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

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  • #46
    No problems. Appreciate the suggestion! And I learned a bit about JFETs in the process!


    • #47
      JFETS are used as switches in this application and are either on or off. If you use your DMM on Diode test, when powered off they should read open circuit Source to Drain. Don't let them confuse you!
      John - I have the board out again at the moment and though I would test the JFETs (J176s) in the circuit just out of curiosity. I have my DMM set to diode test and am placing probes Source to drain, but none of the JFETs on the board are reading open circuit. Most read 0.05v.. I’m guessing doing something wrong.. does this test work in circuit?


      • #48
        Jfets are 'on' til you turn them off. So a low resistance reading from source to drain unpowered is normal.
        I think John just made a typo.
        "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


        • #49
          That's not how JFETs work. SOurce to drain will be a lowish resistance, UNLESS you turn it off with a gate voltage. What resistance? I don't know, I usually expect something like 50-200 ohms. With a gate voltage they go very high resistance, effectively off.

          On diode test 50 ohms or 100 ohms would indeed read like a very low voltage. But flip over to resistance.

          Remember, it takes voltage to turn them OFF, so unpowered, NONE of them would measure open. Unpowered, your JFETs should measure some low resistance
          Education is what you're left with after you have forgotten what you have learned.


          • #50
            Spent some time doing some tube experimentation.

            If I stick a 12AU7 in V2 the hum drops to bearable levels. The hum is still there. Just no where near as noticeable. With a 12AX7 in V2 the hum is a lot louder. Unsurprising I guess given the much lower gain factor of the 12AU7 - so isn’t amplifying the hum as much as a 12AX7. Also use surprisingly the 12AU7 drops the entire volume of the amp around 30%.

            I tried a 12AU7 in V1 and 12AX7 in V2?but hum was still very loud. So have no doubt the hum is emanating from around V2. As tested previously - pulling pin 2 of V2 made no change to hum, so don’t believe there’s any issues there. I lifted and tested a lot of components around pin 3 of V2 and couldn’t find anything.

            So I guess next step is looking more closely at components off pins 6,7 and 8 of V2?

            I suppose I could leave the 12AU7 as a work around and just live with it..


            • #51
              Keep it simple and try not to look too deep.
              Connect a link across R41 (pin 7 to ground). Does the hum stop? If yes, the hum is coming in from there, if not the hum is probably caused by a bad ground for R41 or there is an issue with pin 8. Link pin 7 to the actual chassis at the main smoothing capacitor negative point.
              What happens?
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              • #52
                Checked R41 connections. All seemed fine.

                Linking pin 7 to smoothing capacitor made hum louder - like it was humming in unison.

                Checked / inspected all through pin 8 components and then went back through compoents off pins 2, 3, 7 lifting a whole bunch of components of V2 with no luck. Hum seems to persist and can’t seem to narrow it down to a single component. Starting to think it is “normal” as part of the design.

                I note the damping switch on the back - it makes the hum notably worse when pushed it.. When running it at half power triode mode the hum is not too bad. At full power / pentode mode it’s louder, as would be expected. At full crank in this mode - lots of hum, even with humbucker guitar. Then again it is a 120w high gain amp, so maybe I’m expecting too much.

                Will take a break for a day or so. Do some thinking and reading. Contemplate taking it to a local tech - but if hum is “normal” not sure what else they will do except perhaps mess with the grounding scheme.

                Have certainly appreciated the help and advice to date.


                • #53
                  At this point I would say without a doubt, it is not a faulty resistor or capacitor. It is either a ground in the wrong place, a ground that has lifted somehow or a dodgy valve.
                  Do you have less than 120R between either side of the heater pins and ground and is the value equal?
                  Last edited by Jon Snell; 03-30-2020, 08:39 AM. Reason: Addition of a question
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                  • #54

                    No - not less than 120R. Both measuring 156R


                    • #55
                      From a 200R pot and 100R resistor, that sounds about right.
                      Have you tried adjusting P13 the heater ripple balance control?
                      Attached Files
                      Last edited by Jon Snell; 03-31-2020, 09:22 AM. Reason: Partial schematic added for clarity
                      Support for Fender, Marshall, Mesa, VOX and many more.


                      • #56
                        Hi John

                        Yeah - I’ve played with the hum balance pot. At its current setting it is at the “quietest” I can get it, which is roughly the centre of the pot - but is still loud. At either extreme it hums a lot more though. As an aside - this whole pot was missing when I got the amp. So replaced it with a 200r equivalent and seems to work ok. Just doesn’t dial all of the hum out.

                        Spent today testing every ground I could find on the schematic and all checked out ok. Couldn’t find anything that had lifted. Also kept an eye out for any solder bridges or trace issues while I was at it but all looks pretty good.

                        Tried a bunch a different tubes again thinking I might get lucky, but to no avail.


                        • #57
                          Have you thought about removing the ground side of R11 and putting it at a positive potential, something like 50volts DC above ground? Use a 220k and 47k resistor as a potential divider then place a 47u 63v cap across the 47k to ground.
                          If you have a problem with one of the 6L6s leaking, that may stop the leak or swap out the 6l6s.
                          Without working on it, I don't know what else to suggest.

                          Keep safe and healthy in this difficult time.
                          Last edited by Jon Snell; 03-31-2020, 05:44 PM. Reason: Just an idea
                          Support for Fender, Marshall, Mesa, VOX and many more.


                          • #58

                            I sincerely appreciate your time and expertise. I’ve tried three different sets of matched quads in the power section. A set of 6L6 and a two sets of EL34 - all brand new. So pretty confident it’s not emanating from the power tubes.

                            I’ll give it a little more time, otherwise will just bite the bullet and take it to a local tech to see what they might come up with.

                            You stay safe too!


                            • #59
                              I’ve bitten the bullet and taken it in to a very experienced local amp tech who I have used a few time over past couple of decades. I’ll report back what he comes up with. In talking it through with him at drop off, he suspected it may simply be the design of the amp, but was going to tear it down and take a look anyway.

                              I’m happy either way. This amp was dead and completely non functional when I got it, with a multitude of issues. The fact that she is back to life is a great outcome, and as I said at the start of the thread - I can live with him if need be. Will simply use channel b and dial back the hum using the master volume.


                              • #60
                                Just a quick update for anyone interested. Just took a call from my amp tech and he’s opting out of the repair. Said he’s spent hours on it and can’t figure out what’s happening and where the hum is coming from. Recommended I take it to the other very experienced and reputed amp tech in town.

                                On the one hand makes me feel better that even a very experienced and well regarded tech couldn’t fix it, but on the other hand frustrated that it remains unresolved. Taking it into the other guy next week. Will see how that goes.