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Thread: Sovtek Mig 100 U speaker output issues

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    Member Jesse Pearson's Avatar
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    Sovtek Mig 100 U speaker output issues

    Hi, I purchased a nonworking Sovtek Mig 100 U for $30.00 as a electronics project. I fixed the amp by finding a loose wire and re-soldering it. I ran the amp through a 15" 200 watt Bass cab when working on it and it was really singing through the 15" with a tube screamer pedal after it was fixed. I then replaced all the rusted nuts and washers on the input and output jacks (aluminum washers, all I could find) and now the 8 ohm and 16 ohm output jacks have lost most of their output volume when at full on, but not the 4 ohm output, which is fine through the 8 ohm 15" speaker? I checked for any visible loose connections and found none?. Both the tip and ring of the 4, 8,and 16 ohm output jacks all show zero ohms to chassis ground. This really has me confused. I kinda cranked down on the torque with the nuts at first and have since loosened them back up, don't know if that has something to do with it. I'm trying to exhaust all leads before I take the OT out of circuit to do the load test on. I have been unable to find a schematic for the Mig 100 U modal but have one for the Mig 100 H modal. Any suggestions would be appreciated...p.s. I also cranked the nut on the line out as well and all of the jacks seemed connected in series. I hope I did something stupid I haven't found yet that I can discover and fix. I hope the OT didn't just mess up some how. Thanks

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    Member Jesse Pearson's Avatar
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    Here's the schematic for the Sovtek Mig 100 H
    Edit: I guess I can't load a pdf here? Let me see what I can come up with?

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    Last edited by Jesse Pearson; 08-22-2015 at 07:14 PM.

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    Lifetime Member Enzo's Avatar
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    The secondary (output side) of the output transformer is relatively few windings of heavy wire, and so it has a VERY low resistance for your meter to try and measure. All your meter can do is verify it is not open. It is unlikely the transformer is bad, though possible.

    Since the 4 ohm works, run a signal through the amp, and measure it with your AC voltmeter across the speaker. Now ground the meter, and measure AC voltage at the hot terminal of the speaker just to verify you get the same reading. Now measure at the 16 ohm jack tip contact. Is the voltage there or missing? If it is there, then try a clip lead from the ground terminal of teh 4 ohm jack to the 16 ohm ground terminal.

    Do the transformer wires solder directly to the jacks? Or are ther any connectors or solder junctions in between?

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    Supporting Member Jazz P Bass's Avatar
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    Quote Originally Posted by Jesse Pearson View Post
    Here's the schematic for the Sovtek Mig 100 H
    Attachment 35367
    This link is bad.

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    Member Jesse Pearson's Avatar
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    Thanks J P Bass...I tried to load a pdf. Working on it.

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    Is the schematic attached the one you were looking at?
    The problem seems to be related to the work you did on the jacks, probably related to grounding. The tests & jumpers Enzo mentioned should get it sorted out.
    Attached Files Attached Files

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    Old Timer Tom Phillips's Avatar
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    Quote Originally Posted by Jesse Pearson View Post
    ...Edit: I guess I can't load a pdf here? Let me see what I can come up with?
    The forum software does support .pdf attachments. It will work for you once you figure out the method. You will find FAQs on the subject at Reading and Posting Messages

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    Old Timer Tom Phillips's Avatar
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    We know that the output jacks are grounded as shown in the schematic. However, the schematic doesn't tell you how and where the ground connection is made. The speaker jacks on the Mig 100's that I have seen are plastic jacks. Are yours plastic jacks? If so they don't make the ground connection through the chassis mounting nut. I suggest that you trace the ground wires from the jacks and check the connections. You can also check continuity to the chassis and the speaker as has already been mentioned.

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    Member Jesse Pearson's Avatar
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    G1 thanks for posting the pdf. There is no schematic for the Sovtek 100 U on the web, so I'm in the process of mapping out my own. The Sovtek Mig 100 H looks real close. I think the difference between the two from what I've read is the U modal is a cathode follower and the modal 100 H is plate driven and has more gain. Still trying to understand what that exactly means however, haha. I'll do the tests that Enzo suggested and post the results when I get done. Thanks

    Tom, everything jack wise is metal and seems connected to chassis ground with the ohm meter. I don't understand how both the tip and ring on the speaker outs are showing zero ohms to ground? Could someone explain this to me please, is this right or wrong?

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    Last edited by Jesse Pearson; 08-22-2015 at 07:55 PM.

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    The cathode follower vs plate driven thing is in regard to how the tone stack is driven.

