SB-401 Resistor R17 correct wattage?

[W5EUQ]

Michael,
         I have an original manual on it.  Pictures show R17 as being 
1/2 watt.  Calculations also show that 1/2 watt would work for 680 
ohms.  There is 250 Volts on the input of R17.  It drops the voltage to 
the screens to around 150 to 200 Volts.  Screen current runs from 7 to 
11 ma per tube.  Therefore, for two tubes, max current will be 22 ma.  
Using ohms law,  P=I2 times R   take .022 squared times 680, which 
equals,  0.32912 watts.  Therefore a half watt resistor will work fine 
here. 
What do you think?   Good luck.  HNY,  Bob   W5EUQ  Tampa

Michael Waldrop wrote:
> Good afternoon,
>    
>   I'm working on an SB-401. The final has a resistor, R17 680 ohm, that is in the
>   final amplifier's screen supply. A previous owner installed a 2 watt 680 ohm 
>   resistor, did a horrible job. The resistor has very short leads and it's soldered to
>   a piece of thick wire and I'd like to replace it and clean up the mess. I don't have
>   the original assembly manual. I only have the bama manual which does not include
>   the assembly portion. The schematic is a bit fuzzy. I have a 1/2 watt resistor which
>   I think is the correct wattage but I want to make sure. No I don't want to smoke a
>   resistor just to find out :)
>    
>   Anyway, any suggestions and does anyone have an original SB-401 assembly manual
>   that can verify R17 as being a 1/2 watt 680 ohm resistor? R17 is the resistor shown
>   on figure 2-3, one lead on terminal strip "P" and the other end on the wire between V10 and
>   V11 pins 3. 
>    
>   Another problem it has is no plate current. There's high voltage on the plates of the finals
>   and all the oscillators are working; I can hear them in a seperate receiver loud and clear.
>   I've removed the driver tube, 6CL6, to make sure it's working and it is. I can adjust the signal strength of the SPOT signal in SPOT plus the PLATE control also varies the SPOT signal strength. But, in TUNE, there's no plage current. The relays close, both of them, so it's
>   not the relays (not closing). I suspect both finals have bit the dust.
>    
>   Any suggestions and if someone has a complete assembly manual that can check R17 to
>   verify it's a 1/2 watt resistor. It's the resistor shown in figure 2-3 in the alignment section. I
>   don't have a page number but you'll find it in the alignment section.
>    
>   Happy New Year and thanks.
>    
>   73's
>    
>   Mike
>   W5RKL
>    
>
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