Magnatone 280A (Noble 990)......in Gorgeous Original Condition

well greetings out there in YouTube land and welcome to today's new video which will feature the contents of this box and to be honest with you I have no idea what's in there so let's get it open and see what today's challenge will be as I'm unpacking it I wanted to congratulate the person who wrapped it up very unusual method of the Box body here is complete it completely wraps around a solid double weight cardboard with their removable end and the chassis is safely nestled inside this is a really nice packing job and the best part for me is I'll get to reuse it when I send the amp back to the owner another pleasant surprise the amp owner included couple packs of Kitty treats for Jack and Casey and Ollie and I know since they actually do most of the work around here that they're gonna work extra hard on whatever the contents are this box let me take a second here to address all the budding amp techs out there when you accept shipped amps and they come in and really lousy boxes all torn to pieces and poorly packaged prepare to spend about as much time repacking and return shipping that amp as you spend fixing it okay it's sort of a hidden expense that you might not expect but believe me it's a considerable one okay if you spend an hour to finding a box tape packing the amp all up another 30 or 40 minutes taking that box to FedEx believe me is something you need to factor in on the price that you charge for shipped amp repairs well here's the chassis that was lurking in the box and it is a great big heavy one in beautiful clean condition it is a noble symphonic model 990 anyone looking at this schematic will immediately recognize the symbols for varistors and will know that this is actually a magnet own amplifier and I believe it's a 2 a-okay the 280 used a rather unusual output tube called a six cz5 whereas the 280 a used the 69 73 output tube and as I looked in at the tubes it appears that we have some 69 73 s so I'm guessing that this then is actually a magnet own 280 a which was the absolute top of the mark around the late 1950s it was the best amp that magnets own made so this will not only be an honor to work on but looking at how complex the circuit is I imagine it's going to be somewhat of a challenge okay but that's always the best kind as long as the challenge is overcome and ends up with a good result I don't have a note from the owner telling me what specific things to look for so we'll have to contact them I will say this much when you're sending tubes and you're numbering them to match a tooth chart like that don't put the number on a piece of tape that covers over the amp or X logo because when I pull this tape off almost all of that logo is going to come with it if you're going to label them label them on the packing material on the outside here not on tape on the tube okay because those type of trademarks are really precious and we really need to preserve it looks like the owner also sent some really nice pinup photos here for Jack and Casey and Ollie of the owners Kitty actually two separate kitties okay so we really appreciate the photographs of the furry little rascals and also some pictures of the part of the amp that was not sent showing the super nice I believe Oxford speakers the top of the chassis with all the tubes installed I don't know if he has the tube shields I'll have to check it's good if you go to them to use them and also a picture here showing the speaker compliment with the 2 5 inch tweeters up above now this is a true stereo amp in that the output is completely separate it can be separated too from this pair of speakers and this pair of speakers right here kind of a woofer mid-range and tweeter so it is the ultra deluxe according to magnet own it was suitable for just about every instrument you could think of including accordion which is who I think noble badged these for the only thing I see about it that sends some shivers up my spine is this hideous black tape that has been put on here I guess to show the ideal settings for the knobs boy there must be a better way to do it and I've seen this on other magnet songs to be honest with you I think I bid on this amp about a year or year and a half ago on eBay because I remember thinking it was a magnet tone 280 and a lot of people don't put the name Noble together with magnet own so sometimes you didn't get a pretty good deal on these but I remember that hideous black tape and a wondering if there was going to be any good way to remove the sort of the vestiges of it because god what an eyesore huh but otherwise just flawless and gorgeous plus the extra knob came along in the box with the tubes so let me go in and contact the owner see what it is about this this not ideal in its performance and then we can get started I couldn't resist flipping the chassis over and as you can see this is a very densely populated complex circuit with all sorts of unusual rather large capacitors here all of them strapped with resistors to distribute the voltage evenly amongst them all fender fans will recognize the blue molded capacitors which have a really good reputation we see the selenium rectifier right here with a broken plate which can't be good but it gives us a chance to snort some of that selenium all in all it's as I said complex but unmolested okay and you couldn't ask for better condition I don't think this amps ever been worked on doesn't appear to be if so they were very stealthy about it alrighty let me go find out what's