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    Quote Originally Posted by Jesse Pearson View Post
    I don't understand how both the tip and ring on the speaker outs are showing zero ohms to ground? Could someone explain this to me please, is this right or wrong?
    It's ok, the low resitance of the OT winding is what you are measuring. (see post #3 from Enzo).
    What you are calling "ring" is I think the "sleeve". Like Tom mentioned it is important to know if it has a grounding wire, or is just relying on contact with the chassis (if it is plastic type it will need a ground wire).

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    Member Jesse Pearson's Avatar
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    All the output jacks are metal. A resister is connected to a ground point on the chassis that then goes to one of the line in jack points and then is connected to the sleeve of the line in jack by way of another resister from that point. The black ground wire starts here at the line in jack sleeve and goes to the 16 ohm jack sleeve, then to the 8 ohm jack sleeve and then to the 4 ohm jack sleeve. From there it goes to the power caps/rectifier board to a ground point. I removed all the jacks and reinstalled them again and now the 4 ohm jack has very low volume like the other two jacks? The light bulb current limiter I have the amp going through shines the brightest with the 8 ohm output jack when things are turned up. I poked everything in the amp including the tubes with a chop stick over and over again but there was no change in the sound out put. I was unable to read any a/c or amps from the tips to ground on the jacks? I need a good sleep over this stuff and will pick up after work tomorrow. Thanks

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    Last edited by Jesse Pearson; 08-23-2015 at 07:04 AM.

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    First try at this. If all goes well, there should be an attachment with a schematic that may be of use to you.
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

Name:	Sovtek Mig 100.jpg 
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    I'm wondering if cranking down on those nuts damaged the jacks. Disconnect the wire to the tip on each, then check resistance from tip to sleeve of each jack.

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    The metal Line Out and Speaker Jacks are all insulated from the chassis with plastic shoulder washers. The OT secondary common wire connects to the Line Out sleeve then a wire goes to the 16 ohm jack sleeve, then 8 ohm, then 4 ohm, then to pin 1, pin 1', or pin 1''. Pins 1, 1', and 1'' are all at the same ground point, but it is not connected to the chassis at this point.

    The resistor from the Line Out tip does not go to the chassis. That little connector is insulated from the chassis and a wire hooks the resistor to the 16 ohm tip.

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    Member Jesse Pearson's Avatar
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    Alright, I've finally been able to replicate something in my testing. I took the 8 ohm jack out of it's chassis hole and insulated it in a plastic bag and put the 16 ohm jack and the line out jack in their own insulating plastic bag. I noted that the plastic sleeve on the 8 ohm jack had failed and was cut around its base and was probable allowing the jack to ground to the chassis which was messing up the 4 ohm jack and sucking it's volume down? With only the 4 ohm jack connected and the other jacks in plastic insulating bags, the amp is now sounding like it's 100 watts of volume again. However, both the 8 ohm and 16 ohm jacks when tested in isolation like the 4 ohm jack was, are still producing low volume, which when turning up the volume all the way produces bad hum and kinda crazy distortion as well. I am going to unsolder the 8 ohm, 16 ohm and line out jacks and test for any issues. Jack replacements with insulating plastic washers are probable what the doctor would order. I'm wondering if the OT might have received some kind of damage to it's 8 ohm and 16 ohm winding's from the grounding to the chassis issue or even the phase inverter tube? So the moral of the story for the day is, don't over tighten the nuts on your jacks. I'll let your know what happens when I track down the right replacement parts. Thanks everyone for your help. 66 Kicks, thanks for the schematic.

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    Lifetime Member Enzo's Avatar
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    Try this, unsolder all those speaker jacks. Protect all the bare wire ends. Use clip wires and connect the common and 4 ohm tap wires to a speaker. You don't need any line out jacks or NFB. Does that sound OK? Now clip the speaker to the 8 ohm tap wire and common, sound OK? Now 16 ohm tap and common, OK? That will tell you if the signal is coming out the transformer or not.

    it is way more likely you damaged the jacks than the transformer. The transformer is a hunk of iron with heavy copper wire wrapped around it. it is pretty tough.

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    You shouldn't try playing through the light bulb current limiter. That will limit the output power of the amp and the more volume you try for the more it will limit it. If nothing is shorting out and the amp isn't drawing a lot of current at idle, then you can disconnect it from the current limiter and run it through a speaker and the amp will go to its full power potential since all the voltages will be what they were designed to be.

