[Amps] filament instant heating query

[Jim Thomson]

Date: Fri, 15 Jun 2012 00:45:25 -0400
From: "Roger (K8RI)" <k8ri at rogerhalstead.com>
Subject: Re: [Amps] filament instant heating query
To: amps at contesting.com
Message-ID: <4FDABDE5.3010205 at 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