wrong with it well I can see why one of the knobs was not installed on me in front of the control panel because the little set screws been lost let me dig through my parts drawer here or for set screws and see what I can find but fortunately I was able to find the perfect set screw for the knob so let's reinstall it back here on the speaker selection pot post well while we're waiting to hear back from the owner about exactly what he wants done with this amp let's replace the old original to Chrome power cord let's remove the deathcap and let's replace the selenium rectifier well we've got our three water power cord all installed with the black hot wire going first to the on/off switch then to the fuse and then into the primary of the power transformer the white neutral wire goes here to this newly installed terminal strip and to the other primary wire from the transformer and the green chassis grounding wire is soldered here to a soldered log on the chassis now let's focus on this broken selenium rectifier I have a feeling that happened in transport but regardless it has to go so let's here's the selenium rectifier on schematic using a diode symbol you notice that the bar is toward the transformer well here is the output from the transformer so the band corresponds to the bar okay so we put it in this configuration I've added another terminal strip to make sure nothing's loose and flopping around and make it look original and here is the output from the that winding for our negative DC bias we've got a resistor and we've got a filter cap here so let's see if we've got it wired right I've got the amp plugged into the current limiter I flip on the switch and we see we have about negative 34 volts DC it's always good to check that it's negative DC to be sure you haven't put the diode in backwards because if you run positive DC to the grids of your output tubes you will be kissing them goodbye okay so it looks like everything's working just fine alright I talked with the owner and asked him what his expectations were with this he said that he really wants it to stay as original as possible and to only replace the components that had failed or were failing so it looks like the vast majority of capacitors resistors and other components are going to remain intact but he did say that one of the channels was dead that he was getting absolutely no output believed from channel 2 so let's go ahead and plug in some speakers and see if that's true okay I've got a nicely matched pair of vintage 12-inch Jensen speakers wired up to the cabinet speaker wires these are the wires that are going to go to those two 12 inch and 2 5 inch internal speakers in the magnet own cabinet I've got a 50 cycle tone here on my signal generator so let's turn up the volume [Music] speakers working fine this one's stone dead so it looks like what he said is exactly true let's start checking out all the wiring and see if we can figure out the cause okay I've drawn up a really simple and rather crude diagram here of the speaker connections see here that these are the secondaries of the two output transformers going to channel one hardwire the neutral wire to ground same here then we're gonna send this one up to the contact here the leaf and the ground contact is going to go to the right internal speaker now the reason for that is that if you plug in an external speaker you will lift the leaf off of that grounding leaf disconnect the internal speaker and then only the external speaker will work it's the same on both sides so simply by plugging in your external speakers you're disconnecting the internal speakers but this switch right here can override that and if you close that switch you see that you are bypassing that ground leaf that was disconnected and closing it so that both your internal and external speakers will work so depending on the position of that switch this one right here you can either have external speakers or internal speakers or both however with all that said I can find nothing wrong with any of the contacts to switch anything if I connect directly to the terminal thus the speaker works fine if I trust the system the speaker does not work properly so I'm gonna have to start looking at the wiring and see if I can figure out there's a bad contact or wire that might be broken within its insulation okay we see here that we have two output transformers here is the B+ going in here is the connection to the two plates of the two output tubes so there is the primary and here is the secondary coming out green then will be the amplified signal and black will be the neutral or ground wire you see that the black comes over here and has grounded to this lug whereas this green comes over here to the left-hand terminal we look here at the second output transformer same exact situation we have B+ here here are the twisted leads over to the plates of the second pair of output tubes so on this in this case we have our black coming here to ground just like we did in the first output transformer and the second lead that comes out will be this one back here which is green and comes around to this terminal so we have the output from our two output transformers coming to this terminal for this one and this terminal for the second one well after eliminating about 400 different variables it turns out that one of the black ground wires for the channel that was dead was not properly soldered here it was held in place but it wasn't making good contact okay so now I'm gonna strip these and solder them properly to that