[Amps] How, and at what stage in production, are vac variable

Tue Apr 1 10:09:11 EDT 2014

On 4/1/2014 4:24 AM, Ian White wrote:
> Jim is right, because both ceramic and glass vacuum variables are
> designed to be pumped and sealed just one time - when they are being
> manufactured. Manufacturing involves many one-way steps such as melting,
> welding and vacuum-brazing parts together; and it all has to be done in
> custom made assembly jigs to maintain the tight mechanical tolerances.

I would expect the evacuation to be one of, if not the last step. The 
body has a glass tube sticking out the side through which the cap is 
evacuated.

The evacuation is not a complex process, but it is one fraught with 
dangers for the cap.  Once evacuated, the tube is carefully heated until 
it is soft and the vacuum will pull it closed,  then the tube us heated 
until it can be pulled off while rotating

To evacuate an existing cap the process is much more delicate.  The tip 
of the "tit" is cut off, then a glass tube is fused to the body.  The 
problem is that the area around where the tube is fused must be 
carefully heated to prevent a steep temperature gradient and the 
associated stress which would likely cause the body to crack or break. I 
do not know how much stress this particular glass can take.  It's a fine 
line between where you can pull it off or the vacuum sucks the molten 
glass in, letting the pressure go back to atmospheric.

Most of the work I did required a Hydrogen torch which is a very clean 
"and almost invisible" flame which makes it dangerous.  It also takes a 
lot of Hydrogen compared to Acetylene as IIRC the Hydrogen flame temp is 
much less than Acetylene.

So, "IF" you have the skill to attach the tube, you need the vacuum 
equipment consisting of a mechanical, roughing pump, and a diffusion 
pump. The equipment to accurately measure the vacuum, and the skill to 
do the glass work.

I do not know what pressure they evacuate to, but 10^-1 Torr is about 
where the remaining gas ionizes easily, so "my guess" is probably"
  1 X 10^-5 to 1 X 10^-7 Torr.  Most likely 1 X 10^-6 or better.

The mechanical pumps are unlikely to be the little ones you see in chem 
lab, but 2 stage, rotary vane pumps (6 to 12 liter).  Diffusion pumps 
may be anything from 4" to 10" in diameter with a water cooled back 
gassing baffle. IIRC ours had a 4 stage chimney. Some where around 10 
^-6 or -7 we moved to liquid Nitrogen cooling. Typically 1 X 10^-7 or 
better also required ion pumps.  I doubt they would be necessary

Note: Diffusion pumps work with the scraping action of a high velocity 
gas. At these levels, most of the gas molecules cling to the surface.

The vacuum systems I worked with were for "chambers" that varied from 12 
X 12 X 38 to over 36 X 36 X 48.  The big one contained a 20 KW plus 
Electron beam  and could melt an 8 or 10" dia Silicon Ingot.  It used a 
a 3CW20,000 A7 for control.

So if the copper has not arced, or warped, you still have to evacuate 
and seal the body.  So to evacuate an existing cap would probably take 
several hours, unless the vacuum system were set up to run continuously. 
  With the skill and equipment required, we are most likely looking at a 
minimum of $500 to $1000 dollars, for a "one off" job just to evacuate 
an existing cap.  This is if no glass to metal seals require repair. 
The low number is most likely a bit optimistic.

I'll see if I have the "innards" of a non variable cap and post a 
photo...If I can remember to do so.  It had arced which is quite 
evident. With a deeply pitted surface like this, rebuilding would be a 
lost cause.

73

Roger (K8RI)

>
> Re-manufacturing is only cost effective for large, high-value
> transmitting tubes. For a vacuum variable, the cost would greatly exceed
> the replacement value.
>
> However, as Jim points out, you can easily check the performance of a
> vacuum capacitor with a hi-pot tester. The voltage breakdown of a
> capacitor that has "come up to air" will be greatly reduced (although
> some capacitors may still be usable in lower-voltage applications like
> "C2" in a pi-network).
>

> What specifically is the problem, Chris?
>
>
> 73 from Ian GM3SEK
>
>> -----Original Message-----
>> From: Amps [mailto:amps-bounces at contesting.com] On Behalf Of Jim
>> Thomson
>> Sent: 01 April 2014 05:38
>> To: amps at contesting.com
>> Subject: [Amps] How, and at what stage in production, are vac variable
>>
>> Date: Mon, 31 Mar 2014 12:17:06 +0100
>> From: Chris Wilson <chris at chriswilson.tv>
>> To: amps at contesting.com
>> Subject: [Amps] How, and at what stage in production, are vac variable
>> caps evacuated?
>>
>>
>>
>> Anyone any insight as to how, and at what stage of their production,
>> vacuum variable capacitors are evacuated? Do makers offer any service
>> to check the depression in them and re-evacuate if required? Thanks.
>>
>> --
>>        Best Regards,
>>                    Chris Wilson.
>>
>> ## Dunno about when the air is pumped out and unit sealed.   But if you
>> require a vac cap that has ..gone up to air..or has a lousy break down
> rating
>> as seen on a high pot tester..and want it  pumped out again,..  u will
> be in
>> for a rude
>> awakening...aka  sticker shock.
>>
>> ##  I tell folks to treat vac caps with kid gloves.... leave em in the
> foam lined
>> boxes etc.
>> I have seen  several that have lost their vac  from misuse in the
> workshop.
>> If u have ever
>> seen the price on a new vac cap..it will make u gag..and ditto with
> sending it
>> in to get re-pumped.
>> Much cheaper just to get another surplus vac cap.
>>
>> Jim  VE7RF
>>
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