That same Wikipedia article quotes 10^-7 to 10^-10 torr as being typical
for vacuum tubes. This is consistent with a value of 10^-8 that I'd
heard from another source.
It is also consistent with John's comment that pressures higher than
about 10^-7 torr will lead to problems with arcing, while anything lower
than 10^-10 requires special techniques that are time-consuming,
expensive and not suited to mass production.
The most striking fact of all is that even a "good" vacuum tube still
contains upwards of a BILLION gas molecules. Although these neutral
molecules don't significantly impede the normal operation of an electron
tube, they are also the potential ingredients of a destructive arc which
is why vacuum quality becomes very important in high-voltage
transmitting tubes.
73 from Ian GM3SEK
>-----Original Message-----
>From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Roger
>(K8RI)
>Sent: 21 July 2014 03:02
>To: Hardy Landskov
>Cc: amps@contesting.com
>Subject: Re: [Amps] Ion Pump
>
>On 7/20/2014 8:07 PM, Hardy Landskov wrote:
>
>This is well outside my realm of experience. The claim of 1 X 10^-12
>would be extremely difficult to obtain.
>https://en.wikipedia.org/wiki/Vacuum . So far, I've not found a
>definitive answer to the actual vacuum required in normal production,
>power tubes tat are use at amateur power levels, or several times
higher.
>
>73
>
>Roger (K8RI)
>
>
>
>> Roger,
>> As an EE myself now retired, have any experiments or production units
>> of vacuum tubes been done while the shuttle program was ongoing?
>Maybe
>> there is the vacuum of outer space that might be better than what's
>> achieveable on earth but you might collect particles floating by as
>> well from space junk that would negate the advantages. Just curious
or
>> maybe it's classified....or maybe just impractible.
>> 73 Hardy N7RT
>>
>>
>> ----- Original Message ----- From: "Roger (K8RI)"
>> <k8ri@rogerhalstead.com>
>> To: <amps@contesting.com>
>> Sent: Sunday, July 20, 2014 3:33 PM
>> Subject: Re: [Amps] Ion Pump
>>
>>
>>> On 7/20/2014 3:12 PM, John Lyles wrote:
>>>> Although this doesn't have much to do with the building of
>>>> amplifiers, it does shed insight into some of the complications of
>>>> making good tubes and how the technology there has impacted other
>>>> industries.
>>>>
>>>> General Electric was one of the first to recognize that vacuum
tubes
>>>> (their Pliotrons and Kenotrons) required better vacuum than the
>>>> Audion's had that were made by DeForest companies. So naturally, a
>>>> lot of development came from GE and also benefited their Xray
tubes.
>>>>
>>>> K8RI's experience (below) is helpful here. I am in RF engineering
>>>> and only observe vacuum experts doing their stuff from the
>>>> sidelines. We work hand in hand, of course, in not applying RF
power
>>>> until vacuum is sufficient. I know of several instances at particle
>>>> accelerator laboratories where oil contamination from diffusion
>>>> pumps created a costly mess. I didn't think we have them for
pumping
>>>> between 10^-1 and -6. The big roughing pumps are indeed rotary. We
>>>> use cryo pumps and ion pumps, maybe the cryos are for in-between? I
>>>> know they have to be regenerated regularly.
>>>
>>> I should add that the chilled water or liquid N2 "cold traps" are
>>> also called "Back gassing traps" whose purpose is to prevent the
>>> gasses in the pumps from getting into the chambers on which they are
>>> pumping. The liquid N2 traps also aid the pumping as they remove
>>> many molecules of gas. When you fill the N2 trap, the vacuum goes
>>> down almost immediately, but they are not filled until there is a
>>> vacuum on the pump or any moisture would freeze to the baffle and
>>> then slowly sublime, slowing pumping substantially.
>>>
>>> The diffusion pump is a very simple device, but might take a bit of
>>> space to explain.
>>>
>>> 73
>>>
>>> Roger (K8RI)
>>>
>>>>
>>>> Our accelerator cavities need to be in the low 10^-7 Torr or we get
>>>> many discharges from the extreme RF voltages there. Electron
>>>> accelerators need much better vacuum. I believe that big tubes must
>>>> be better than -6 to minimize damaging flashovers during
conditioning.
>>>>
>>>> 73
>>>> John
>>>> K5PRO
>>>>
>>>>> From: "Roger (K8RI)" <k8ri@rogerhalstead.com>
>>>>> Roughing pumps do not normally get anywhere near the vacuum
>needed to
>>>>> start an Ion pump. We used a roughing pump to get to 10^-1 Torr,
>>>>> then
>>>>> a diffusion pump to make 10^-6, (-7.with a liquid N2 cold trap) At
>>>>> that
>>>>> point we'd start the ion pump. Once it was working, we'd valve
off
>>>>> the
>>>>> diffusion pump inlet as the ion pump would not work in parallel
>>>>> with the
>>>>> diffusion pump. It would pull oil, or mercury fumes out of the
>>>>> diffusion
>>>>> pump.
>>>>>
>>>>> We used ion pumps on a mass spectrometer. It created a beam of
ions
>>>>> containing the material to be analyzed. This beam passed between
>two
>>>>> curved and charged plates inside a powerful electromagnet. IIRC
the
>>>>> poles were about 10" in diameter with the magnet weighing around a
>>>>> ton.
>>>>> However the last time I worked on that was "maybe in or around
>1980.
>>>>>
>>>>> An ion pump is basically a big diode that collects ions on the
>>>>> surface, or getter. They have no exhaust. Starting one at the
>>>>> typical
>>>>> 10^-1 Torr of a roughing pump would quickly contaminate the ion
>pump.
>>>>> Penning tubes will light up with plasma around 1 X 10^-1. The
>>>>> ionization
>>>>> will go out around 1 X 10^-2. The ion pump would need much better
>>>>> than
>>>>> that to start.
>>>>
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