ground lug and I think both of our speaker is gonna work just fine it's time to see if our repairs have worked remember I had to resolder the channel to ground wire for the speakers the speakers are now connected to the internal or cabinet speaker wires from the amplifier circuit we have the 50 cycle tone being piped in let's turn it on and see if both speakers now have equal volume [Music] sounds pretty equal to me I would say that our problem [Applause] [Music] [Applause] [Music] [Applause] [Music] [Applause] [Music] okay I'll flip the standby switch off you notice we're starting to get a little bit of illumination back here in the current limiter I'm going to guess that that's normal because this is a pretty husky amp circuit but we'll definitely keep an eye on that just for the fun of it let's see how much current this thing does draw I'm using my ammeter here I've got the amp plugged into it and it's plugged in to the current limiter we're on power on that's also in standby mode so were drawing very little current here it's just to heat up the filaments now when I go to full operation I'm using what 1.25 amps okay so really that's a small draw for a amp like this with four output tubes 69 70 I can also equate that with that much glow in the bulb so the fact that the bulb almost never illuminates tells me that most of the amps that I'm working on or drawing like 1 ampere or less current so really then I don't think the draw is very significant for we continue let's stop for a second and take a look at the tube set that came with this amplifier and this really is a wonderful tube set 12ax7 s our amp per X the rest of the tubes are either GE s or the 69 73 s are all a matching set of RCA s so let's hope that these are all good tubes well as predicted both triodes of the 3 12ax7 s and the 3 6 c g7 s all checked out just fine now that the amp is working I wanted to check the plate dissipation on the 69 73 outlet tubes they seemed rather warm to me and sure enough as you can see I have measured the resistance of the half windings of both of the output transformers remember this is a true stereo amp to separate output transformers you see the voltage drop across the resistance and then I've calculated the plate current and plate dissipation now the maximum for the 69 73 is 12 watts we know since this is fixed bias we go with 70% of that so we should be a little below eight point four so with that in mind we'll see here what the plate dissipation is of our four output tubes high eleven point six eight a perfect match but still way too high over what three point two wants this one is the only one that's even close to being acceptable and it's one and a half over and look at this beast you could fry eggs on that one so we're going to have to increase the amount of negative DC that we apply to the grids of these tubes to try to bring these values down now one thing that's kind of unusual is that the amount of negative DC bias being applied to the griz right now is in agreement with the schematic now all I can think of is that they didn't observe the 70 percent rule back in the early 60s if the tubes were biased anywhere near acceptable around 12 watts and you can see they are 11 6 8 low and that was a little high but if you're going to accept 12 watts as your plate dissipation then this is set fine but we know from bitter experience that you've got to reduce a plate dissipation with fixed bias circuits okay so I'm going to then follow the modern thinking and reduce the plate dissipation in all four of the output tubes now we've already overhauled the negative DC bias circuit we see here is our diode that wreck to finds it we come over here this is what distributes it this green wire distributes it to the grids but we see back here our 47 K resistor to ground now what we need to do is increase its value so more of the negative DC goes to the grids and less goes to ground now to adjust the bias of the output tubes I remove that 47 K resistor to ground from the negative DC bias apply I've replaced it with a 100 K caught which will allow me to dial in exactly the resistance needed to get the plate dissipation values under the eight point four want a limit that we've set for them then once I have found what that exact resistance is I will install a fixed bias resistor in its place before I continue with the biasing procedure I wanted to mention some steps we can take when we have for out the tubes to better balance them we see here on the left and as previously mentioned that the matching as far as plate dissipation is perfect words on the right side it is very imperfect we have one tube that's operating at about nine point nine four watts and the other is at thirteen point four seven difference of around three and a half wants okay the culprit here is it depends on how you look at it either a tube with a rather weak cathode or tube with a very very strong and intact I thought the better tube is able to produce greater plate current than the weaker tube now the way to better neutralize this excessive difference is to take one of the tubes from the left I know it hurts us to break up a perfectly matched pair but if we take one of these tubes and pit it against the hotter tube we'll find that the average plate dissipation over here and when we move the wick or tube over here that the average plate dissipation on the left will be about to say let's take a look then at the post switching results and see what we think now we have eleven point seven Watts versus nine point seven four