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  • Amp Hum

    I'm trying to reduce some hum in my 20 year old Carvin Belair. I think it's 60 cycle hum and is present without the guitar plugged into the amp. It's not at an intolerable level, but still annoying. I've replaced the filter capacitors and plate resistors, which seems to have eliminated some other random noises the amp was making. I've tried using other electrical outlets in the house, but it didn't make a difference. Also tried different tubes. Haven't tried a 7025, maybe that might help. The tube sockets seem OK.

    I put an audio probe on the AC side of the first decoupling capacitor and think I can hear the hum at this point. Does that mean the hum is originating at V1? A DMM didn't show any DC leakage. An oscilloscope shows a distorted sine wave. All this was done with the amp on, but no signal into the amp. One clue might be the volume control. At zero volume the hum is at it's loudest. It decreases as you move to 3. After that it quickly increases back to its original level.

    Does anybody have a suggestion on how to track down the source of this hum?



  • #2
    Question, did you buy a matched set of power tubes and bias them when you installed them?
    This would be my first check. It looks like the amp has a Bias pot similar to a Fender amp for adjusting zero hum.

    Second would be to signal trace the amp and find out at which stage the hum begins. If you have a clean signal coming out of a preamp tube, then a him going into the next, the issue may be a leaky coupling cap.

    If you have a clean input and hum on the output, the tube may not bet getting clean DC to bias it.
    Usually power supply filter caps fix that issue but your full wave rectifier may not be doing its job well. You can get a weak diode and have a half wave issue.

    Given your symptom with the volume control it does sound like a coupling cap though, You have DC leakage between stages which would account for the hum changes when the volume pot tapes the signal to ground.

    The volume on that amp comes after the first gain stage so you have full signal before it gets to that pot.
    If you're going to shoot blind I'd replace C2, C3 and C5. maybe C1 and C8 wouldn't hurt either. These small caps can be bought for $.50 each so doing a bunch of them at the same time is easier then taking the amp down multiple times. You don't want to flex the board cables too many times so doing a whole section at once is best. This schematic has a bunch of voltage check points that might help you narrow it down too.

    I have seen amps where it had caps replaced and someone accidentally swapped am electrolytic power caps polarities around. I was surprised the thing would even run like that without blowing the cap but it did. I figured the cap was toast so I changed it immediately with the others with the correct polarities and the amp was much quieter.

    Other possibilities, I suppose it could be the inverter tube going bad too. They can do some nasty thing to the amp because they are supposed to provide equal and opposite waveforms to the power tubes. If its uneven you get hum. Old tube amps used tube cover on the preamp tubes to minimize hum from the transformers. Sovetek 12AX7-LPS The spiral filament reduces hum in AC filament amps if the hum is coming from the filament supply.
    Then amp has a reverb tank. If its got plugs, try unplugging it and see if the hum changes. You may have a damaged tank coil and its injecting hum into the circuit.
    Last edited by WRGKMC; 09-01-2016, 03:20 PM.


    • #3
      Thanks for the info WRGKMC. It has new matched power tubes and I set the bias. I also tried a new inverter tube (12ax7), but it didn't help. My reverb tank is bad. However, pulling that tube did not affect the hum, so I guess the tank is not the problem.

      It's funny you mentioned a cap being installed backwards. I replaced C21 and C22. Those are two caps in the circuit for the DC filaments on V1 and V2 (the other tubes are AC). I'm almost positive they were installed backwards. I made a mental note on where negative was oriented on the board when I desoldered them. When I went to reinstall the new caps, the + symbol on the board was where I remembered the - side on the cap was sitting. I thought surely I was mistaken because the amp wouldn't work correctly if they were backwards. Anyway, I put them in according to what the board was showing.

      You also mentioned the voltage test points. The DC voltage on the preamp plate resistors was good, except on V2a, which is R13. It was about 50v lower that the number on the schematic. This is channel 2 on the amp, which is a bit quieter than channel 1, so I was not too concerned about the lower voltage. Should I be concerned? I did replace all of the preamp plate resistors with better, low noise Dale resistors.

