[TenTec] Centurion & Tubes

[Bob McGraw K4TAX]

A comment on 3-500 type tubes.  These types of tubes have an extremely 
high vacuum.   NOS units are subject to air leakage into the envelope 
around the pin seals.  The coated anode in the 3-500Z and 3-500ZG serves 
as the getter to burn the extraneous air out of the envelope.   It is a 
very incorrect belief of many hams that having the filament on for a few 
hours will burn the air out. This is not true with this type of tube 
thus having only the filament on serves no useful purpose.  The only 
correct way to burn the air out is to have the tube operational and with 
enough voltage and current to cause the anode to show color.   It is 
this heat and chemical action of the anode coating that removes the 
extraneous air from the envelope.

Other smaller tubes may have a getter that is ignited by external 
microwave energy.  Once the getter is ignited, it is consumed, and any 
leakage from that point forward will only deteriorate the tube.  Again, 
having the filament on for some hours does no good in this regard.

External anode tubes, such as the 3CXxxx and 4CXxxx types, are different 
designs and thus running the heater will remove any remnants of air 
leakage as the getter is part of the cathode coating.

If you intend to prepare a 3-500ZG or such, a filament supply, an 
adjustable bias supply, and an adjustable HV supply, with proper 
metering for each supply is the correct way to static burn in this type 
tube.  The values are adjusted such that anode does show color.    Just 
putting the tube in an amplifier and hitting the switch will most likely 
produce a flash, bang, pop and the smoke may then exit the box.

73

Bob, K4TAX


On 8/3/2018 8:30 PM, Stan Gammons wrote:
> I think most/all of the 3-500's are made in China now.  One can still 
> find some Eimac's that may still be good after all these years.  The 
> near 40 year old one I have may still be good.  Need to put it in my 
> L-4B some times and see.  This is what we get for outsourcing so much 
> of our manufacturing to China.  I've said for many years that that was 
> a bad idea.  At one time Amperex tubes were made in France.  Don't 
> know if that's still the case.  I'm thinking no.  Maybe we can talk 
> CPI/Eimac into making 3-500's here again? Probably not, but might be 
> worth a try.
>
> 73
>
> Stan
> KM4HQE
>
>
> On 08/03/2018 08:12 PM, Bob McGraw K4TAX wrote:
>> Yes, thanks to the new tariffs we can expect tube prices to go 
>> through the roof.   Most tubes are made in China and branded under a 
>> US company.   a.k.a. RF Parts.
>>
>> Bob, K4TAX
>>
>>
>> Sent from my iPhone
>>
>>> On Aug 3, 2018, at 5:03 PM, Carter <k8vt at ameritech.net> wrote:
>>>
>>>> On 8/3/2018 4:45 PM, Greg S via TenTec wrote:
>>>> There will be a price increase SOON. RF Parts apparently will no 
>>>> longer be importing Chinese tubes.
>>> Hmmm...bad news, as I thought China was the *only* one making 
>>> 3-500s. If not China, who else?
>>>
>>> Carter
>>> K8VT
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