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  • Power Resistors in Guitar Amps

    After watching all of Uncle Doug's videos, I expanded my viewing to a couple of other guys who are repairing guitar amps on YouTube. While watching one yesterday, I saw something for which I'd like your input. The technician replaced some 1 watt carbon comp resistors in a tube amp with some 1/2 watt metal oxide resistors. His comment was something on the order of, "This tube circuit doesn't need 1 watt resistors. It doesn't use that much power." As far as power is concerned, he's correct, but what about voltage? Most 1 watt metal oxide resistors are only rated for 300 volts, if my research is accurate. One watt carbon comps are rated at 500 volts. My guess is that many one-watt resistors in guitar amps are there for voltage purposes, not for power purposes. Am I missing something? I confess that I never thought about the voltage rating on a resistor until about two years ago. Then again, the issue only pops up with high voltage circuits.

  • #2
    The thing to remember is that a resistor's voltage rating is the voltage drop across the resistor. Rarely do you have a resistor go from B+ directly to ground. Because there is 500V on one side of the resistor does not mean it's dropping 500V. For instance, look at a typical preamp plate resistor. There is supply on one side and plate voltage on the other side. Subtract one from the other to calculate the actual voltage drop across the resistor.
    "Yeah, well, you know, that's just, like, your opinion, man."

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    • #3
      A bleeder resistor (220K-470K) to ground is a good example of a large voltage drop.
      --
      I build and repair guitar amps
      http://amps.monkeymatic.com

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      • #4
        I suspect you are looking at the preamp tube plate resistors. Like Dude says, they may start with 350v on one end, but that pl;ate is not grounded. The resistor itself likely has more like 150-200v across it tops.

        Carbon comp 1 watt probably used for noise reasons. The film resistors are quieter than comp types, so no need for larger. COnsider the average 100k plate resistor. In a 12AX7, a typical current flow is about 1 milliamp. Oh maybe 2 or even 3, or for that matter even less, like 0.7ma. But 1ma is close enough. 1ma through 100k ohms will drop 100v. 2ma would drop 200v. before very long you run out of volts to drop. If you could manage to drop 300v across it, you'd be dissipating 0.9 watts in that resistor. But that would be a steady drop. No one has that. My 1ma, 100v drop asks the resistor to shed only 0.1 watt

        Even if you amp has 500v in it somewhere, it is not going to be across that resistor.

        I think bleeders are about the only place resistors are from B+ to ground. A 470k bleeding a 500v supply has about half a watt steady dissipation, so maybe a 2w resistor would be a good size. 220k bleeder on that 500v is a little over a watt, so 3-5 watts would be my choice.
        Education is what you're left with after you have forgotten what you have learned.

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        • #5
          All good observations. I think the lower noise of the 1 watt carbon comp makes sense. Are there any considerations for the plate resistor being sized for 1 watt in case the tube shorts?

          Should a replacement plate resistor be metal film or metal oxide? I've heard these online repair guys use both names. Isn't metal oxide the best choice?

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          • #6
            I'll disagree with the others and say you're better off using 1W than 1/2W in many positions. They can withstand more voltage, are less stressed in terms of power rating, and sometimes have less noise.
            As far as the voltage ratings, I think the AC (signal) is a factor that has not been mentioned. Best example I can think of is the HotRod/Blues series amps where the PI plate resistors are often failing and need to be replaced with 1W types. The only reason I can think that causes the failures is voltage rating with regard to signal.

            An added bonus is they likely have less flimsy, larger diameter leads (than 1/2W). This can be important with modern shrinking parts size. And again, body length is directly proportional to voltage rating.
            "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
              Well the cost differential is minimal, and larger certainly can't hurt.

              Shorts? Again, I am assuming you mean 12AX7 plates. And no one I know runs them on 500v, 300-350 maybe more like it. It is hard for a plate in a tube to find a path to ground to short to. But even if it does, that 100k resistor will limit the current to 3 or 3.5ma in the example. It is not a failure I am concerned about..Power tubes are a different story. And as to voltage, that 12AX7 grounded plate will still only put the 300-350v across the part. There is no transformer in the preamp, so the voltage across the plate resistor is limited by the B+ voltage and ground.

              Now the HR DeVille is unusual in that it runs the PI on 440v, so you may have a point. Though I suspect even at full clip the voltage never gets down to zero.
              Education is what you're left with after you have forgotten what you have learned.

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              • #8
                OK. Firstly, so as not to be a hypocrite, when I replace those Hot Rod plate resistors, I do use 1W because I have them, they're cheap, and it's a little extra insurance. That said, I think the problem with those amps roaching plate resistors is likely crappy tubes- or maybe even crappy resistors, but I don't think it's resistor wattage being too low. I've had lots of those Fender branded preamp tubes bad. If you look at signal levels there and calculate power dissipation for the AC signal content, it adds something like .003 watts based on the roughly 15VAC signal level stated on the schematic, which shouldn't be an issue. And, yes it will be slightly more if you crank the amp- but still...... I'll add: If you look at any old Fender schematic, it says, "ALL RESISTORS 1/2W 10% TOLERANCE IF NOT SPECIFIED", which includes preamp plate resistors. I don't see those bad very often. I do agree about lead size and there's no reason not to use 1W resistors. I'm just saying that it shouldn't be necessary.
                "Yeah, well, you know, that's just, like, your opinion, man."

