Lane's Corner - One Tech's Blog

Discussion in 'The Workbench' started by Lane Sparber, Dec 20, 2010.

  1. MM54

    MM54 New Member

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    Very nice, I like terminal strips for some reason, they make circuits look so small and efficient :lol:

    Can't wait for some clips :dude:
     
  2. MajorNut1967

    MajorNut1967 New Member

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    Isn't that choice to do it the old way!
     
  3. TwinACStacks

    TwinACStacks New Member

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    Re: Rangemaster Kit Assembly

    :) How is it You Brit Blokes always have the NEWEST stuff?:D:D

    All Us Scots ever do Is make Whiskey, Fight, Breed animals and Fuck our Womins.

    On second thought that ain't half-bad.

    Major, be a good Laddie and toss me that Caber over there. Yeah, the BIG one....

    :lol::lol: TWIN
     
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  4. MajorNut1967

    MajorNut1967 New Member

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    Re: Rangemaster Kit Assembly

    Roger that mate! What's a Caber? LOL Actually we have a Highland games down in Turakina about 5hrs from Auckland.
     
  5. SigurdTheGreat

    SigurdTheGreat New Member

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    Nice one, Lane!

    As it reportedly worked like a charm with a 2203, it got added to my project waiting list. Indeed, I had been searching for an OD for my JMP 2203, but I think I'll make one of those instead.
     
  6. Lane Sparber

    Lane Sparber Well-Known Member VIP Member

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    I highly recommend it. I love mine! :)

    -Lane
     
  7. TwinACStacks

    TwinACStacks New Member

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    Lane, Did you order yours from Britain on that website?

    :):) TWIN
     
  8. Lane Sparber

    Lane Sparber Well-Known Member VIP Member

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    Yeah, the one I linked to. I believe they also have one up on the US version of eBay. They're the same company that built that Top Boost module my other client bought. That's how I found this kit, actually! :)

    -Lane
     
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  9. Lane Sparber

    Lane Sparber Well-Known Member VIP Member

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    Hello, Marshall Forum family!

    I am back temporarily to deliver on a promise made to certain MF members on Facebook. Some of you wanted a blog about an amp repair I was planning to perform on a piece that I literally discovered on the street one afternoon.

    I was walking home in a light drizzling rain when I saw that one of my neighbors had thrown out a slew of stereo gear. Among that gear was a Harman-Kardon “Allegro” integrated tube amp. This is a MONO 15w class AB amplifier that uses two EL84 tubes in it’s cathode biased output section. It’s the little brother to the H-K “Ballad” stereo amp in my listening room! Well, I don’t usually pride myself on being a dumpster-diver, but I literally couldn’t help myself. I swiped this little gem later that night under cover of darkness. I brought it home eagerly, but as I had a pile of amps to work on and this was a personal project, it had to wait patiently until I had the time.

    Well, the wife took both kids out yesterday for errands and I had an empty shop - and I had the afternoon to myself. I was SUPPOSED to relax and take it easy, but my soldering iron literally called me into the workshop to finish this little project while I can - before I’m hit with the next incoming round of broken amps. Thus, I eagerly made my way downstairs to see what this diminutive monster had in store for me.

    I wanted to establish a single over-arcing ground rule for this repair to make it interesting: I was limiting my budget to $50.00. As this was literally a found item, I didn’t want to sink a lot of money into it, and it would be a good exercise in economy and good decision-making, as this also kept me from going nuts and modding the SNOT out of it! :naughty:

    The night I…um…”rescued” this amp, I took it apart to make a quick part order. To my dismay, all of the original Mullard tubes were shattered and/or not working, so that was the first hurdle. I had used-but-good (mostly NOS) tubes in my bin that would work, but the sticking point was the EZ80 9-pin rectifier tube. That’s a fairly rare tube, so I needed to buy one. Damn. No one I checked with had one, so it was off to eBay land! $14.99 + $5.50 for shipping. Total so far? $20.49

    It obviously needed a re-cap, as all of the electrolytics were original, and in a single “3 in 1” cardboard tube. It had clearly failed. There are two options in this instance: Take the cap apart and install new radial caps INSIDE the old tube thus preserving the “look” of the amp’s interior (this approach is favored by many old-time radio enthusiasts), or just use discrete caps soldered directly onto the board. I chose the latter. There were only two filter caps plus the cathode bypass cap on the power tubes. As there was VERY little space on the board to fit these caps, I chose radials, and I also had to pay close attention to the physical size of the new caps I was going to order so that they would fit and play nicely with the other components fighting for the same real estate.