that's about two wants difference and on the right we have eight point three four versus nine point eight which is about one-and-a-half Watts difference now as you can see the error on both sides is about equal and all four tubes I think will work better than the gross mismatch that we had before with the perfect match versus the very very imperfect match I swapped the output tubes around trying to prevent any one of them from having a really grossly elevated a plate current and I greatly increased the resistor to ground in the negative DC bias supply and as a result I end up with these voltage drops these clay currents and these plate dissipation as you can see we're much closer to eighty percent of Max rather than seventy percent this one is just a current hog and there's not much I can do to calm it down so this is what I'm gonna live with and probably the life expectancy of this particular tube will be a little shorter than the others but it's the best I can do with what we've got in the circuit now one thing really caught my eye when I first looked at this chassis from behind and that is there are four input jacks which is fairly typical but one of them has an extra leaf and that extra leaf contacts the shield or ground of the quarter-inch input jack now what purpose could that possibly serve and is this even an original input Jack let's take a look at the schematic and it is original if you see here there is the extra leaf whereas the other three Jack's don't have it now what is the purpose of it well if we follow down here to the oscillator of the vibrato circuit we see that there is a switch it says instrument or foot switch now those of you familiar with Fender tremolos know that you have to have a foot switch installed to give ground to the tremolo circuit well magnet own went one better yes the foot switch can provide that ground but if you have this switch in the instrument instrument position the quarter-inch Jack can provide that ground to that extra leaf so there's two ways then of activating the vibrato plug your quarter inch jack into this socket or have a foot switch in the grounded position and here you can see the white wire from that grounding leaf coming over here to your foot switch or instrument switch which allows you to provide the ground to the vibrato a circuit either by the foot switch which is right here or by plugging into that quarter inch jack and so therefore the reality in this case really does pretty well resemble the schematic okay you see there's the white wire coming down here to the switch and here's the foot switch plug right here you can't help but be impressed by the beautiful wiring job that was done notice that the polarity of all capacitors is observed the values are upward facing you so that you don't have to unsolder to figure out what value the capacitor is everything is very neat and in good geometrical alignment not only that but the whole preamp section and output tube section is mounted on a rubber isolated sub chassis to cut down on vibrational feedback to the tubes now those of you familiar with high watt amplifier wiring know how the military-spec wiring was done in them generally by former military technicians and everything is geometric right angles and bundled now such is not the case here with the magnet own because we do get a little bit of that plate of linguine look right here but other than the point-to-point wiring that sort of draped between sections this is in my opinion one of the best constructed amplifiers that I have ever worked on a truly quality work for a quality instrument for those of you wondering what varistor look like here they are and there's eight of them in this circuit they look sort of like the old dog-bone resistors but there are no markings at all anywhere on them and I think it was sort of a proprietary secret exactly how they were achieving the vibrato effect in these amplifiers so they bought these varistors and they are unmarked and to this day present a great challenge for those who are trying to emulate the varistor circuit from these early magnet own amps now I'm not going to go into great detail about how the vibrato circuit works in Magnezone amps I've already done that in a preceding video on the panoramic amp that was also made by Magneton I'm gonna put a link in the video description so that you can refer to that video and hear the detailed description of how this vibrato circuit functions - I felt it would be dunt to put such a lengthy description into almost consecutive videos so I hope you understand and will refer to that link if you want to gain more information on how the vibrato circuit funk I will say in conclusion that the difference between the vibrato circuit that we have here and the tremolo circuit that were much more familiar with in other amps is that tremolo rhythmically modulates the volume of the signal very much like you're turning the volume control up and down and you get the typical tremolo effect vibrato however uses an oscillator circuit much like the tremolo does except that that circuit interacts with the varistors to modulate rhythmically the frequency or pitch of the signal so instead of volume modulation with tremolo we're going to get pitch modulation with our vibrato and I will tell you as you will see shortly it sounds unlike anything you've probably ever heard before you'll be able to tell immediately the difference audibly between this and what you're accustomed to and a tremolo effect and I think you'll find this to be very interesting