[Amps] SB220 step start blows fuses - c'mon

[Dave Hachadorian]

C'mon guys, this is only an SB-220.  It's a barebones amplifier 
with no complicated protective circuits.  It's easy to work on, 
since it was built as a kit.

To troubleshoot this amp, you should need only one spare fuse. 
Somewhere in the procedure below, the fuse will blow, and you 
will know where the problem is.

First, remove the surge limiter circuit.  It is a troublesome, 
unnecessary item.  Restore to factory configuration.

Disconnect the primaries of the plate and fil/bias transformer 
and see if the fuse blows.

Connect the primary of the fil/bias transformer, with secondaries 
disconnected, see if the fuse blows.

Connect the bias secondary of the fil/bias transformer and see if 
the fuse blows.

Connect the filament secondary of the fil/bias transformer, with 
the tubes removed.  See if the fuse blows.

Plug in the tubes.  See if the fuse blows.

Connect the primary of the plate transformer with secondary 
disconnected, see if the fuse blows.

Connect the secondary of the plate transformer, with the B+ line 
to the tubes disconnected.  See if the tube blows.

Connect the b+ line to the tubes and see if the fuse blows.

It's a lot more fun to work on Tetrode amps with all kinds of 
digital and analog protective circuitry!

Dave Hachadorian, K6LL
Big Bear Lake, CA


-----Original Message----- 
From: donroden at hiwaay.net
Sent: Monday, July 10, 2017 5:50 AM
To: amps at contesting.com
Subject: Re: [Amps] SB220 step start blows fuses

Agree with Gary !!

Make a tester out of a garage light with a 300 or 500 watt 
halogen element.
( or pair )   relatively inexpensive and I always need an extra 
light
when looking for things in my garage.

Don W4DNR



Quoting Gary Schafer <garyschafer at largeriver.net>:

> You don't have a big enough bulb.
>
> I use two 300 watt bulbs in parallel. 300 watt bulbs are about 
> the largest
> you can find easily.
>
> I made up the bulb unit using a square electrical box and 
> mounted two
> outdoor light sockets on it, an outlet and a regular house 
> switch to short
> out the bulbs when I want to bypass the series bulbs.
> It works great for checking amps. It is really nice if you have 
> an
> intermittent arc or want to let suspect capacitors cook for 
> awhile with the
> amp is in standby as no harm is done if something arcs, the 
> bulbs just light
> up bright.
>
> I sometimes plug the light bulb setup into a variac too.
>
> 73
>
> Gary  K4FMX
>
>> -----Original Message-----
>> From: Amps [mailto:amps-bounces at contesting.com] On Behalf Of 
>> Warren Volz
>> Sent: Sunday, July 09, 2017 9:53 PM
>> To: amps; gudguyham at aol.com
>> 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.
>> >
>> > 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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>> target="_blank">http://lists.contesting.com/mailman/listinfo/amps</a>
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DonR
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