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Re: [Amps] SB220 step start blows fuses

To: "'Jim Garland'" <>, <>, <>, <>
Subject: Re: [Amps] SB220 step start blows fuses
From: "Gary Schafer" <>
Date: Mon, 10 Jul 2017 12:46:52 -0500
List-post: <>
That's a lot of fuses.

Gary  K4FMX

> -----Original Message-----
> From: Amps [] On Behalf Of Jim Garland
> Sent: Monday, July 10, 2017 9:11 AM
> To:;;
> Subject: Re: [Amps] SB220 step start blows fuses
> I've been reading this thread for some time, and while I agree with many
> of the suggestions (i.e., light bulb testers, variacs), I don't believe
> any of these are necessary to diagnose this fault. The SB220 is a
> straightforward, simple amplifier, and catastrophic faults that blow
> fuses aren't normally hard to diagnose, even without any test equipment.
> Looking at the schematic, here's what I would do.
> 1. Disconnect the red secondary plate transformer wire. Turn on the amp
> and see if the fan operates, the tubes light up, and that no fuses blow.
> If all looks good, that rules out a plate xfmr problem (keep your
> fingers crossed) and a filament xfmr problem (ditto), but leaves the
> possibility of a problem in the HV filter bank or an HV short in the
> tubes. If the fuse still blows, that means the plate xfmr is shorted, or
> that there's a short in the filament xfmr, or that a tube filament is
> shorted.
> 2. If the fuse still blows in step (1), then pull the tubes, leave the
> red HV secondary wire disconnected, and repeat the test. If now the fuse
> doesn't blow, then that rules out a short in either the plate
> transformer or filament transformer and points the problem to a filament
> short in a tube. If a fuse still blows with the tubes pulled, then we
> still have to suspect the filament xfmr or plate xfmr.
> 3 If the fuse still blows in step (2) (with the tubes unplugged), then
> you have little choice but to separately disconnect a primary wire from
> the plate xfmr and filament xfmr to see which one is bad. If the fuse
> only blows with the tubes plugged in, but doesn't blow with the tubes
> unplugged, (with the HV secondary wire disconnected) we know that the
> problem is a shorted tube filament.
> The most likely scenario is that the tubes will light and that the fan
> will work with the red plate xfmr secondary wire disconnected. If that's
> the case, then you need to pin down whether the problem is an HV short
> in a tube or a problem in the HV filter circuit. The easiest thing to do
> is to pull the plate caps off the tubes and hook back up the red
> secondary wire. If you turn on the amp and no fuses blow and the HV
> meter shows plate voltage, then you've got at least one bad tube. If the
> fuse blows with the plate caps disconnected, then you've got an HV
> filter problem. Look for shorted diodes in the rectifier bank. You may
> also have bad filter caps, but they're probably just open and not
> shorted. If this were my amp, I replace all the filter caps and diodes
> with modern components, just as a precautionary measure. That will
> probably fix your problem.
> 73,
> Jim W8ZR
> On 7/10/2017 3:08 AM, gudguyham--- via Amps wrote:
> > 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.
> >
> > -Warren
> >
> >> 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.
> >> REPLY:
> >>
> >> 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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