    I have a MIG 100 U here that I fixed several years ago. I decided it sound shitty though once I had it all fixed and I gutted it, kept the PT and got a Heyboer choke and OT for a Vox AC100 and made the amp into a clone but added a second higher gain channel to it also. I still have the original choke, OT, and board here if you run into any problems I can take a look at the board and parts, and I am into selling what I have also if you end up needing the parts. The MIG 100 H is very similar and both amps are similar to Marshall designs.

    If I recall correctly, the original jacks were plastic jacks like what Marshall uses, but they had the 'metal' looking screw on the front of them. They are not like a switchcraft jack however. I would have to go look into the box to be sure but I can do that tomorrow if you want. Any jack with a plastic 'sleeve' as you mention is not going to be a metal jack like a Switchcraft.

    I am with Enzo here.....it is very unlikely that you blew out the OT. Make sure to run the amp with a load and you should be just fine. Despite many shortcomings in those amps, the Russian OT's are pretty overbuilt.

    Greg

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    Old Timer J M Fahey's Avatar
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    As a side note, when you damage an OT because of lack of connection to a speaker, the insulation breakdown/internal short happens in the primary , where hundreds and even over a thousand volts can be found (specially if unloaded), never on the secondary where we are talking tens of Volts.

    So if primary were damaged, all output taps would sound ugly, but since at least the 4 ohms one does not, then the OT is fine.
    Also suspect poor jacks wiring.

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    One or more of those jacks is damaged. If they were all fine, the insulator breakage would only cause a ground loop/hum issue.
    You will probably find one (or more) of them has resistance between tip and sleeve.
    Replacing them with plastic type should solve your issues. From the sounds of 66Kicks post, that's the type they would have been originally anyway.

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    Old Timer Tom Phillips's Avatar
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    Jesse,

    Are the speaker out jacks on your MIG 100U specifically labeled 4, 8 and 16 Ohms?

    I ask because I found the attached photo which shows a 100U model that had two of the three speaker jacks connected to the 4 Ohm tap and the third jack connected to the 8 Ohm tap. (I didn't date code the amp but I took the photo in 2000 when the MIG-100U passed through the shop so I know the amp was manufactured in 2000 or earlier.)
    Click image for larger version. 

Name:	MIG-100U_04A.JPG 
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    No one has come up with a 100U schematic yet but I found the attached schematic which shows a version of a MIG 100* that used an OT with just a 4 and 8 Ohm secondary. The schematic file is missing the right edge so we can't see the jack wiring or the complete model designation in the title block.
    This information may not solve your specific problem but it may help clarify the wiring if it does turn out that you don't have a 16 Ohm tap.
    Otherwise, it's just some interesting historical information that reinforces the fact that musical instrument amp manufacturers are not strict when it comes to configuration control.
    Sovtek-Mig100-schematic Partial.pdf

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    Last edited by Tom Phillips; 08-25-2015 at 12:13 AM.

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    Quote Originally Posted by g1 View Post
    Replacing them with plastic type should solve your issues. From the sounds of 66Kicks post, that's the type they would have been originally anyway.
    I didn't mean to imply that they had plastic Cliff Jacks originally. Every stock Mig 100 U that I've seen had those rough metal Switchcraft knock-offs. They had imitations of Switchcraft's #11, #12A, and #12B in those amps and they would use any one of those when they needed a #11. They had a different solder lug order, which made it easy to connect to the wrong lug if you're used to Switchcraft. It was also easy to spin the wafers when backing them up with a screwdriver between the lugs.

    I've only been shocked twice in 45 years of working with tube amps, and the second time was by a Mig 100 U. They had no bleeders on the first set of filter caps and they used a DPDT switch for standby so that the unused lugs were hot when off. I brushed my finger across an unused lug while touching the chassis with my other hand.

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    Quote Originally Posted by 66 Kicks View Post
    I didn't mean to imply that they had plastic Cliff Jacks originally.
    Sorry, I meant to say that you had mentioned they were all insulated.
    I should have said the jacks should be replaced either with metal types and insulating shoulder washers, or plastic types.
    By the way, welcome to the forum, and thanks for the input.

