Date: Fri, 15 Jun 2012 00:45:25 -0400
From: "Roger (K8RI)" <k8ri@rogerhalstead.com>
Subject: Re: [Amps] filament instant heating query
To: amps@contesting.com
Message-ID: <4FDABDE5.3010205@rogerhalstead.com>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
On 6/14/2012 11:59 PM, Charles Harpole wrote:
> to check on a supposition............
>
>
>
> It appears possible that the sudden application of voltage to a tube filament
> causes degradation of the filament.
>
>
There are others on here who can probably explain this better than I, but...
This is typically a problem with filaments that draw a lot of current.
At turn on the resistance is a fraction of that once the filament is up
to temperature. If the current is not limited the sudden rise in
*current* well beyond the operating current produces a very strong
magnetic field that interacts with the current flowing in the filament
which can cause the filament to move mechanically.
Take a 4CX-3000. The filament runs 9V at 41.5 Amps which is just shy of
375 watts, but the cold, inrush current can be several times higher if
the transformer can provide it. That would cause the filament to
"jump". A number of the larger QRO amps run much higher current than
the 4CX3000. This is the reason for "step start" or ramping up the
filament voltage so there is no sudden very high current pulse at turn on.
73
Roger (K8RI)
## real simple. Cold resistance of a thoriated tungsten tube is one twelth
of its hot resistance.
If u believe a fil xfmr will limit the inrush current to no more than double
the normal operating current,
then go for it..and suffer the problems.
## a real simple step start with just ONE resistor in EITHER leg of the 240
line is all you need.
Then you can kill 2 x birds with one stone..and step start both the HV supply
and the fil. Just
set it for 2-6 secs, b4 you short out the resistor, no brainer, and no rocket
science.
## as regards tube life.... when the fil is lit... the clock begins ticking.
Turn on a sb-220..with no HV...
and no drive applied..and no cables going in-out....and the tubes will be soft
in the same number of hrs as a
working sb-220 with hv and drive applied.
## In the telco I worked at for 34 yrs.. everything is powered up and running
full bore 24-7-365..never shut off...
and its all on UPS since day 1, including microwave and cell sites. Equipment
runs just fine.
## running xcvrs at half power or less is another wasted effort. Typ xcvr
operates at only 50% eff..when run
at full power output. Run it at half power output....and eff drops to 35%.
Run it at qtr power output....and eff
is in the basement. You save nothing.
Jim VE7RF
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