    It’s worthwhile at this point to note that the first filter cap after the rectifier tube was a 40uf/375v cap. Now, the most common and closest values to the original component (in radial caps) nowadays are 33uf and 47uf. I went with the 33uf. Why go with the LOWER capacitance when we all know that -to a point at least- more capacitance equals a tighter, more focused sound; something ESPECIALLY desirable in a high-fi amp? Because a quick check of the spec sheet for the EZ80 lists 50uf as the MAXIMUM capacitance value that it wants to “see” after it. The 47uf cap could, with it’s standard 20% tolerance, go as high as 56uf. That’s obviously over the limit for this rectifier tube. Would it still work perfectly? Most likely. Did I want to risk it with my new twenty-dollar rectifier tube? Most definitely I did NOT. Hence, I went with the 33uf.

    Finally, the two resistors in the power tubes’ “balance” circuit had burned themselves literally in half. Something catastrophic had happened inside this chassis! I didn’t have those 330 ohm 1 watt resistors in stock either.

    In addition to all of that, this amp also needed a new fuse and pilot light - both of which I thankfully had PLENTY of in stock. However, it also had two molded paper coupling caps, which DID have to go, as paper caps don’t age or hold their value well. Thus, my Mouser order contained 3 electrolytic radial capacitors, two Sprague “Orange Drop” coupling caps, and the two new power tube balance resistors. The total cost at Mouser plus shipping came to $17.91

    Parts total: $20.49 + $17.91 = $38.40. Not bad. I even had some “padding” there for later just in case I needed anything else.

    Now on to the exciting stuff…the actual repair. Here’s what I found when I pulled the lid off of the chassis and removed the old tubes and broken glass:

    [​IMG]

    Pretty clean, all things considered! Note the area to the middle left of the drawing where the multi section capacitor’s black/red/yellow/green wires join the board. This is the entirety of the space I had to mount all 3 new electrolytic caps!

    Here’s a close-up of the old electrolytic cap can. Note the mis-shapen wax bulge on the right side of the cap where the leads emerge – this thing had DEFINITELY joined the choir invisible!

    [​IMG]

    Here are the two fried power tube balance resistors:

    [​IMG]

    Finally, here are the two ancient paper coupling caps that had to go:

    [​IMG]

    [​IMG]

    In addition, when that electrolytic cap can fried, it literally melted, sending molten wax down the chassis and into the volume pot, which also contains the power switch. This component is common in old hi-fi equipment like this – basically, when you turn the volume all the way down, a little extra pressure will engage the power switch and shut the amp off. The wax had rendered this pot absolutely immobile (unless I heated the shaft with a soldering iron). I was going to have to perform the INCREDIBLY risky job of removing this delicate component, dismantling it, and manually removing the wax. Here it is disassembled and ready for repair:

    [​IMG]

    I removed the wax by applying heat indirectly with my Zippo and quickly removing the molten wax with Q-Tips. This had to be done separately on the shaft/wiper assembly, carbon ring, and body of the pot. It took a while, but when I was done it turned smoothly and tested fine on the meter. I then reinstalled it into the amp.

    Next up I installed the new fuse, pilot light, and 330 ohm balance resistors. This brought me to the stickiest problem - figuring out the best way to mount the new electrolytic caps. After entertaining several mounting options, I decided to mount one filter cap vertically and one filter cap horizontally, securing them both with hot glue. This way, they would take up the least possible amount of real estate and at the same time maintain maximum distance from the sensitive preamp circuitry less than an inch away! Here are two angles of the finished, mounted caps:

    [​IMG]

    [​IMG]

    Now, you might have noticed that there is almost NO space left to mount the cathode bypass cap, which theoretically goes right next to those caps. My solution to this problem was to “cheat!” I mounted it to the other side of the circuit board…underneath the other components:

    [​IMG]