      Replacing the small caps you mentioned seems like a good idea at this point. I will do that.


      • #4

        Those caps might surely do it. You'd have all kinds of ripple in the heater circuit which would likely induce hum in the preamp tubes. Manufacturers do make modifications and maybe the board was printed wrong. (I always mark the polarity on the board with a felt tip pen before I pull caps just to be sure, and also confirm things with the schematic if I have one)
        I'd check to make sure the positive side of the caps connect to R19 and the negative side to R20

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        I cant see the one channel very well in the schematic. It looks like channel 1 has 155Vdc and channel 2 uses 230Vdc plate voltages. If the plate voltage is 50V lower I'd be concerned. That's 33% off on the 1st channel or around 20% on the other .

        I'd go here and test the voltage. See if you have 325 at point D and 335 at point C. If the voltages are good here then maybe R2 and R8 are dropping more voltage then they should. Again, those caps I mentioned before may be the cause of the voltage being off.

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        If the voltage is off at D or C then you have to check your components working back to the power supply.
        You can try pulling the tubes to unload the supply is its low there too. I'd also check those points with the meter set for AC. If there is AC mixed with DC it will contaminate your audio signal.

        Again, it may be a weak bridge rectifier diode. You can set your meter for AC so it will measure 1/2 waves then test across each of the bridge rectifier diodes. You're simply looking to see is the readings are similar/balanced. If ones way off you may have a dud there.
        Last edited by WRGKMC; 09-02-2016, 07:52 AM.


        • #5
          I did trace the filament caps back to the resistors when they were soldered back in, just to be sure everything was correct.

          The schematic you have appears to be for the Vintage 33. It's basically the same as the Belair, but the Belair does have some higher values, mainly the B+. Point D on your schematic is 325V and the Belair schematic is 385V. My amp is only reading 349V.

          The voltage coming out of the secondary is 300VAC, whereas the schematic shows 350V. This is with 116V at the power switch. Raising the mains voltage to 120V with a variac increases B+ a little, but still short of the 385V on the schematic.

          So, the secondary winding is about 50VAC less than what it probably should be. Both of the wires from the secondary have the same voltage and the amp still sounds good. Still, I will probably drop in a new transformer. I bought this amp used and it sounded terrible. I want to get it back to its original condition. It sounds pretty good now, but has a little more hum than it should.

          I mentioned that one of the plate resistors is 50V lower than the other on V2. I was wrong. It actually measures 100V lower. They should both be the same. These resistors are new and right on spec. B+ to both resistors is also the same. Since the resistor is good, does that mean the current through this part of the circuit must be off in order for the 100V deficiency to occur? I will replace the coupling capacitor (C18) and see if that corrects the problem. I will also change that tube, clean the socket and reflow some solder on the pins. Can the cathode resistor be an issue here?

          The reverb circuit is also a problem area. Its measuring 90V too high on one plate resistor and 50V too high on the other, compared to the schematic. And that's with the B+ being lower than it should be. I guess this is the real problem area to tackle first.

          Thanks for you help WRGKMC.

          Also, everyone doing this knows to be carefull. It's worth repeating again. I was distracted last night when taking some measurements. Had a pen in one hand and a probe in the other. I put the probe down for a minute, and then went to measure VAC at the power switch. My meter did not show any volts, so I looked back over at the switch and saw that I was probing it with the pen! Thankfully the pen had a plastic case and rubber grip. Also, my free hand is never touching the amp or anything else. I could not believe I did something so stupid. Might of been a different outcome if I was probing the standby switch.


          • #6
            Also, I will pull the tubes as you suggested and recheck the voltages.


            • #7
              I been zapped so many times I glow in the dark.

              What kills you is the path to ground. If you have one hand on the chassis, then probe with your other hand and get zapped, that produces a current path across the chest which can stop your heart. If you use just one hand, you can be zapped but its all a matter or how much resistance between you and ground there is. Standing on a dam floor with bare feet being the worst. If you wear sneakers and are standing while working you have the least amount of conductance to ground. Of course you can get it between one finger and another. It will smart like hell but unless you have a bad heart and react from the shock, its unlikely to kill you.

