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Re: [Amps] Ameritron Amps

To: Steve Thompson <g8gsq@eltac.co.uk>
Subject: Re: [Amps] Ameritron Amps
From: R L Measures <r@somis.org>
Date: Mon, 31 Jul 2006 06:19:18 -0700
List-post: <mailto:amps@contesting.com>
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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