[Amps] SB 220 Exploding Resistor

[Jim W7RY]

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.


73
Jim W7RY



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

Jim,

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


Don W4DNR