[Heathkit] sb200 R15 question

[Carl]

What you mentioned are the 33 Ohm 1W resistors at the grid socket of 
each tube. Their purpose is to act as a parasitic suppressor as well as 
offer some degree of forced tube matching.

R-15 is the grid metering resistor located on a terminal strip in the 
lower region of the amp......it cant really be called a chassis.

Carl
KM1H


----- Original Message ----- 
From: <KHeck73 at aol.com>
To: <w5rkl at yahoo.com>; <heathkit at mailman.qth.net>; <ka9cch at arrl.net>
Sent: Thursday, September 25, 2008 6:46 PM
Subject: Re: [Heathkit] sb200 R15 question


> I'm sending this a second time as the first one wasn't plain  text...
>
> Many years ago (more than I care to think about), my high school 
> station  had
> an HW-101 running into an SB-200. Once or twice a year somebody would 
> try
> loading up the amplifier full power with the bandswitch set in the 
> wrong
> position vs. the band set on the transceiver. This would blow one (or 
> two?)  'grid
> bias' resistors. I don't have a schematic in front of me, but I recall 
> them
> being 1/2 watt carbon. This situation happened more than once. It 
> didn't  seem
> to hurt the amp, and probably prevented further damage elsewhere 
> inside.  So,
> if it was R15 that I remember, I'd consider the 1/2 watt rating a 
> safety
> feature not to be changed It inexpensively tells you if something is 
> set wrong
> (with a pop and then the smell, because I did it once  ;).
>
> -Karl.
>
> In a message dated 9/25/2008 10:48:40 A.M.  Eastern Daylight Time,
> _w5rkl at yahoo.com_ (mailto:w5rkl at yahoo.com)   writes:
> R15 along with R16 and the bias supply through the relay is part of 
> the
> standby cut off bias as well as the normal transmit -2VDC operating 
> bias.
> Changing the values of one or both resistors can and will affect the 
> cut off and
> operating bias voltage to the grids of the tubes.
>
> I would not  substitute the original 1.5 ohm 1/2 watt resistor with a 
> 2 ohm
> resistor. A 2 ohm  resistor is an increase in resistance of "40%" (1.5 
> + 40% =
> 2.1ohms) of the  original value. I would order a new resistor from 
> Mouser or
> your favorite parts  outlet instead of using a 2 ohm resistor.
>
> Why did R15 burn open? A  shorted tube could cause R15 to burn  open..
>
> 73
> Mike
> W5RKL
>
>
>
> --- On Wed, 9/24/08, labernathy <_ka9cch at arrl.net_ 
> (mailto:ka9cch at arrl.net) >
> wrote:
>
> From: labernathy <_ka9cch at arrl.net_ (mailto:ka9cch at arrl.net) >
> Subject: [Heathkit]  sb200 R15 question
> To: _heathkit at mailman.qth.net_ (mailto:heathkit at mailman.qth.net)
> Date:  Wednesday, September 24, 2008, 8:35 PM
>
> I'm working on an old SB200. In the process of changing out the bias
> cap  (C3) and C19, I found that r15 (1.5 ohm 1/2 watt) was in two 
> pieces.
>
> I have some questions:
> 1. Any ideas as to what would cause this resistor  to overheat and 
> break?
>
> 2. How critical is the resistance, will 2 ohms work just as well?
>
> 3. Is this resistor expected to open as a "safety device"? I know
> that  in some Kenwood xcvrs, a resistor doubles as a fuse and changing
> to a higher  wattage unit can shift the failure to an expensive part.
>
>
>
>
>
>
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