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[Amps] SB220 step start blows fuses

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Subject: [Amps] SB220 step start blows fuses
From: "Jim Thomson" <>
Date: Tue, 11 Jul 2017 08:16:20 -0700
List-post: <>
Date: Sun, 9 Jul 2017 20:52:40 -0600
From: Warren Volz <>
To: amps <>,
Subject: Re: [Amps] SB220 step start blows fuses

I may shortly be looking to purchase a variac.

After building a dim bulb tester this weekend I have a minor update.

With 120V connected, the HV primary disconnected and one tube in a socket 
(doesn?t matter which I use) the 100W bulb in my tester lights up bright. No 
fan and the meter lights aren?t on. If I take out the tube the fan and meter 
lights work. So I guess I have a shorted tube? Would it be good to double check 
the DC/AC voltage on the filament supply with the tubes plugged in? That would 
show any sags that might exist.

When I rebuilt this, I did the following modifications:
- W7RY board with fuse option installed
- ES4SB220 filter capacitor board installed with new electrolytics 
- RM220 replacement metering board installed
- Grids grounded
- Molex connector on the HV transformer primary windings


##  Each tube draws  75 watts of fil power...and  150 watts with both tubes.  
That  75 watts of fil power from just the one tube plugged in....also has to 
ultimately  flow through your 100 watt bulb. 
Your 100 watt bulb has a hot resistance of 144 ohms.  No wonder it lights 
up...with one tube plugged in.   Hot resistance of each tubes fil is just .33 
ohm.   .167 ohm for the pair. 

##  Ok, did the SB-220  work just fine  BEFORE  you did the above mods ??      
If so, then one or more  mods has likely caused the issue.   On  a side note, 
if the step start is done correctly, and step start resistor inserted into one
leg of the 240 vac primary..... or hot leg if wired for 120 vac,   you will be 
able to step start the HV supply and both fils...simultaneously. 

Jim  VE7RF 
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