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                • #9
                  Metal Film resistors of sufficient wattage also are good choices, and lower noise than carbon comp.

                  https://www.vishay.com/docs/31027/cmfmil.pdf

                  In this CMF Vishay-Dale series, the CMF70 carries a max working voltage rating of 500V, wattage rating is 0.75W. I used to have Mepco, Ohmite, Philips, IRC and other thick engineering catalogs, though now they're tucked away in boxes in storage. From a noise standpoint, larger wattage resistors tend to be lower noise than lower wattage, plus the max working voltage rating also goes up with the higher wattage part. If size is of no concern (as well as cost), I'd chose the larger wattage part as a general rule.

                  https://www.vishay.com/docs/28768/mb_vg06.pdf

                  This series also has high working voltage rating, as well as some interesting graphs in the noise and nonlinearity characteristics. I've never used these, so no idea how expensive or available, but looked interesting on Vishay's website.

                  I found some additional files on the basics of resistors:

                  basics of Resistors.pdf

                  RF Electronics -Resistors.pdf
                  Last edited by nevetslab; 08-01-2020, 07:31 PM.
                  Logic is an organized way of going wrong with confidence

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                  • #10
                    I think R.G. has posted here about the suitability of metal oxide resistors for tube circuits. While some metal oxide resistors have a spec at something like 350V, not all do. Some are rated for KV's. And while some CC resistors have a spec of 500V, not all do. Most are rated at 350V. In fact the specs for both are not dissimilar as far as available options when you shop. My understanding is that metal oxide is a good choice for HV resistors because they're non inductive and typically come with a flame proof or at least flame retardant spec. I think they also exhibit better thermal specs for stability and derate too. My point is, just because one offering of CC resistor has a 500V spec and one offering of MO resistor has a 300V spec doesn't tell the whole story here.

                    If this is about power rail resistors I've heard many arguments in favor of metal oxide and I generally use those. If this is about plate resistors the points being made above about the actual voltage drop across the resistor can be considered and then it's usually NOISE that's the primary concern. Thermal (johnson) noise and "shot" noise are said to be the primary problems with CC resistors. I think I read that the quietest resistors for plate circuits are actually wire wound. Some posters have sited their inductive properties, though I don't expect that is significant at audio frequencies. Maybe there's more to it and potential oscillation frequencies can be a problem. But this is starting to go beyond my station now.
                    "Take two placebos, works twice as well." Enzo

                    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

                    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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                    • #11
                      The noise generated by PI plate resistors is irrelevant. It will only be "amplified" by the power stage, typically having a voltage gain smaller than 1.
                      Last edited by Helmholtz; 08-01-2020, 04:33 PM.
                      - Own Opinions Only -

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                      • #12
                        Having found some additional files on Resistor Basics and models, I tried to attach them here, but was unable to get past the constant request for URL. I went to see if I could edit the previous post of 7/31/20 @ 9:18PM, and to my amazement, I WAS allowed to add those attachments, same method, using the Upload Attachments key below this composing block. No idea why it works in Editing, but won't in a new post.
                        Logic is an organized way of going wrong with confidence

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                        • #13
                          I found & downloaded an 1980 Allen Bradley Component Catalog, and have the Carbon Comp Resistor Data Sheets & Technical specs/testing data on them here:

                          Allen Bradley Carbon Comp Resistors.pdf
                          Logic is an organized way of going wrong with confidence

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                          • #14
                            Originally posted by Helmholtz View Post
                            The noise generated by PI plate resistors is irrelevant. It will only be "amplified" by the power stage, typically having a voltage gain smaller than 1.
                            So would you routinely change 100K 1 Watt carbon composition resistors in an input stage to metal film or metal oxide?

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                            • #15
                              From Randall Aiken's page:

                              In general, for low-noise design:
                              • Keep resistance values low, because thermal noise is directly proportional to resistance value.
                              • Wirewound resistors are the best choice for noise, followed by metal film, metal oxide, carbon film, and lastly, carbon composition. However, wirewound resistors are not readily available in large resistance values, and are usually inductive, which can cause instability problems in some cases. Bear in mind, however, that many people prefer the "sound" of carbon comps, claiming they sound warmer than film or wirewound types. This is possibly due to distortions generated by the modulation of the contact noise current by the AC signal. Since this noise has a 1/f frequency characteristic (similar to pink noise), it is more pleasing to the ear than white noise. However, pleasing noise is still noise, and in my opinion, it should be reduced to the lowest possible level. The signal distortion is a different topic altogether.
                              • Use the largest practical wattage resistors (unless you are using wirewound resistors) because contact noise is decreased in a larger geometry material.
                              It looks to me that, according to Randall, something like 1W or 2W metal film is the best available option for early stage preamp plate resistors for lowest noise.
                              Last edited by Chuck H; 08-02-2020, 02:55 PM.
                              "Take two placebos, works twice as well." Enzo

                              "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

                              "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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