[Amps] SB220 step start blows fuses

Jim Garland 4cx250b at miamioh.edu
Mon Jul 10 10:10:54 EDT 2017

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 

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.

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 <warren at warrenvolz.com>
> To: amps <amps at contesting.com>; gudguyham <gudguyham at aol.com>
> Cc: dezrat <dezrat at outlook.com>
> 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 <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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