[Amps] 5 Minutes for Ameritron 811H to warm up?

[Roger (K8RI)]

When it comes to seals, as noted, glass to metal is an art. At best, the 
glass is not a perfect bond to the metal.  We are talking clearances 
that are so small, a molecule of gas may take months or years to work 
it's way from one end of the seal to the other and those molecules are 
always moving.

OTOH Ceramic to metal can easily be automated using a precision pulse to 
heat the surfaces.  IIRC the two surfaces are coated with a mix of flux 
ans silver solder. The surfaces are placed into intimate contact with 
each other and a pulse of heat melts the solder into a molecular contact 
with each surface and bonds them together producing a gas tight seal.

This many forms of this process are readily adapted to the batch 
process.  Problems often come from starting up the production line and 
getting all the settings correct.

Higher prices are due in part to the higher precision and placement of 
parts. as well as the tiny and fragile wires used in the grids and 
screens.    I wonder why we don't see Pentodes in production any more.  
IIRC they are just a Tetrode with a grounded screen added to eliminate 
secondary emission.


73

Roger (K8RI)


On 1/26/2015 12:21 AM, John Lyles wrote:
> Ceramic/metal construction is usually very vacuum tight, with 
> exceptions due to mfr mistakes. Somewhere I have a paper from 
> Eitel-McCoulough extolling the virtues of their new "CX" tubes back then.
>
> Glass to metal seals are the earliest type, learning first from the 
> Edison bulb ideas to Mr. Deforest, with improvements by Mr. 
> Housekeeper at Westinghouse and a long line of GE improvements. 
> Sealing metal to molten glass edge is an art, and requires good glass 
> blowers/turner.
> I don't believe that vacuum seal leaks are due to glass 'sagging' over 
> time, but just due to longevity of those seals themselves. Corrosion 
> of the metal is one factor. Temperature extremes between the metal and 
> glass is another (from overloading).
>
> The major tube manufacturers like Eimac/CPI, RCA/Photonis/Burle and 
> Thales/Thomson have dropped all glass tubes from production in the 
> past decade for reasons that it was getting very difficult and costly 
> to by the glass blank cylinders from companies like Shott Glass of 
> Germany. I know one scientific user in UK that uses a big glass 
> pulsed-megawatt triode, they had to by a 20 year supply of the glass 
> for a major tube manufacturer to guarantee a supply of the tubes. 
> Where I work, we needed a source for 120 KV switch tubes, small planar 
> triodes not much large than ham sized. We requested glass, like Eimac 
> used to make in Salt Lake City, for reasons of improved HV standoff 
> (when in oil), not with RF of course. They did make them for us in 
> Palo Alto, once we had need for more than 50 of them.
>
> For ceramic/metal, there has been many decades for improvement. The 
> biggest US-produced VHF gg triode still in production is the the RCA 
> 7835, which has pressed seals of metal to ceramic. These tubes come 
> with 2 l/sec vacion appendage pumps connected to them. No 
> metalization, just tremendous mechanical force keeps those tubes 
> tight. Eimac makes the 4CM2500KG tetrode, their largest for RF, with 
> metallized ceramic seals. Thales does same for their big tetrodes for 
> 500 kW and up, like TH558, TH576, and Th628. Metallized ceramic, when 
> done right, is very good for vacuum tightness. A mechanism allows for 
> flexure of the two components - which are closely matched thermally, 
> where RF current is carried through good conductors inside. The 
> metallization allows for the seal to be brazed tight.
>
> BTW, its amazing to see those big tubes being lit during their 
> production process, where they are run very hot. Thoriated Tungsten 
> runs about 1900 Kelvin. I saw a 7835 in Pennsylvania at the factory 
> where the ceramic was bright pink, almost white. It looked sort of 
> transparent. The tube has a 30 kW filament,no kidding, 5 Volts DC at 
> 6800 Amps.
>
> I love ceramic metal tubes. As others have said, they aren't as 
> beautiful as glass, but a lot more rugged.
>
> 73
> John
> K5PRO
>
>
>
>
>
>
>
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