[Amps] SB 220 Exploding Resistor

[Carl]

That is only good in early amps, a Heath bulletin had a simple wiring 
change, no convoluted voodoo mod needed.

Only needed when on 240V

Carl
KM1H


----- Original Message ----- 
From: "Jim W7RY" <w7ry at centurytel.net>
To: <donroden at hiwaay.net>
Cc: "AMPS" <amps at contesting.com>
Sent: Monday, January 18, 2016 10:56 AM
Subject: Re: [Amps] SB 220 Exploding Resistor


> 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
>
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