    Next, those two paper coupling caps had to go. Here are the new Orange Drop caps installed in their new homes:

    [​IMG]

    [​IMG]

    At this stage of the repair, I was ready to form the electrolytics on the Variac and test this amp out, but instead I was provided with yet another challenge. This amp, like many older radios/stereos of the era was equipped with a “safety” feature wherein the power cord was riveted to the outer chassis, so that when the cover was removed, the power cable came with it. Thus, you could not physically power the amp on when the cover was removed. Well, this just wouldn’t do, so I needed to come up with a workaround. The solution? Fortunately the designers of this particular amp saw fit to include an auxiliary outlet on the back of the chassis. EUREKA! I could feed the power INTO this outlet and use it for the exact opposite of it’s intended purpose! This way, I could get power into the amp while it was disassembled. I stripped the insulation from an old zip cord power cable I had and tinned the ends with solder to avoid stray strands of wire shorting anything out. I fed them right into the auxiliary outlet and plugged the other end into my Variac. Worked like a charm:

    [​IMG]

    WARNING!! The above step and it’s solution are VERY dangerous. Do NOT, I repeat NOT try this at home unless you have a thorough knowledge of electrical theory and safety procedures. Trained professionals ONLY. ;)

    Once fired up, to my amazement, it ACTUALLY WORKED! Without it’s outer covers, it was a bit noisy, but it still sang and chimed and sounded fantastic. All voltages checked out too! Sometimes, you just get lucky.

    It was time for cleanup. I didn’t feel like cutting the metal straps and removing the old multi section capacitor, so I just clipped the bare leads and cable-tied the wires to the cap’s body. Now it was totally removed from the circuit and out of the way. It won’t be hurting anyone anymore. ;)

    [​IMG]

    Here’s a master shot of the finished underside of the chassis:

    [​IMG]

    Next, here is the amp all fired up for testing. Note the trusty iPod feeding it on the left side of the picture:

    [​IMG]

    All ready to rock!

    [​IMG]

    I spent the entirety of the remaining afternoon CRANKING Beatles’ mono mixes through this beast, and it was an incredible listening experience, to say the least. Tube audio amps, like their guitar counterparts, just sound better. If you ever get the chance, try it yourself and hear the difference!

    Now I just have to decide what to do with this amp. Any ideas? Any takers? ;)

    As a quick aside, I am still in my recuperative phase. I need this time to heal and get my head together, and already I do feel much better. I WILL be back eventually, but for now I’m just not ready to come back on a more permanent basis.

    Thank you all SO MUCH for your kind words and concern. I assure you that these sentiments mean the WORLD to me, and have aided in my recovery to an un-measureable degree.

    Cheers, and sayonara again for now!

    Most importantly:

    HAPPY HALLOWEEN!

    -Lane
     
    Last edited: Oct 31, 2011
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  10. cylon185

    cylon185 New Member

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    Great stuff as always Lane
     
  11. thrawn86

    thrawn86 Well-Known Member

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    Good read, Lane. Please take your time and heal properly. One of the worst things you can do is rush your recovery since you feel better, and end up lengthening the process. We miss you bro, but we'll be waiting here. :thumb:
     
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  12. Les Moore

    Les Moore Well-Known Member

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    I read somewhere that you left the forum. Can´t say how glad I am to see that that was wrong. Truly.
    Sad to read you´re ill. I think you rock. Get well soon :)
     
  13. Lane Sparber

    Lane Sparber Well-Known Member VIP Member

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    Thanks, Les.

    I am in a MUCH better place now. I will probably be back - albeit on a more limited basis - some time after the new year. It seems that there are a few new guys handling things pretty well, and Joey's back (sort of) so you've all been in good hands. :)

    In the meantime, I just competed a wacky Marshall repair that I'm thinking of doing a new blog about, so maybe that will be up in the next week or two. I consider writing these blogs therapeutic...don't know what THAT says about me! :naughty:

    Cheers, and thanks for the concern, everyone! :D

    -Lane
     
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  14. TwinACStacks

    TwinACStacks New Member

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    :) I do miss you, better-looking Little Bro. Say Hi to Mom for me.

    :lol::lol: TWIN
     
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  15. Lane Sparber

    Lane Sparber Well-Known Member VIP Member

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    Hello again, faithful blog readers!

