[TenTec] Centurion

[Robert & Linda McGraw K4TAX]

Well using a 3-500Z, with 5 volts at 14.6 amps that's 73 watts.  Half that 
power makes a good soldering pencil quite hot.  {40 watts @ 750 degrees 
melts solder and some melts at a much lower temp, as low as 350 degrees} 
Add a bit of contact resistance between the socket and the tube pin and one 
has heat.  How much depends on the resistance between the socket contact and 
the tube pin.  A blower will keep things cool assuring long tube life.  The 
warm-up time on a 3-500Z is about 1/2 second with a 15 amp 5.0 volt 
transformer.  Most consider these as "instant on" tubes.  No warm-up 
required.

I commented earlier about the temperature maximum for the base of a 3-500Z. 
It is 200 degrees C or about 425 degrees F.  Most solder melts between 350 
and 750 degrees F.

Again, most amps that use 3-500Z's apply filament voltage and HV at the same 
time.  Some may use a step start circuit on the input to limit in-rush 
current.  According to the EIMAC book, the 3-500Z can absorb an in-rush 
current of 2 times the rated current without damage to the tube.  Now, IF 
the amp is designed correctly, thus the filament transformer is properly 
rated such that it provides its own current limit.  The typical problem lies 
in a case where the designer uses a transformer with an excessive current 
rating thus permitting excessive current during start-up.

A pair of 3-500Z's will draw roughly 30 amps of filament current during 
operation.  With a transformer that is CCS rated at 30 amps there is no 
problem and no step-start is needed.  Keep in mind that a CCS rating does 
include an adequate margin of safety.  As to the HV supply, again with a 
series current limiting resistor in the B+ there is no problem with 
excessive current to the filter network on start-up and thus no concern of 
damage to the HV transformer.  It is only when systems are "over designed" 
does the problem raise its ugly head.

Now, back to your point of concern.  I'd put the tubes in the amp and run 
them.  The spares would stay in the box and on the shelf for that time if 
and when they are needed.  Keep in mind that using the amp day in and day 
out, keeping grid current and plate current under maximum values, and with 
an amp so designed to be free of parasitic oscillations, the tubes should 
last 10 to 15 years.


73
Bob, K4TAX

----- Original Message ----- 
From: "Gary Hoffman" <ghoffman at spacetech.com>
To: "Discussion of Ten-Tec Equipment" <tentec at contesting.com>
Sent: Sunday, December 11, 2005 9:06 PM
Subject: Re: [TenTec] Centurion


> Hmmmm.....and I was just getting around to thinking about building a 
> "test"
> socket setup.
>
> Are you saying that filament heat alone was enough to melt the solder ?  I
> would have thought that it would take the red hot anode to get it that hot 
> ?
>
> Now as regards running them in the amp to heat them up, this I understand.
> However, it takes the anode heat to get a 3-500Z hot enough to activate 
> the
> getter anyway, does it not ?  So it seems to me that the only preventative
> for a 3-500Z type tube is to heat it often enough that when you do apply 
> the
> high voltage, the gas buildup has not been allowed to get so bad that it
> causes a failure...right ?   Otherwise, you would have to apply high 
> voltage
> gradually, meaning you would have to install temporary resistors in the 
> high
> voltage leads, as discussed elsewhere.
>
> If there is yet another way, I don't see it off-hand.
>
> 73 de Gary