SB-401 Resistor R17 correct wattage?

Mon Jan 1 10:46:41 EST 2007

Hello Bob,

  Thanks for the R17 info. I've replaced the resistor with a 1/2 watt 680 ohm
  resistor. I also found the 1 uh driver plate RFC was opened. Lucky, I had 
  a spare from an old Apache TX-1 parts chassis. Replaced it and now I 
  have plate current again hi hi.. With plate current I can TUNE the final
  but the output is very low, less than 10 watts. The driver tunes nicely
  and very stable in a seperate receiver. I doubt it is anything but possibly
  the finals. 

  The next problem is getting the power output up to 100 watts. The CW GAIN
  control increases the plate current but only about 50 mils. There's no idle
  plate current in sideband and the output in TUNE and CW is less than 10 watts.
  The driver has plenty of drive. At this stage, I'm do not believe it's the driver as
  all mixer and driver coils have been adjusted to maximum. Pulling the driver tube
  to adjust the final idle current has no affect, it's still zero. I've replaced all the
  electrolytics, including the HV and LV supply caps. I'm using my own designed
  HV and LV supply replacement printed circuit boards. The front panel meter shows
  800 volts, right on the money which, prior to replacing the caps, it showed 850VDC.
  I believe the problem with low output is the neutralization capacitor's setting. As for
  no idle current, that could be the final tubes so I'll swap my SB-400 6146A's to
  see if that improves the output. Did the SB-401 originally come with 6146/6146A's
  or were they 6146B's? I believe they were 6146/6146A's. 

  Any suggestions are greatly appreciated. Happy New Year to everyone and thank
  you for all your help.

  73's
  Mike
  W5RKL

W5EUQ <w5euq at verizon.net> wrote:
  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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