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@centurytel.net>
To: <donroden@hiwaay.net>
Cc: "AMPS" <amps@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@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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