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  • Resistors

    The attached photo is a work-in-progress conversion of a Hammond AO-35. In the photo, both R4 and R5 are 100k ohms; however, they are physically different. Both are original, I've seen similar resistors in photos from other conversions. Looking at the schematic, R4 sees 90 volts and R5 sees 240 volts. I'm guessing that R4 is a higher quality resistor. Can someone tell me what the difference is between the resistors?

    The amp has three resistors that are like R4, One is shown as R2 and the other was removed.
    Click image for larger version

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  • #2
    R5 is a carbon composition type resistor and I'm not sure but R4 looks like possibly an old style metal film? Not sure why they would use a different type though... probably the metal film was satisfying a particular spec that the CC didn't quite make?
    ~Semi-No0b Hobbyist~

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    • #3
      R5 looks like a 1W carbon composition. R4 looks like a film type construction (metal or carbon on a ceramic core) of unknown power rating, perhaps 1/2 watt. At 240V R5 would dissipate just over 1/2 watt. At 90V R4 would dissipate 0.081 watt.
      WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
      REMEMBER: Everybody knows that smokin' ain't allowed in school !

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      • #4
        R4 is stamped CGW N20 10% and 100k. I did a search, but what do you expect for a 50+ year old resistor.

        According to the schematic I'm using, one of them needs to be removed. The circuit feeds the plate for 1/2 half of a 12AX7 and will see about 240 volts. Based on the comments, sounds like R5 would be the one to reuse.

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        • #5
          I wouldn't put too much thought into it. There was nothing special about the amp, just a basic generic amp. If a film resistor might be slightly less noise than a composition one, they might have used it for that, otherwise, I doubt you would see much difference. Modern resistors are dirt cheap anyway.
          Education is what you're left with after you have forgotten what you have learned.

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          • #6
            Unless you bought it new and never had it repaired, you can't really be 100% sure it's original. Could have been replaced while still under warranty some 50yrs ago . Or, they may have run out of one type during assembly and used another type.
            But I think loudthud is probably correct they are for different wattage requirements.
            "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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            • #7
              Thanks for the help

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              • #8
                Not that it's a big deal, but you might want to clean up some of that frass. Looks like an ant colony lived in there. I've had some Hammonds loaded with it too. A q-tip and rubbing alcohol works pretty well.

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                • #9
                  Thanks, I cleaned the tube sockets and the "frass" before I powered up the amp. Actually, i realized after reading all the responses, that the schematic clearly shows the resistors in question to be "low-noise" type.

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                  • #10
                    Did you know "frass" is a real word? I didn't.
                    https://www.google.com/search?q=defi...hrome&ie=UTF-8
                    DON'T FEED THE TROLLS!

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                    • #11
                      Originally posted by rjb View Post
                      Did you know "frass" is a real word? I didn't.
                      Sure did. In "Sideways" one of the vineyards the guys drove past had their sign at the end of the entrance: "Frass Valley". Got a chuckle out of that. I guess they grow their grapevines in bug poop, for that special terroir!
                      Enjoy. Every. Sandwich.

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