[Amps] Soft Start

[Jim Thomson]

Date: Wed, 27 Dec 2017 20:34:31 +0000 (UTC)
From: Bryan Swadener <bswadener at yahoo.com>
To: "amps at contesting.com" <amps at contesting.com>
Subject: Re: [Amps] Soft Start

<Though, in some cases (such as in the SB-220), there's little/no benefit, 
as the filament transformer limits the current. In those cases, you're
mainly extending the life of the ON/OFF switch from yuuuge inrush current
caused by overly large HV filter capacitance. 

vy 73, 
Bryan WA7PRC 


Date: Wed, 27 Dec 2017 06:25:27 -0600 
From: Jim W7RY 
Subject: Re: [Amps] Soft Start 

<Soft start (I call it step start) does reduce filament inrush current. As 
long as it's on the filament transformer primary. 

Which it is in the SB200, SB220, TL922, L4B and TenTec Titan... And many 
others. 

73 
Jim W7RY

##   Dead  simplest method for step start is a 25 ohm, 50-100 watt resistor in ONE leg
of the 240 line..for about 1-2 secs.    Then you not only step start the HV supply,
you also step start the  fil.   On dual 3-500Z amps, this works  superb.  With the
step start 25 ohm resistor in the one leg of the  240 line, the  fils will come up to
apx  2/3  to  3/4 brilliance... then when resistor shunted, fils go to full brilliance. 
Forget the miller larsen effect,   you have passed through it so fast, its a non issue.  

##  In just about all cases, the fil xfmr will limit inrush to no more than double the rated
fil current.   Be careful though.  Dont oversize the fil xfmr.  Larry, W7IUV used a 30A fil
xfmr on a  single  HB 4-400A.  Resulted in a fil to grid short on several new tubes. 
The V drop across the fil pins may be down to almost nothing, but the current through the fil
is still sky high.  Try shorting out a fil xfmr sec sometime...with a peak reading  AC ammeter.
You will be in for an eye opener.  Typ tubes like  3-500Z  have a cold resistance that is only
1/10  to  1/12 of its hot resistance. 

##  I use an external step start setup.  One dual pole contactor applies the 240 vac.   2nd contactor, 
spst, shunts the 25 ohm resistor.   Separate toggle switch for each  contactor.  IE:    I manually apply the
240 vac..and also manually shunt the resistor.   Both toggles mounted in a small box near the amp.  Step
start resistor and pair of contactors in another, larger box, located by input to hv supply.  Drake amps
all use an external HV supply box. 

Jim   VE7RF