On Jul 31, 2006, at 1:01 AM, Steve Thompson wrote:
>
>>>>>>> When the Chinese were making 3-500Z tubes with regular glass
>>>>>>> and holes were sucking in the sides, Rich concluded it was a
>>>>>>> VHF parasitic that couldn't be seen or measured. This VHF
>>>>>>> oscillation supposedly caused dielectric heating of glass at
>>>>>>> VHF.
>>>>>>
>>>>>> Correct. Rus Healy at QST was the first who told me of this
>>>>>> phenomenon. However, it took Eimac a while to come up with a
>>>>>> glass
>>>>>> recipe that would not melt at VHF, so I was not surprised that
>>>>>> the
>>>>>> Chinese initially encountered the same difficulty.
>>>>>
>>>>> Do you know when this was? I'd like to know which date codes
>>>>> to avoid.
>>>>
>>>>
>>>> Early 1991.
>>>
>>> Sorry, I think I didn't make myself clear. When did Eimac change
>>> their
>>> recipe?
>>>
>> I don't know. However, probably around the time that the 15E came
>> out.
> Isn't that about 25 years before 3-500Z was introduced?
Pretty close, Steve. The 15E was developed by Eimac in 1940 for use
in the AN/APS-18 radar set.
>
> In the 40s they were making lots of devices running high power at high
> VHF/low UHF so they must have had good enough glass. I think 3-500Z
> was
> introduced in the late 60s - their knowledge of glass for tubes
> rated at
> 110MHz was well established by then.
Agreed, but the Chinese apparently had no knowledge of how to make
low D-factor glass, so it took some more trial and error.
>
> If they had invented glass clever enough to be ok at 30-50MHz but melt
> in fields at 100odd MHz, I imagine they would have patented it.
As I understand it, the recipe is similar to Pyrex in that it uses
boron as a fluxing agent instead of lead.
> In fact
> it sounds like it would be a good way to damp out parasitics.
Molten glass is definitely not good news in this application.
>
> I can see an oscillation causing very high dissipation,
Push-pull parasitics, yes, push-push parasitics no. To explain:
With a push-pull parasite, the VHF energy from one tube alternately
pushes energy into the other tube,and vice-versa, so both tubes have
high dissipation until the operator stops transmitting. However,
since there is a load on each half-cycle, there is no wild and crazy
arcing, and grid-I is not excessive. If you want to see a push-pull
parasitic, short out both parasitic suppressors in a SB-220,
transmit, and with a litle bit of luck you will see both anodes turn
bright red, The frequency is c. 50MHz. .
However, Push-push parasitics tend to run amuck because they are
not loaded.
> and insufficient
> cooling then leading to problems, but magic glass - no. As always, I'm
> wide open to correction.
Lead-fluxed glass is not magic, it has a higher expansion rate, it
doesn't happen to have a good D-factor, but it's cheaper to make.
>
> Anyone got any experience of what it takes to melt a 3-500Z?
>
> Steve
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R L MEASURES, AG6K. 805-386-3734
r@somis.org
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