considering the filament and the grid cover the whole plate structure.
Am I stupid or what
Anyway.......I cant sleep until I figure this out

BTW..........the toobs have 3400 VDC on them and I idle them at 50 mills.

Bill

[WA1GFZ]

I bet the area of the plate with the least mass so heat doesn't sink away as well.

[w4bfs]

Bill ... I ran some experiments with the 833 a few years ago and as the plate dissipation approached a relatively low figure (80 Watts or so) I noticed what you mentioned as a small area that increased in size with increasing Pd ....  I have seen localized heating with beam power tubes in the target areas .... I'm not sure what the plate material is in this tube but am assuming its molybedeum since it can operate orange-red ... only thing I can offer is that the center area of the plate would tend to get the largest number of filiamentary electrons ....interesting question .... 73 ....John

[Opcom]

the hottest glow is in the middle of the fire..

[K3ZS]

This is probably a stupid answer, since it is a beam power tube, the electron beam is focused on the middle of the plate so that part heats up more.

[WBear2GCR]

Bill,

In the meantime, I would guess that you might benefit from dropping the quiescent dissapation of the 833s by putting more negative bias on them. I don't see any reason that sitting around they should be glowing at all. If you want to run them in AB, they don't need to be biased high into class A region at all... in fact you'd do well audio-wise to have them darn close to class B, especially if they have feedback wrapped around them.

                 _-_-bear

[kc2ifr]

So far so good guys except the 833 is a power triode, not a beam power triode.
And Bear, for class B operation at the voltages I running, they recommend  idling the toob at 100 mills for 2 toobs.

See pdf file below.....

Bill

[W1AEX]

As I modulate the size of the color in the middle of the plate grows to about the size of a silver dollar. The question is........why is the color circular in the middle of a square plate?

Hi Bill,

I don't have an explanation for this behavior. I can only report that I see the same localized glow that you are seeing when the variac is cranked to supply anything above 2500 vdc. My plate supply delivers 2900 vdc at maximum smoke, and I idle my 833's at 90 mills no matter what the plate voltage is set at. They are strange but impressive tubes that can produce gobs of power, and it's worth the trouble to cut windows in the front of the rack so they can be seen!

Interesting question. I'd love to know the reason for this as well.

73,

Rob W1AEX

[k4kyv]

Tubes  like 4-400's have circular plates and spiral filaments right in the middle.  They are symmetrical in design, so that the electrons radiate radially in all directions from the hot filament.  833A's have a zig-zag shaped filament laid out in a single plane, surrounded by the grid and plate structure, each a flat thin structure.  Most of the electrons flow towards the flat part of the plate.  Very few go off towards the edge.

811A's, 8005's and most graphite plate tubes  like  the ones made by RCA are structured similarly to the 833-A.  This includes screen grid tubes like the 813.

That explains why 833A's can be mounted horizontally as long as the plane of  the plate/grid/filament structure is oriented vertically.  Tubes with the spiral filament like 4-400's, 250TH's, etc. must never be mounted horizontally.

[kc2ifr]

Rob,
I certainly agree about a peek a boo window to see these toobs in action.......thats half the fun of running toob rigs on am. I think this attitude is an "AM" thing! Other toobs that get me excited are the 4x1 and 304tl's.
Now that I think about it.....maybe I should get a life......

[KE6DF]

I thought I had a good explanation, but there was a hold in my logic so I deleted it.

Oh Well....

[w4bfs]

I have done some more thinking about this and I believe the plate structure as Don K4KYV described it will indeed form a roughly circular 'hot spot'.  811's also tend to do this and even burn thru with heavy handed operation.

If you were to calculate the electric field intensity between the rectangular plate thru the planar grid to the filament
it would not be uniform nor rectangular (fringing effects).  Maybe someone with intact calculus skills could help here.

If this is the reason for 'hot spotting' maybe this is what gave impetus to development of radial structures that could develop higher power densities at higher frequencies

73   John

[WBear2GCR]

So far so good guys except the 833 is a power triode, not a beam power triode.
And Bear, for class B operation at the voltages I running, they recommend  idling the toob at 100 mills for 2 toobs.

See pdf file below.....

Bill

Holy Jokers Batman!

Why that's 300watts quiescent current!!

Dunno why one needs that much... the curves look linear down low... hmmmm...
Think I'd try to run it at about a third of that meeself... see if there is a problem with xover distortion - doubt there will be.

I have no experience with these tubes, so maybe someone who knows them "up close and personal" can shed some light on the issue...

(is that 50ma per tube or 100 ma per tube? ...think it ought to be 50 ma per tube??)

                     _-_-bear

               _-_-bear

[Opcom]

300W quiescent - -makes a good bleeder! The book does say 100mA for a pair. I always consider that the manufacturer would like to sell more tubes, and/or the ratings shown are to demonstrate the tube's performance where one or more parameters are maxed out. Very serious applications where failure cannot be tolerated, like military rigs, often run at about half the heat one way or another.