[Amps] SB 220 Exploding Resistor

[Louis Parascondola]

Very possible and after repair the amp should be brought up slowly on a variac watching the grid current and plate current meters while doing so.  Use one tube at a time in  the beginning.  With each tube watch for any plate current or positive and most likely negative going grid current.  If you see movementmovement ofveiter meter, stop.  The tube is bad.  Remove and do next tube same way.  If you don't do this, the amp will bow same parts once again.  

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-----Original Message-----
From: Jim W7RY <w7ry at centurytel.net>
To: Tom Hellem <tom.hellem at gmail.com>; Amps <Amps at contesting.com>
Sent: Mon, Jan 18, 2016 08:46 AM
Subject: Re: [Amps] SB 220 Exploding Resistor


You either had (momentary arc) or have a shorted tube.

Be careful with the way the bias on the relay is wired. If the cathode 
shorts to ground, you can burn up the filament transformer by shorting out 
the 120 VDC bias circuit.

Replacing the small VHF choke with a 10 ohm 10 watt wire wound resistor is 
common practice.

Bottom line.. You probably have a shorted tube.

73
Jim W7RY


-----Original Message----- 
From: Tom Hellem
Sent: Sunday, January 17, 2016 8:05 AM
To: <a href="mailto:Amps at contesting.com">Amps at contesting.com</a>
Subject: [Amps] SB 220 Exploding Resistor

Hi-
My SB 220 has what appears to be a glass coated wirewound resistor in
series with the anode B+
supply lead just before the RF chokes. It is not on the schematic so must
have been a modification
done at some point. It blew up in spectacular fashion one day and I
hesitate to just replace it without
having some idea of what may have been the root cause. I do not have much
of an electronics background
and would appreciate if someone could suggest some things that I should
check out before I try to put it
back on line.

73 To All on The List.

Tom
K0SN
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