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Sovtek Mig 100 U speaker output issues

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  • #31
    Good thing you got it all working correctly! Those are loud amps and they are very similar to the 2203 circuit for sure! I like the glued on grille cloth on the head...looks good!

    Greg

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    • #32
      Hi, I have a question about amp hum. When nothing is plugged into the Sovtek amp or I have the guitar plugged into the high gain jack, I get a pretty good hum. But, when I plugged the guitar into the low gain jack, it's quite as a mouse? I know that the low gain jack engaged bypasses a couple of power tubes, which in this case makes the hum go away? Would this mean that one or both of the two power tubes that the low power jack doesn't use might be going bad? If it was a filter capacitor starting to go then it would be happening with the low gain jack as well, which it's not. Just to let you know, I have the pre-amp volume all the way up for max distortion gain at low volumes, but considering it's a 100 watt amp, that's still pretty loud. The fact it's not doing the hum thang when connected to the low gain jack means something but what? Thanks

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      • #33
        Yes, plugging in the Normal input bypasses the first tube.
        Probably a shielding/grounding problem there, rather than a "bad part".

        An experienced Tech might solve it quickly, problem is finding the trouble and explaining what to do by remote control, such things are solved "hands on".
        Juan Manuel Fahey

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        • #34
          Plug into the high gain amp, and you get hum. Now turn the volume control on the guitar to zero. Does the hum remain or go away? If it goes away, your pickups are making the hum. let us say that is the case. Does backing away from the amp as far as possible reduce the hum? And does turning side to side - aiming the guitar different directions - make the hum increase or decrease? it is real common for guitars to pick up the magnetic field from the amp power transformer.

          The high gain input has a whole extra gain stage over the low gain. So it naturally amplifies any hum and noise that much more than the other channel.
          Education is what you're left with after you have forgotten what you have learned.

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          • #35
            The amp has this loud hum/hiss issue when nothing is plugged into the amp or is plugged into the high gain channel, same quality and level of hum/hiss there. It completely goes away when plugged into the low gain input and then sounds fine? So it's probable not EMI from the guitar pickups. Since the first power tube and high gain jack gets bypassed when the low gain jack is plugged in and the hum/hiss completely goes away, then what ever it is, has to do with that first power tube and or the high gain jack circuit, right? I've been researching everything I can find on the web about tube hum/hiss and there are a lot of different things that can cause it. Would you suggest after powering down to pull the first power tube out and then plug into the high gain jack, would the amp still work just like the low gain jack without damaging anything? I'm thinking either the first power tube starting to go or a grounding/shielding issue maybe with the first power tube socket wires after I repaired the output jacks? I tie wrapped the wires after the output jacks repair, maybe something related to that? Filament wire too close to a cathode wire or something like that?

            Hi Mr Fahey, I hear you with the hands on issue. If you want to learn to swim, you gotta get wet. haha. I love learning about electronics at this stage in my life and enjoy fixing any problems. Reminds me of working on a car yourself, kinda a pain at times but feels good when you fix the problem. Thanks for your input.

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            • #36
              Well I'm looking at the schematic and for the life of me I can't see how the first power tube gets bypassed by the low gain jack when engaged? It looks like the V1 pre-amp tube 12ax7 gets bypassed? I had read somewhere that the low gain jack bypassed a couple of power tubes but never followed the schematic to verify this back then. I mentioned that in a earlier post here which Fahey answered when he referred to "the first tube". Don't know if Fahey was referring to V1 or not, but it looks like it's the first pre-amp tube that gets bypassed by the low gain jack and the power tubes don't get by-passed at all? Would this be correct then? If so, that clears up a lot here as far as a diagnosis goes to isolate where to look.

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              • #37
                So it's probable not EMI from the guitar pickups.
                Don't find an excuse for NOT doing a test, just do it please
                At least to properly discard that possibility.

                FWIW the first stage adds 50X gain , so what's unbearable can become bearable and yet tell us nothing.

                That said, yes, you can have heater to cathode problems, (try a new 12AX7 there) , first stage poor grounding, bad shielding, poor wire layout, you name it.

                Let's go through all of them and keep discarding until, like Sherlock himself said: "once you discard all possibilities, the one remaining must be the right one, however improbable it might seem" .... or something like that
                Juan Manuel Fahey

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                • #38
                  How did you decide the low gain jack bypassed any power tubes? That is not the case. It bypasses the first triode gain stage in the preamp. If the amp has a loud hum with nothing plugged into the high gain, then start there, fix that first, worry what it does when plugged into later. yes, plugging into the low gain disconnects that extra triode stage. So your hum is coming from that stage. if not the tube, then perhaps a jack grounding issue, possible wiring dress issue. I am sure Juan referred to the first preamp tube, as he is familiar with this sort of circuit. Marshall did it decades ago. PV VTM did it decades ago.
                  Education is what you're left with after you have forgotten what you have learned.

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                  • #39
                    Yes, now I'm getting hip. The Sovtek is pretty close to the JCM 800 and like the JCM 800, the high input uses both gain stages of V1 (the first preamp tube) while the low input only uses the second stage. Marshall has used this circuit layout since late '76/early '77. The earlier ('75/'76) master volume amps used impedance matching for differing input sensitivity...So, the first input stage of V1 is the problem concerning the hum. Either the tube or the wiring or a resistor maybe? Cool

                    Hey Fahey, I believe I stated earlier that when the guitar is not plugged into the amp, the same hum/hiss is still present, that would make a test for EMI from the guitar pickup and or cord a moot issue at that juncture I would think. I'm playing a strat, so yea, there is going to be something noise wise coming from the single coil pickup on the high gain for sure, but you'd never be able to know how much until the V1 noise issue is resolved.
                    Last edited by Jesse Pearson; 09-29-2015, 05:33 PM.