especially if you've never heard it before and I will point out the oscillator circuit in this magnet own amp it's going to use one triode of a 12 ax7 and you see we have a point of 1.01 and 0.022 oscillation loop here is the 12 ax7 and here are the 3 capacitors the point of 1.01 and 0.02 to securely connect it between two terminal strips and connect it to that trial to the 12 ax7 one final observation about the circuitry we see that we have sort of like on fender inputs where depending on which input jack you select you'll have diff and input impedance therefore if you plug in to the upper Jack's you'll have high gain and you'll have lower gain on the lower Jack's we'll see that that is the case when we go into our audio demonstration ok I have installed a three wire power cord with proper wiring for the primary circuit replace the selenium rectifier with a good stout diode and rewire the negative DC by a circuit with new components we have altered the bias resistor so that the plate dissipation of those four sixty nine seventy three output tubes are now within reasonable limits and rearrange them so that there's they're sort of balanced it's not exactly perfect we've also solved that a problem with a mysterious failure of one Bank of speakers to work by reconnecting the ground wires for those speakers I've gone through and tested the resistors and replaced several of them I found that the capacitors all seem to be in real good shape so I'm gonna honor the owner's request to leave this just as original as I can and let's focus on a really extraordinary audio demo for this okay because it has the two separate output transformers I'm going to connect it to two very different speakers and then we're gonna collect a stereo recording in other words one channel from it won't be these speakers it'll you'll see what I'm going to use but we're going to have a left and right channel they will be different and I'm going to incorporate this in the video so that you have a videos a stereo soundtrack for this video that is going to give you a difference between the two speakers which should give you a lot of depth and complexity on the tone or at least let's hope okay so let's turn this around and get ready for our audio demonstration well alcoholics we have a really special audio in store for you today I'm going to try something really different I'm using an sm58 microphone on a 15-inch Elna co speaker in one of my home-built apps and we're using using the 12-inch Jensen ceramic speaker over here with the SM 57 we'll be recording using the two external speaker outlets bearing in mind that each one of these will come from a channel that has its own dedicated output transformer I'm thinking that the combination of the Elna coal and the ceramic should really give us a full-body tone okay we're gonna start off here in the vibrato socket if you recall and I discussed this this is the one with that extra leaf that's going to turn on our vibrato however the first few tests that we're going to do the vibrato is going to be turned completely off then we'll turn it on for a contrast the tone controls for a little extra clarity are going to treble it about nine o'clock here and the base will be at the neutral twelve o'clock position I'm going to leave the bright switch off okay and we're going to be recording two separate channels here on our tascam a channel recorder you see I have the two channels ready okay if you're ready I'll and Jack are all tuned up and ready to go so let's get started with our audio demo [Music] [Music] [Music] [Music] [Music] you [Music] [Music] [Music] [Music] [Music] [Music] [Music] [Music] [Music] [Music] [Music] [Music] [Music] [Laughter] [Music] [Music] [Music] [Music] [Music] well that's about it for this very special video on the noble super 990 slash magnet own 280a I hope you've enjoyed it and that you'll stay tuned for future videos I want to take a few seconds to thank all my patreon patrons and PayPal contributors for keeping us on the air and advertising free should you like to join them you'll find links in the video description that will help you to do so and now for our part 2 video I spent up a couple weeks in April putting a whole new front end on the 1930 chopped and channeled model a coupe and I'm going to take you through that step by step so that you'll see all the procedures that were involved and see what the end product looks like that sounds interesting stay tuned because we're here we go well today we're going to begin a very challenging job this is the front suspension on the 1930 chopped and channeled model a coupe and I'm going to replace the straight axle the leaf spring the radius rods on the side the spindles and the disc brake units that are on both wheels okay it's going to be fairly challenging because all of the distances are going to be changed the new front axle is a little narrower so the tie rod will have to be shortened and new brackets will have to be welded on the frame back here for the longer split wishbone radius arms so I guess the first order of business is going to be to remove all of the parts that are going to be replaced I'm also going to remove the aluminum friction shocks I don't think there'll be room for them because the new spring is going to arch up and probably they would interfere with it so I'm gonna remove them and see if there's another way that I can mount them okay so let's get started step one of course is to remove the tires and wheels so that we have free access to the front end next let's remove