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Re: [Amps] SB220 step start blows fuses

Subject: Re: [Amps] SB220 step start blows fuses
Date: Mon, 10 Jul 2017 07:31:13 -0500
List-post: <>
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 <>:

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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