    Today’s offering is about a 1974 Marshall model 1959 that recently came into my shop. The owner had bought this in a lot with two other amps for the princely sum of 500 bucks. That’s right, he got this vintage Marshall head for about $166 dollars. Why can’t this stuff happen to me?!?

    Anyway, he brought it into my shop for a checkup, as it had been worked on, and he wanted to make sure it was bone stock and in good health. This client is in a major touring band and runs a busy recording studio on the side, so we needed to make this amp as “bullet-proof” as possible. The first thing I noticed when plugged in was that the amp was VERY noisy for a ‘59, and, well, it just didn’t sound right to me. It also distorted around “2” on both volumes, and anyone who has EVER been lucky enough to own a 1959 (my personal favorite Marshall model, for those who care), that just ain’t right. Externally, it also had one mismatched speaker jack that the client had tried out on his own, and he found that this jack didn’t seem to do anything at all. Curious. It also had all of it’s original DALY filter caps, which I initially thought was the cause of all of that noise. Those caps’ days were numbered.

    Upon disassembling the amp, I noticed that the power transformer had been replaced at some point with another Dagnall. I STILL can’t find much out about this PT, and, regrettably I didn’t take a clear pic of it. It was MASSIVE, and new holes had to be drilled in the chassis to accommodate it. It had a sticker on it that identified it as a “T3556” model, but almost NOTHING about it is on the web. To complicate matters further, someone had written “Fender” on it with a non-existent part number. I found ONE picture of a “Dagnall T3556” transformer on the internet, and it looked NOTHING like this one. This oddball tranny also featured cloth covered wire, which I’d NEVER seen on a Dagnall. Part of me thinks that someone just stuck that ID sticker on to fool people, although I could be wrong.

    This amp had to be modded in SEVERAL ways to accommodate this transformer, because all of it’s secondaries were NOT spec for this amp:

    1. The heater winding had no center tap, meaning that a false center tap had to be created using two 100 ohm resistors from each leg to ground – Fender style. The original tech had installed these resistors RIGHT ACROSS THE V8 POWER TUBE SOCKET! BAD idea. THAT was what was creating a lot of that excess noise.

    2. The HT winding was 185-0-185 at a 120v input. That’s too high for vintage Marshall spec, which is about 175-0-175 The plate voltage on the tubes was over 500vdc. These tubes were the original GE 6550s, and they seemed to have taken it well, but it was time for a change.

    3. The bias supply winding was wound for roughly half of the voltage of an original Marshall 100w Dagnall PT. This winding needs to be 100vac for spec, and this one was 55v, so MAJOR mods to the bias supply had been done.

    Thus we can see that this transformer was a poor choice for almost every conceivable reason. It needed to come out. I chose a Hammond replacement that was wound to vintage specs, with the addition of extra voltage taps on the primary for 220 and 240vac, respectively.

    Finally, all of the 1 watt 10K B+ decoupling resistors had been replaced with ½ watt 15k resistors. Why was THIS done? I have no idea. Maybe it was to drop the preamp voltages back down to spec due to the higher B+ winding, but even so, 1 watt resistors should have been used.

    So, here’s the laundry list of repairs for this amp:

    1. Install new power transformer and reverse all mods done to facilitate the incorrect one.

    2. Change the power section over to EL34 specs. This, IMHO, is CRUCIAL in a vintage non-master volume Marshall that shipped with 6550s; much more important and effective than doing this in a master volume Marshall from the same era. Why? Because non master volume amps get a significant amount of their distortion tone from over-driving the POWER section, and a 6550, to my ears, sounds MARKEDLY different from an EL34 in this respect. For true Marshall tone in an amp like this, it should be EL34s all the way!

    3. Replace the incorrect value and wattage B+ decopling resistors with the correct spec resistors.

    4. Replace the weird mis-matched speaker jack with a new, matching socket.

    5. with a modern switch for drastically improved reliability.

    6. Disable the dangerous polarity selector switch.

    7. Fully re-cap the amp and bias it up.

    Now, I try to give EVERYONE the benefit of the doubt, but over-all, the work done to this amp, while electrically sound, was amateurish at BEST. Sloppy, above-the-board mods, cold solder joints, and messy, carelessly arranged wiring abounded. Let’s take a look!

