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Re: [Amps] SB220 step start blows fuses

Subject: Re: [Amps] SB220 step start blows fuses
From: gudguyham--- via Amps <>
Date: Mon, 10 Jul 2017 05:08:26 -0400
List-post: <>
If you have no HV on the tube all you are doing is lighting the filament unless 
you have a HARD short that would be detectable with an ohm meter.  
I am thinking that you have the fan hooked up wrong and somehow have the fan 
wired both into the plate transformer and filament transformer out of phase or 
something weird.  If you have the fan wired correctly it should go on no matter 
what.  It should have nothing to do with the tube being in the socket. Can I 
assume that before you embarked on this total rebuild you did that the amp 
worked OK?  If it did, then you screwed up someplace when installing the mods.  
I am only hoping you did not burn up one of the transformers.

-----Original Message-----
From: Warren Volz <>
To: amps <>; gudguyham <>
Cc: dezrat <>
Sent: Sun, Jul 9, 2017 10:52 pm
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.


> On Jul 7, 2017, at 12:58 PM, gudguyham--- via Amps <> 
> 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 <>
> To: Amps group <>
> 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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