[Amps] Ameritron Amps

[Steve Thompson]

>>>>>> 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?

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.

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. In fact 
it sounds like it would be a good way to damp out parasitics.

I can see an oscillation causing very high dissipation, and insufficient 
cooling then leading to problems, but magic glass - no. As always, I'm 
wide open to correction.

Anyone got any experience of what it takes to melt a 3-500Z?

Steve