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Ampeg VT-120 Restoration / Repair - hum on channel C

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  • #16
    Hi John,

    I tagged a 10UF / 450v across C9 in case this was the culprit.

    Unfortunately made no different to the hum at all.

    Open to any other suggestions.


    • #17
      No real change with guitar (early 70’s tele) plugged in and guitar volume increase
      Only the hum level with guitar volume at zero matters. The guitar is only used to short the amp's input via the vol pot.
      - Own Opinions Only -


      • #18
        So have spent a couple of hours doing some measuring and investigating to try and further narrow things down.

        Below are V1 and V2 voltages noting we are looking are the preamp. These are a little low in some areas but not sure if they’re low enough to be concerning.

        Also pin 2 of V2... should there be any DC voltage here at all?

        1 - 175vdc
        2 - 0vdc
        3 - 1.2vdc
        4 / 5 - -1.2mvdc
        5 - 2.35v
        6 - 174vdc
        7 - 0vdc
        8 - 1.23vdc
        9 - -1.3mvdc

        1 - 152vdc
        2 - 1.4vdc
        3 - 2.4vdc
        4 /5 - - 1.3mvdc
        6 - 147vdc
        7 - 0vdc
        8 - 0.98vdc
        9 - -1.3mvdc

        Plate voltage on pin 3 of power tubes - 488vdc

        Bias is interesting. The amp has two test points. The reading between Test Points 1 and Test Points 2 is 4.6vdc using my Fluke multimeter. It is my understanding that this should be ideally 0.08 to 0.12vdc

        However junction of R9 and R10 reads -47vdc which is in line with what the schematic says.

        Using a euro tubes style bias probe the plate voltage is the same as above but”cathode current” is 2.7mv, which seems to support the reading at R9 / R10 and is in the ballpark in terms of recommended bias.

        Seems the bias test points might not be accurate. The 1ohm resistors that were here were well and truly underrated and all burnt up, which scorched the board as well. I replaced with 2W 1ohm resistors and triple checked all traces for continuity and seemed to be ok. Will double check the work here.

        In terms of power tubes, I fitted a new matched quad of Ruby (Shuguang) EL34BHT - these are meant to be great for high voltage applications. Given I happened to have two matched quads of this type sitting around I tried both and hum remained. This amp came with EL34s fitted and according to the manual can handle 6L6 and EL34. So simply for interest also tried a matched quad of Ruby 6L6GC and this also made no difference to the hum. So I think safe to say no issue with the power tubes.

        Of note, the bias pot makes a significant difference to hum level, but probably not surprising. At the recommended bias setting hum is quite loud. At colder settings it is lower but still quite noticeable.


        • #19
          I’ll also add some key power supply voltages:

          C7 - 380v
          C8 - 298v

          These seem to be in spec according to voltages marked on the schematic. Makes me wonder why for V1 and V2 pins 1/6 why the voltages are 30-40v down on what the schematic says?


          • #20
            Here’s a link to a very short video to demonstrate the hum. The amp starts in channel C, then I briefly switch to channel B and then back to C. You can hear the very low hum on channel C. Not present on Channel B in the video as the volume for channel B is at zero - just to show with and without hum.

            Last edited by q9522678; 03-26-2020, 10:17 AM.


            • #21
              I am surprised.
              You say that if you remove V1 it stops and Gain C pot has no effect therefore either the hum is from a poor ground point on that pot or a leaky valve or a problem with the FET.
              Link R33 and if the hum stays it is either a bad ground point , a faulty valve or the FET/driver.
              Try lifting C22.
              That should isolate a few issues.
              Keep an organised mind and don't lose sight of the fault. Some people go around in circles and confuse themselves.
              Take notes if you need to.
              Good luck.

              Don't forget Gain A & B & C come from the same point!
              Last edited by Jon Snell; 03-26-2020, 09:19 AM. Reason: Addition of data
              Support for Fender, Marshall, Mesa, VOX and many more.


              • #22
                Thanks John,

                I’ll give these things a go and report back. I appreciate the thoughts and advice.

                I’m always confusing myself! I’m more musician than electronics whiz but generally know enough to get by and not kill myself. I just hate seeing these old amps put out to pasture and have had pretty good success over the past few years in getting these old things back to full functionality and use.

                A lot of the stuff I do is out of interest and curiosity regarding the inner functioning of the amps I restore as well. Makes me wish I studied electronics as a youngster! But enjoy the process nonetheless.
                Last edited by q9522678; 03-26-2020, 09:39 AM.


                • #23
                  Keep logical and you will fix it.
                  Best regards from the UK and keep safe and well.
                  Support for Fender, Marshall, Mesa, VOX and many more.


                  • #24
                    When he reported no hum on B and A it was because he had the masters for those at zero. When he advances the masters, THEN those other channels gains advance the hum.
                    Education is what you're left with after you have forgotten what you have learned.


                    • #25
                      Link pin 7 of V1 to ground and the hum should stop.
                      If it does, there is a problem with the grounds on the input jack sockets.
                      Support for Fender, Marshall, Mesa, VOX and many more.


                      • #26
                        Also pin 2 of V2... should there be any DC voltage here at all?
                        Should be around 2V. What's the input resistance of your meter?
                        - Own Opinions Only -


                        • #27
                          Did a bit more investigating this morning.

                          I jumpered R33 and the hum remained unchanged.

                          I tried lifting C22 and the hum remained unchanged.

                          I linked pin 7 of V1 to ground - hum remained unchanged.

                          Checked continuity of ground pin on all gain controls and they all looked good.

                          Also checked continuity of input jack sockets to ground and also appears good. I’d already reflowed solder on all of these connections just to be sure and hit all jacks and sockets with deoxit.

                          So after some head scratching based on my earlier advice to you with V1 removed hum dissappeared. Decided to try and remove V1 again and hum remained! I could have sworn it stopped when I removed it the other night...perhaps was suffering tube disorientation!

                          So then went ahead and removed V2 and hum definitely stopped when V2 was removed. Amp was pretty much silent other than some minor hum that you’d expect in a tube amp of this era. So perhaps something in / around V2?
                          Last edited by q9522678; 03-27-2020, 09:18 AM.


                          • #28
                            Hi Helmholtz - good to hear from you. I hope you’re looking after yourself through these crazy times.

                            Voltage on pin 2 of V2 is around 1.4v so reasonably close?


                            • #29
                              Voltage on pin 2 of V2 is around 1.4v so reasonably close?
                              Grid voltage at pin of V2 should be identical to the voltage between R50 and R51. If lower than that your meter loads down grid voltage. If identical it means a plate current of 0.64 mA. Consequently plate resistor R56 (100k) must drop around 64V. What is supply voltage at node D ?
                              - Own Opinions Only -


                              • #30
                                Hi Helmholtz,

                                Reading at pin 2 of V2 is 1.4vdc. The junction of f R51 and R52 measures 2.3vdv so perhaps the meter is loading down grid voltage noting there’s a 0.9v difference.

                                Supply voltage at node D is 293vdc.