[Amps] SB220 step start blows fuses

donroden at hiwaay.net donroden at hiwaay.net
Mon Jul 10 08:31:13 EDT 2017

I think you are seeing a normal condition . Measure the voltage at the  
input of the Amp.    Use a light wattage that gives at least 1/2  
voltage at the amp.
Your filament transformer appears to be fine.  Turn on the HV with the  
tubes out and test again.


Quoting Warren Volz <warren at warrenvolz.com>:

> 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.
> -Warren
>> On Jul 7, 2017, at 12:58 PM, gudguyham--- via Amps  
>> <amps at contesting.com> wrote:
>> I know I sound like a broken record but yes, a Variac is the way to  
>> go.  Especially with amps that can have power supply problems and  
>> gassy tubes.  You will hear the plate transformer groan and won't  
>> see HV climbing on the meter if there is a problem in the PS long  
>> before the smoke Genie appears.  My Variac gets used daily several  
>> times after doing ANY work on an amp.  It's saved me loads of time  
>> fixing blown parts that would normally occur if I had just turned  
>> an amp on with problems.  Best investment I ever made.  0-250 volts  
>> at 20 amps.
>> Sent from AOL Mobile Mail
>> -----Original Message-----
>> From: Bill Turner <dezrat at outlook.com>
>> To: Amps group <amps at contesting.com>
>> Sent: Fri, Jul 7, 2017 12:34 PM
>> Subject: Re: [Amps] SB220 step start blows fuses
>> ------------ ORIGINAL MESSAGE ------------(may be snipped)
>> On Fri, 7 Jul 2017 10:21:36 -0400, Mark B. wrote:
>>> A variac varies voltage.   It is not designed to limit current.
>> At the very low end of its range, a Variac limits current just fine. I
>> worked as a calibration technician at Tektronix for years, taking
>> brand new scopes off the assembly line and powering them up for the
>> first time. Believe me, a Variac is the only way to go with a unit
>> which could have any number of problems at first. Over the years I
>> powered up literally thousands of scopes this way and never a problem
>> with current limiting.
>> 73, Bill W6WRT
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