              I did have an electronics instructor who was leaning up against a bench while working on a Television flyback transformer. He said he had to skip having sex for a few weeks.


              • #8
                Did you check/replace the filter capacitor for the negative grid supply for the power tubes?
                "Isn't it a pity, isn't it a shame,
                how we break each other's hearts
                and cause each other pain"


                • #9

                  I don't know a whole lot about amp circuits. I think for the Belair that capacitor is C38. I will put it on the list of caps to replace next month when I get some time off work. Thanks for the suggestion.


                  • #10
                    Something less obvious you could check is the input jack. I'm not familiar with that amp, but many have a jack with a switch so when there is nothing plugged into it, the switch closes the circuit which eliminates any hum. On older equipment those jack-switches can get stuck open, but are easy to clean/fix once you've determined that's the problem.


                    • DeepEnd
                      DeepEnd commented
                      Editing a comment
                      Verne, this thread is nearly a year old. The OP is still active but I suspect he's either solved the problem or given up by now.

                  • #11

                    Thanks for the suggestion. It didn't solve the problem. The hum is still there.

                    DeepEnd is right, I've given up on this amp. The biggest issue now is some really excessive voltages. It has already burned up two 1958 RCA preamp tubes.

                    Anyway, the amp sits in the corner for now. Later this year I plan on converting it to a point-to-point fender style amp.


                    • #12
                      I have a 1986 Carvin X-100 B, that replaced a Marshall JCM 800.
                      The distortion channel is weaker than a than the Marshall, but put a pedal in front of the Carvin / dial in the pedal and the Carvin has more dynamics than the Marshall and just as vicious. Carvin's clean channel is as great as any legendary "Fender Clean " tones.
                      One bad thing about Carvin, they tend to treat you like an Orphan on his 18th birthday, when it comes to servicing your older amp.
                      I wish they would allow, us loyal owners of older Carvin amps, to send in our older amps to be checked up or serviced to old specs. They tend to ask you to have it locally serviced 😶, which I don't like because in my area, we have a lot of quack amp repair bozos, who don't do amp repair to old specs and do shoddy / questionable amp work, at best.
                      Last edited by AJ6stringsting; 07-21-2017, 02:03 PM.
                      How many guitarists does it take to screw in a lightbulb ? Five , one to screw it in , hit the switch and four to sit around bragging how much better they could have done it !!!! 😱👹😲


                      • RGx2
                        RGx2 commented
                        Editing a comment
                        You're right. The clean tone on the Belair is really good.

                    • #13
                      Carvin makes some great gear but for some odd reason resale value (if you need to) is horrible. Not sure why that is as they've been around forever and can stand toe-to-toe with the best of them.


                      • AJ6stringsting
                        AJ6stringsting commented
                        Editing a comment
                        Very true, my X-100 B was a replacement for my Marshall JCM 800, after I got the tone zeroed in, the JCM gathered dust.
                        You can find X-100 B amps all day on Reverb or eBay for under $ 500.00.

                    • #14
                      I only paid $150 for it. It was in excellent cosmetic condition, but buzzed really loud. I soldered in new plate resistors and that fixed most of the problems except of some hum that should be a little lower.

                      B+ voltages seem OK, but some voltages in the preamp section were 400+ volts. Way over spec. I checked and / or changed the resistors and capacitors in the preamp circuits with the high voltages. That didn't help.

                      I wanted to change out the tube sockets as well. However, they are soldered on both sides of the board and I don't know how to desolder those.

                      So, for $150 I've got 80% of the parts needed for a DIY amp build. I will make it point-to-point. Just haven't decided on which preamp circuit to clone.


                      • #15
                        Originally posted by Verne Andru View Post
                        Carvin makes some great gear but for some odd reason resale value (if you need to) is horrible. Not sure why that is as they've been around forever and can stand toe-to-toe with the best of them.
                        Which makes them good buys on the used market. I don't recall what I paid for my LB75, but it wasn't much.
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