    Here’s how the T3556 transformer was wired into the circuit. The wiring is sloppy and poorly done, but in this pic you CAN make out the cloth covered wire. Weird, no?

    [​IMG]

    Here are the 100 ohm heater supply false center tap resistors wired RIGHT across the V8 power tube socket [shudder]:

    [​IMG]

    The mis-matched modern style output jack:

    [​IMG]

    Here’s the inside of the chassis, showing said mis-matched jack. HMMMMM….wonder why it didn’t work? IT WAS NEVER EVEN CONNECTED!

    [​IMG]

    At some point, the original Erie 330uf cathode bypass cap on Channel 2 had failed. Here’s how it was dealt with:

    [​IMG]

    Have a look at the bias supply on the right side of this shot. As previously stated, it had been modded to allow enough voltage out of that lower voltage bias tap to tame the 6550s, but the work was EMBARASSINGLY sloppy and the solder joints were ice cold. They actually broke apart as soon as I touched them. There's even solder dribble on the bias supply caps! Sigh. Also, on both sides of this pic you can see where all four of the 10k 1 watt B+ decoupling resistors have been replaced with ½ watt 15k resistors in the lazy, “above the board” splice’n’dice fashion. Lastly, note that the grid leak resistors to the power tubes (should be 150k for Marshall 6550 spec) are 82ks!

    [​IMG]

    Here’s a closer look at that modded bias supply:

    [​IMG]

    Below you can see the chassis after the removal of the old transformer. Note all of the extra holes that had to be drilled out for that old transformer. In the picture, I’ve already moved the rubber grommets on the left to the inner, original holes, and had yet to install the one on the right side (the T3556 had all of it’s wires exit on one side, as opposed to the more standard array using both sides of the transformer):

    [​IMG]

    Unfortunately, the Hammond replacement transformer was STILL a little too narrow to fit into the vintage holes. It wasn’t a problem that some washers from the local hardware store couldn’t solve!

    [​IMG]

    Here’s the underside of the chassis after mounting the new transformer, but before creating the wiring harness:

    [​IMG]

    All wired up! Note at the bottom center of the picture that I heat-shrinked off and labeled the additional 220 and 240v primary taps (the blue and purple wires, respectively) just in case the owner decides to move to Europe at some point. ;)

    [​IMG]

    Here’s how the V8 socket with turned out with cleaned up wiring – and no extra resistors!

    [​IMG]

    With the new transformer installed and tested, it was time to move on to the re-cap and board repairs. I personally prefer to just un-bolt the pots rather than de-soldering them to get at the underside of the board. This pic should illustrate what I mean. Also, the jack and impedance selector were replaced at about this time. At the back of the pic you can see the new buss wiring I installed for the output jacks:

    [​IMG]

    Here’s the back of the chassis showing the new output jack and impedance selector:

    [​IMG]

    Now, this wouldn’t be a TRUE overblown and boring Lane’s blog entry without the gratuitous cap-forming shot, would it? You KNOW you want it!

    [​IMG]

    Finally, we have the finished, cleaned up and re-capped board. Spot the differences! I know that those new blue Panasonic B+ decoupling resistors LOOK small, but they’re actually TWO watts and are rated for 600v! Cool!

    [​IMG]

    [​IMG]

    Ready to rock!!

    [​IMG]

    After re-building a good bit of this amp, it finally sounded LOUD, punchy, and did everything a healthy, vintage 1959 is supposed to do. All voltages checked out, and it sounded AWESOME. This was a hard one to give back to the owner. I especially felt a kinship with this amp because it’s the same vintage I am – a 1974 model!!

    Thanks for reading this and sticking it out! I hope you were able to get something out of my experiences here and found at least some of it useful. It is, after all, why I do these.

    Also, HAPPY THANKSGIVING! I am thankful for this forum, the fact that I was given my own little corner here to play in, and all of the wonderful techs and friends I’ve made here. Have a great holiday!

    Until next time…and KEEP IT CRANKED!

