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Re: [Amps] SB 220 Exploding Resistor

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Subject: Re: [Amps] SB 220 Exploding Resistor
From: "Jim W7RY" <>
Reply-to: Jim W7RY <>
Date: Mon, 18 Jan 2016 07:56:40 -0800
List-post: <">>
Yes there is. Move R27 to me in series with the 120 volt bias and the center 
tap of the filament transformer. So if a there is a filament to grid short, it 
wont short out the 120 volt bias supply.

>From Rich Measures web site...

If one of the tubes happens to develop a filament-to-grid short: Since each 
grid is DC-grounded, a shorted tube also short-circuits the +110V power supply. 
This power supply is powered by the unfused filament-transformer. Thus, if a 
filament-to-grid short occurs, and the amplifier is not switched off promptly, 
the filament-transformer will literally melt-down and short-out. The black tar 
that comes out of the overheating transformer makes an unpleasant mess inside 
the amplifier. This potential source of grief can be eliminated if the relay is 
rewired as shown in Diagram 1. The new circuit uses resistor-cutoff bias, 
utilizing the existing 100k ohm resistor [R27], which is rewired to relay 
terminal #5. The current through this resistor, during receive, is normally 
less than 0.25mA [P<0.007W], so the 1/2W rating is more than adequate. (In 
other words, it is risky to connect a power supply to the bias contacts of any 
grounded-grid amplifier.

Jim W7RY

-----Original Message----- 
Sent: Monday, January 18, 2016 7:24 AM 
To: Jim W7RY 
Subject: Re: [Amps] SB 220 Exploding Resistor 


I have a SB220 upside down and I'm guessing that there is a  
modification to prevent damaging the filament transformer ( ?? )


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