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    Here is a schematic that came with a Mig 100 U.
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

Name:	%22SOVTEK%22 MIG 100 LEAD TUBE AMP #2.jpg 
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    Member Jesse Pearson's Avatar
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    Hey fellas, I believe I finally fixed the amp. Turns out the shoulder washers that were insulating the output jacks had deteriorated so much that they pulled apart and or just fell off on one side exposing the the jacks to the chassis when I replaced the rusted washers and nuts on the output jacks (I had no idea that's what they were used for). This created a ground loop I believe and caused the huge volume drop. After I un-soldered all the jack ground lines and tested the jacks individually, I decided to just connect the 4 ohm jack (in a plastic bag) ground line to the line input jack sleeve where the yellow OT ground lead was connected and which is where the black ground line from all the jacks had been connected, leaving the 8 ohm and 16 ohm grounds unconnected. That worked so I added the 8 ohm jack (in a plastic bag) ground line and that worked, so I then added the 16 ohm (in a plastic bag) ground line and everything worked just fine, WTH? I thought I had already done a plastic bag isolation test before, but maybe the 4 ohm was still connected to the chassis because that was kinda working. I also noted that the NFB was connected to the 16 ohm jack, which I would think was probable done to give more headroom and a more dynamic feel like Marshall used to do in the early 60" with their NFB? I think the NFB is normally connected to the 8 ohm jack for most new tube amps anymore isn't it? I just ordered 8 packs of shoulder washer pairs for around $10.00 from "Studio Sound Electronics". The OT must be o.k. (right on with the right on's)! So I gotta say I learned a bunch from all this which is what it's all about. Hopefully when the isolating washers come in and the amp is all put back together it will still work the same as the last test (I'm ready for some serious pay back at my rehearsal studio concerning the death metal guys) haha. Thanks for the help guys, appreciate it...P.S. 66 kicks, that's a great find.

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    Last edited by Jesse Pearson; 08-25-2015 at 08:40 PM.

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    Lifetime Member Enzo's Avatar
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    The NFB is part of the circuit, not a thing on its own, so on any given amp design, it is tapped off wherever they wanted it. it is nothing more than the output signal, sampled and sent back to an earlier stage. The higher the tap impedance, the larger the signal voltage sample. But that feeds into the PI or nearby stage, and other resistors are involved to form a voltage divider, so the tapped voltage and that other circuit combine for a total NFB level. 8 ohm or 16 ohm can have the same final result depending on the resistor values back in the PI. Don't read more into it than that.

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    Member Jesse Pearson's Avatar
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    Thanks Enzo, I didn't even know what the heck the NFB was until I started looking it up after reading your previous post. The OT secondary winding's on the Sovtek Mig 100 U is as follows: 4 ohm Red, 8 ohm white, 16 ohm blue, line out Yellow ground, #1 5881 pin 3 small wire purple, #3 5881 pin 3 small wire yellow. Is the line out yellow big wire the OT ground wire?

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    Last edited by Jesse Pearson; 08-25-2015 at 08:18 PM.

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    I'm a little confused about the last post & post #25, but I think when you say the yellow is going to the line out jack, you mean it's going to the sleeve of the line out jack, and carries on through to the circuit board ground. Is this correct?
    If so, then yes, the big yellow is the OT common (ground) wire.
    Saying "line out, yellow" gives the impression the yellow is going to the line out tip, which I don't believe is the case.

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    Member Jesse Pearson's Avatar
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    Cool G1, thanks for that clarification. The big yellow OT ground wire is indeed connected to the line out jack sleeve and continues to all the other speaker output jack sleeve's via a black ground wire till it reaches the power section pc board common ground point. It really pays to learn about grounds/common/reference points when trying to diagnose a problem.

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    Member Jesse Pearson's Avatar
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    O.K. The amp is up and running. I installed extra long insulated output jacks and that took care of the last of my problems. I bought an empty 2x12 speaker cab off of Craigslist for $40.00 and rebuilt it and loaded it with Celestion 16 ohm Vintage 60 - 12" speakers wired for 120 watts. The speaker cab turned out to be an old Fender Super Sonic 60 ext cab. I used old vintage Marshall Salt and Pepper grill cloth on the cab and glued some extra on the amp head to match. I'm pretty impressed with this amp. I can get that Robin Trower "Bridge of Sighs" sound without any pedals hooked up. And did I mention loud, holy moly! These amps are great amps. I think the "Sovtek Mig 100 U" modal stands for "unmodded" clone of the Marshall JCM 800 2203. Here are some pics ...
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Name:	Sovtek from side close #3.jpg 
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Name:	Sovtek with Strat.jpg 
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    Last edited by Jesse Pearson; 09-25-2015 at 04:33 AM.

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    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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    Member Jesse Pearson's Avatar
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    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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    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".

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    Juan Manuel Fahey

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    Lifetime Member Enzo's Avatar
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    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.

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    Education is what you're left with after you have forgotten what you have learned.

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    Member Jesse Pearson's Avatar
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    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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