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73 SVT Pre Amp Resistor

Discussion in 'Amps and Cabs [BG]' started by JOE EDO, Aug 1, 2012.


  1. JOE EDO

    JOE EDO Supporting Member

    Aug 28, 2007
    Central New York
    Note the large browned out resistor next to the blue caps..
    Can anyone tell me the value and where I can find one of these? Looks like it's old and prob has drifted a bit.

    IMG_2381.
     
  2. Bassoballs

    Bassoballs

    Nov 14, 2009
    Mid Michigan
    5W 10K or 5W 8.2K varied from year
     
  3. --Vissinger --

    --Vissinger --

    Jan 31, 2010
    If the color bands are gray, red, brown it's 820 ohms ... It looks like a bleeder resistor for the caps, so probably the value isn't very important.

    If it's gray, red, purple, it's much larger. I don't remember the value for purple. 7 I think ... so it would be about 820 Meg which seems pretty odd.

    If the bands are a different color, check a resistor value chart.
     
  4. JOE EDO

    JOE EDO Supporting Member

    Aug 28, 2007
    Central New York
    Thank you gentlemen!
     
  5. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    In your amp it is 8.2K / 5W 5%. The first band is clearly gray, so gray red orange gold. The discoloration is a clear sign that it wouldn't hurt to bump up the wattage rating for the replacement. I use 10W. Best to make sure that no other wires are touching it. That sensitive white shielded wire should be moved.
     
  6. Make sure that white lead is away from the twisted heater wires also. And stop sneezing on those caps ;)
     
  7. JOE EDO

    JOE EDO Supporting Member

    Aug 28, 2007
    Central New York
    That's 1970's Mojo ectoplasm!:bassist:

    Thanks again guys. Really getting an education here from some SVT gurus!
     
  8. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    I was going to say the same thing that Jon did. I use that F&T cap. Originally they used a 2X20uF 450V cap in the amp. If you can find the space to mount it (it is longer than the original) you should be able to use the one that you bought.

    The thing that I don't like about some (not all) of the vintage style tubular capacitors is that they are made up of two small caps glued into a cardboard tube. They pack it with sand. The problem is, if one of the caps blows, you won't see any evidence of it. Normally electrolytic caps have blow out valves that bulge or pop if there is a problem. This could lead to a dangerous situation if it is in the cardboard tube. Now having said all that, people have been using these caps for years without any problems. I just like to err on the side of caution.

    Below is a pic of an original cap with the outer shell removed. The two tabs at the end were wired together putting the two internal caps in parallel, making it like a single 40uF, 450V cap.


    [​IMG]

     
  9. Bassoballs

    Bassoballs

    Nov 14, 2009
    Mid Michigan
    Joe, the cap you ordered will work just fine. It is very similar to the style cap your amp would have had originally.
     
  10. JOE EDO

    JOE EDO Supporting Member

    Aug 28, 2007
    Central New York
    Well the double cap was back ordered so I cancelled that and went with the47mf/500 volt. I also upped the resistor to a 10 watt 8.2K, instead of the stock 5 watter. Have not fired up the amp yet as I hope to get a few buy in's that I did this right (lest the novice solder joints!) Yes, I also got that white shielded wire up away from the others.

    IMG_2717.

    IMG_2704.
     
  11. Sorry you MUST fix those solder joints. I can't tell if the connection even looks okay on the right side of the lower pic?
    Note: Solder makes a electrical connection not a mechanical connection. Wrap the leads at least 1/2 way around the terminal lugs, squeezed tight with needle nose pliers before you apply solder. Leads just poked through are asking for future failure. Don't over-use the solder as it can hide a bad joint.
    Not nit-picking, just trying by remote control to help and provide a little education.
    I small dab of solder on the iron, then apply that dab to the leads and lug and feed the solder from the opposite side. When the joint is hot enough to flow the solder the connection will be good.

    Can't tell in the pic if the polarity is correct on the cap (can't see the + or -).
     
  12. JOE EDO

    JOE EDO Supporting Member

    Aug 28, 2007
    Central New York
    B! Your words are needed and heeded and not taken personal at all. Thanks and I will pull those joints and get a solid connection as you suggest and then solder with the method you suggest.

    A few more shots

    IMG_2716.

    IMG_2717.
     
  13. JOE EDO

    JOE EDO Supporting Member

    Aug 28, 2007
    Central New York
    I did freshen up the face plate....new knobs on order too!

    IMG_2713.

    IMG_2348.
     
  14. Thanks for taking it the right way :). I see the + on the cap is correct and you might have had sparks from one at least one end of the resistors leads :)

    The face lift looks sweet!
     
  15. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    +1

    This is a harder joint to solder than most due to the larger gauge wires and solder terminals. What might help in this case is to dab on some solder paste onto the wires before soldering. You can get it a Radio Shack. This helps clean the joint and flow the solder. There will be less time with the iron on the joint. Those bigger wires take more heat when soldering, they act like a heat sink, making it harder to do a good job. The flux makes it easier. Some solders are rosin core, they have flux inside. I find that it still helps to add the extra flux. You can clean up the excess flux with isopropyl alcohol (drug store) after soldering. BTW, RS sells a nice tin/lead/silver solder.

    Whatever you do, don't breath the fumes when you are soldering.
     
  16. Sniffing fumes used to be the best part :eek: :D
     
  17. JOE EDO

    JOE EDO Supporting Member

    Aug 28, 2007
    Central New York
    Up at 5 AM to get this done right (yes..I had nightmares of bad joints!) I kept in mind that I was to ensure a solid mechanical connection and solder being the substance that provides the "electrical union." Pull the connections and made sure each lead had a solid connection to the lug/ each other (pliers method). I then tried to pull them off the lug/apart ...gently... and they did not budge! I was ready to flow....and the results:

    IMG_2721.

    IMG_2720.
     
  18. I can see the joint in the bottom pic clearly. MUCH better :)! Good job, sorry for the sleep loss :oops:. I would be confident to road that joint, would like to see the wire's joint improved. Lots of globbing there? It's all about preventing any source of potential trouble. :)
     

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