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                    • #40
                      Hi Enzo, I posted before I was able to see your last post. So it's clear we're on the same page, great. Concerning the power tube misunderstanding, that came from me reading a post on another forum talking about the Sovtek Mig 100 watt amp and me not verifying what the guy was saying by looking at the Sovtek schematic. Fahey got me looking in the right direction by looking at the schematic and researching the JCM 800 2203 on the web more closely. Everything is getting a little clearer now for sure. Thanks guys

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                      • #41
                        Originally posted by Jesse Pearson View Post
                        I believe I stated earlier that when the guitar is not plugged into the amp, the same hum/hiss is still present, that would make a test for EMI from the guitar pickup and or cord a moot issue at that juncture I would think.

                        As to the EMI and hi gain input jack, it is still important to do the test with the guitar plugged in and guitar volume all the way down.
                        The jack will normally have a shorting switch so the input is grounded when nothing is plugged in. If that switch does not function, then it's like having a cord plugged in there with no guitar at the other end. Turn up the volume with an open cord and it will hum bad.
                        So please do the test.
                        Originally posted by Enzo
                        I have a sign in my shop that says, "Never think up reasons not to check something."


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                        • #42
                          Hey Fahey, I believe I stated earlier that when the guitar is not plugged into the amp, the same hum/hiss is still present, that would make a test for EMI from the guitar pickup and or cord a moot issue at that juncture I would think.
                          Again looking for excuses not to do it

                          Yes, I agree, it looks like a waste of time.

                          Yet, I have, many times, (same as others here) been bitten by that same approach.

                          The point is, that by doing such tests (or others in that vein), it MAY lead to a "duh !!!! " or "WTF" moment

                          Such as?
                          Such as, maybe, noticing that when you plug your guitar there for the "useless" test, say, that the jack nut is loose, the jack turns in its hole and when doing so loses grounding ... or... pulls some wire ... or ... chassis bends slightly and something else makes/breaks contact or .... I think you get it by now.
                          Sh*t happens and sometimes you find something you weren't exactly looking for.

                          Besides that, hum can not be same sound, intensity and quality whether a guitar is plugged or not , at least some difference must exist.

                          Ok, good luck

                          EDIT: just one Real World example, not "something read on a Forum" but which happened to me:

                          La Renga, one of Argentina's most popular Rock bands,
                          https://www.youtube.com/watch?v=d2a95fNQ7QE <-- free sample
                          was playing our World Cup River Plate stadium, 42000 ticket paying people there, 2 nights in a row (Friday/Saturday) and they had a miserable Friday: guitar had to be mixed low and dull because there was an unstoppable cheesy FM radio getting in , they couldn't find it and had to play anyway.
                          To boot it was not a constant interference but it came and went on its own evil will, or so it seemed.

                          On Saturday sound check they were desperate, rang me and sent a Limo to pick me up, to "see what I could do".
                          Well, I found it, and at the most unexpected place: in the huge pedalboard, one Boss Chorus or Flanger , the more modern type which has a plastic case, sprayed inside with some conductive paint for shielding.
                          The much abused jack had scratched a groove around itself in the plastic base, losing ground connection.

                          Invisible to plain sight, and , if plugged in the signal chain it would also fool the meter, since it would show continuity to ground after all ... only such ground was found downrange or uprange, and obviously allowing strong RF to get in.

                          But the main point is that they didn't care much about that pedal because "it was not used on most songs, it was off almost all the time, interference appeared with/without it " , so "it couldn't be responsible, why care?..... "
                          Ring a bell ?
                          Last edited by J M Fahey; 09-29-2015, 06:56 PM.
                          Juan Manuel Fahey

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                          • #43
                            Is this 60Hz or 120Hz hum? V1 has a regulated DC heater supply with full wave rectification, so it can generate both 60Hz and 120Hz problems when things go wrong with it. Would Sherlock have caught that before eliminating heaters as a possibility?

                            Sovtek's have a hiss on high gain, it's just the nature of the beast. You may be able to tame it a little, but it's always going to be there.

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                            • #44
                              Never think up reasons not to check something.

                              Standard advice in my shop. And especially if it takes but ten seconds to try.
                              Education is what you're left with after you have forgotten what you have learned.

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                              • #45
                                o. k. I just got back to the studio and ran the guitar cord test with the volume down. It's now acting like the expected standard EMI from the strat pickup, but not as loud as the other day? The only thing different is I'm using a different guitar cord. It's not bad Hum/Hiss with the guitar volume down all the way at typical stage volume settings on the amp with the high gain jack. Transient problem or just me freaking out, who knows. But Juan was right in the beginning about doing all the simple tests and not skipping steps based on assumptions from other indicators. I know you guys are seasoned pro's that's why I'm thankful for your help and the fact you guys share your knowledge. And as always, I learned some new things along the way. Right on
                                Last edited by Jesse Pearson; 09-29-2015, 11:34 PM.

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