these those hairpin radius rods I released the tie rod here from the steering arms on both sides and I had to loosen the grille shell and lift it up a little bit so I could get at the nuts that hold the spring into the spring perch so that's the next job is to remove the spring and drop it I also separated the brake lines from the frame so that they wouldn't be stressed when the axle and spring were dropped days work is finished the whole front end and all the steering steering arms and everything then removed some of the new parts that started showing up already let me go inside and show them to you here's the chrome straight axle absolutely gorgeous it's like jewelry way too pretty to put on a car and here are the split wishbones that are drilled for that industrial look and they're chromed also and now I'm gonna have to wait for several more boxes of parts before we can start assembling this now it's time to install the new front end this first step was to weld in the holes that I drilled for the friction shocks and grind and paint the surface now with those holes filled and the frame and ground smooth and painted it's trying to install the transverse leaf spring it has to be absolutely parallel to the front of the frame and it is held in the spring perch with brand new grade 8 bolts and all the proper hardware and lock washers now the next job is pretty tough I've got to use the cutoff wheel to remove the bracket for the old radius rods and then relocate it way back here wherever those split wishbone eye bolts are going to end up okay I'll have to install them and then figure out where the bracket has to be hopefully I can salvage this bracket and just reinstall it back here after putting some graphite lubricant on the bolts that go through the shackles they have a really nice thick nylon bushings also this piece right here is screwed into the split wishbone you can see the end of it here I'm going to get an acorn nut to cover that and I've got the hydraulic jack holding it up in place and then at the rear I've got it tied and that will be how I figure out exactly where to locate that rear bracket and weld it and so this is the new drilled split wishbone a radius arm and as you can see it tilts back at the top that gives you your caster on your straight axle I'm going to have to cut the bracket off and on the other side and mount that one just the same and then we'll know exactly where the axle is going to be and exactly where that rear mounting bracket is going to be also I think the wishbone would look a whole lot better if within the holes inside this area here was painted black then you just have your holes kind of set against a black background I think it'd look a lot better well we've got a lot done today we have both of the radius rods installed other than the rear brackets which will have to be positioned and welded painted black inside to emphasize the holes the original radius rod brackets have been cut off get the spring installed with new hard and this just gorgeous I beam front axle I wanted it out the reason I went with a spring behind is to just position this forward and like it's just the featured piece here okay it's just to me that's just gorgeous sitting out there and it's way ahead of the frame I call it a suicide type front end so that's where we're gonna stop today tomorrow we will install in well those brackets and I'm trying to think there's a bunch of other things Oh probably install the spindles on the ends of the straight axle okay so I'll see you again tomorrow here the radius arm brackets have been welded to the frame and the well ground down and everything painted the heim joint here has been adjusted exactly to square the straight axle in perfect parallelism with the front of the frame and everything's been bolted down tight now it's time to install the spindles on that straight axle first off this adapter here for the rotors had to be pressed on to the spindle shaft for the modern bearings that will be used and then it all had to be painted and we're going to put this on to this straight axle and then slide the kingpin in from the top making sure that we passed through the thrust bearing right down here at the bottom and then we're going to insert this Cotter key into the straight axle tube to lock in right here and make sure that that kingpin doesn't come out when they're cruising down the interstate at 80 miles an hour okay so that's our next step here we have the spindle installed on the end of the straight axle it's theirs more going on than what you might imagine you have to hone the bushings in the spindle to exactly fit with like zero clearance the kingpin then you have to fit the Cotter to that notch in the kingpin and when you're through there's some resistance which will go away as you drive the car but absolutely no play in any direction okay so that's it also some people like to Chrome everything and I think really that defeats the purpose to me there should be some contrast to the chrome here and there and I think the black and chrome looks better than chrome and Chrome okay also it's cheaper well that's it for the part one a video of the front end replacement on the 1930 Ford coupe hope you enjoyed it I will include the part 2 video as the second feature in our next amp video so please stay tuned thanks so much for watching

Channel: Uncle Doug

Views: 37,623

Rating: 4.954896 out of 5

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Id: fqi6yQw_SvM

Length: 57min 2sec (3422 seconds)

Published: Thu Jun 04 2020