[Amps] SB220 step start blows fuses

Mark Bitterlich markbitterlich at embarqmail.com
Thu Jul 6 19:14:06 EDT 2017

With all the answers from the real experts at hand, I feel safe offering my 
2 cents on the topic.

If you suffer from an amplifier (or anything else) blowing fuses, like this 
amp is doing, you need to isolate the problem as Jim has recommended.  An 
alternative way to accomplish that end is to set your amp up with a current 
limiting device so that you do not exceed the fuse rating and thus blow the 
fuse.  You can then turn on the amp and make meter readings to find out 
where the voltage drop is.  Of course you need to know how to use a meter 

How do you build a current limiter for your amp?  Well it is actually dead 
simple.   Use a light bulb in series with the power lead to the amp. Since 
you are running this amp on 120 volts AC, you only need one light bulb.  If 
you were running 220, you'd need two.

Consider:  If you want to limit current to 1/2 amp total, you would use a 60 
watt 120 volt common light bulb in series with the power lead.  Use bigger 
bulbs as necessary to allow increased current flow.  Of course finding 100 
watt light bulbs these days is problematical but the concept is 
straightforward and substitutes can be used.

If you turn on the amp and the bulb goes full bright, then you know you have 
a bad short to ground.  If you do not have a short, the bulb will barely 
glow.  In the meantime, running your tests will not result in a trashcan of 
blown fuses and you will not be stressing other components.

Long story short, light bulbs in series with the main voltage act as perfect 
current limiters.  This concept also works with any power feed, circuit 
breaker, etc.  Put the concept into your toolbox of tricks and gadgets, it 
works well.


----- Original Message ----- 
From: "Jim Thomson" <jim.thom at telus.net>
To: <amps at contesting.com>
Sent: Thursday, July 06, 2017 4:42 PM
Subject: [Amps] SB220 step start blows fuses

> Date: Thu, 6 Jul 2017 00:07:50 -0600
> From: Warren Volz <warren at warrenvolz.com>
> To: amps <amps at contesting.com>
> Subject: Re: [Amps] SB220 step start blows fuses
> <Without tubes and with the HV transformer unplugged, the fan and meter 
> lights work. In addition, the step-start fuses don?t blow!
> <That leads me to suspect the tubes + HV supply. I checked the windings on 
> the HV transformer to look for shorts:
> BLK-YEL = .7 Ohm
> BLK-BLKYEL = .6 Ohm
> BLKYEL-YEL = 0 Ohm
> BLKGRN - BLU = .8 Ohm
> BLKGRN - BLKRED = .6 Ohm
> BLU - BLKRED = 0 Ohm
> Do any of those look off?
> Also, would these steps (or something similar) make sense?
> 1) Re-enable HV supply, power on, if fuses are ok power off and go to step 
> 2
> 2) Once HV dissipates, add tube 1, if fuses are ok, power off and go to 
> step 3
> 3) Assume tube 2 is shorted, else tube 1 is bad
> -Warren
> ###  With  NOTHING  connected to the SEC winding of the plate xfmr,  you 
> should see  aprx
> 9-10 ohms of dc resistance.    You could also hook it all back up, then, 
> with tubes REMOVED
> from sockets...OR disconnect the B+ from the top of each tube..... try it 
> again.    If normal B+  appears,
> its one or both tubes.   You really need to isolate stuff.   IF  plate 
> xfmr is good, the trbl could be in the
> FW Doubler  diodes.   IF diodes are ok,  it could be the 8 x filter caps, 
> or eq resistors across each filter cap.
> It could be a bad HV  interlock assy,  it could be the plate choke assy, 
> it could be anything..including a
> internal shorted, or partially shorted  plate xfmr.   Or  any HV inter 
> connecting wiring between  any of the above
> components.   Or it could be your new  step start assy itself.
> Jim  VE7RF
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