    -Lane
     
    Last edited: Nov 24, 2011
  16. Kunnz

    Kunnz Active Member

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    ▀▄▀▄▀▄▀▄█▓▒░ 1st Class, # 1 ░▒▓█▀▄▀▄▀▄▀▄▀
     
  17. MM54

    MM54 New Member

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    Excellent work as always, Lane :cheers:

    I've used those little blue Panasonic resistors in a million different things, they're pretty awesome!
     
  18. Lane Sparber

    Lane Sparber Well-Known Member VIP Member

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    Hello again blog readers!

    Today, I have another “by-request” offering for you. This entry isn’t so much a “how-to” missive like my others. Rather, this installment will document a basic repair/restoration on a fairly rare and VERY cool 1966 twin chassis Gretsch 6159 “Bass” Amp. This amp was one of many related models made by Valco for various distributors –Gretsch and Supro being two of the more famous imprints. Rumor has it that Jimmy Page recorded the first Led Zeppelin album in it’s entirety with a Supro cousin of this amp – how cool is that?!

    [​IMG]

    This is a REALLY cool little beast. Basically, it has 4 inputs – 2 bass inputs, an “Accordion” input (yes, you read that right…”Accordion”), and a guitar input. It features a tremolo circuit on the accordion/guitar channel as well. Here’s the schematic:

    [​IMG]

    Check out that output section! What we have here is a pair of 6L6GC tubes in a cathode bias configuration. They’re biased fairly hotly at about 28 watts a tube according to the schematic, and my real-world measurements bore this out. To those who would ask me how I got that dissipation rating, here’s how - the info needed is all right there on the schematic:

    30 volts at the power tubes’ cathodes divided by the 200 ohm cathode resistor=.15 amps or 150mA total draw (75mA per tube). So, we take 150mA multiplied by 370 volts (400 volts at the plate – 30v mean listed cathode voltage) = 55.5 total watts, or 27.75 watts per tube! That’s pretty close to the maximum dissipation rating of a 6L6GC at 30w, no?

    When I received this amp, it was missing a power tube and needed a complete re-tube, had a two-conductor power cable and “death cap” installed, had a blown pilot light and, finally, it needed new caps. This is the story of how I approached the repair.

    Here’s the control panel (I’ve already removed the lower chassis containing the power and output stages in this picture). Note that “Accordion” input! You can also see the Tolex at the bottom of the cabinet has all but detached around the base.

    [​IMG]

    I wanted to tack that Tolex back down first and let it dry while I performed the repair, so I used my Luthier’s clamps (and a couple of industrial spring clamps) to fix the Tolex in place while the glue dried. In this picture, the glue and clamps are applied and we’re just waiting on the glue:
    [​IMG]

    The upper chassis, which contains the preamp circuit, was REMARKABLY clean inside. Note the shielding plates soldered to the input jacks. The large white caps are the dried-up electrolytics that need replacing. Also, the soon-to-be-removed “death cap” is that HUGE ceramic cap on the bottom right side of the first picture.

    [​IMG]

    [​IMG]

    [​IMG]

    Here’s a top view of the lower chassis as I found it. Note the mounting hole punched into the chassis. This hole was originally intended for the rectifier tube in a previous iteration of this model. By ’66, when this amp was made, they’d already switched to solid state rectification. On this particular amp, however, they used an older lower chassis. A previous tech had simply cable-tied the main and preamp filter caps to these mounting holes. This was crude, but HIGHLY effective!

    [​IMG]

    [​IMG]

    At the bottom of the above pic, that blue mini LCR was a dual cap that housed the preamp filters. The single Sprague Atom cap cable tied to it’s left in the pictures is the first main filter cap. Overall, this amp is criminally under-filtered for a bass amp. The highest filter cap value is just 20uf! As the owner liked the amp’s extremely spongy and uses it primarily for harmonica, I kept that scheme for the most part. I DID more than double the value of the screen filter cap to 47uf, however. This was because in the case of a modern bass being plugged into this amp, I wanted it to be able to hold together at a bit better at higher volumes without turning completely to mush!

    The large silver filter cap for the power tubes’ screens (upper right in the above pic) was original, and needed to come out. I’ll never understand why some techs will replace SOME caps and not others. I just don’t get it. Can someone maybe enlighten me?

    Another minor detail worthy of note is that there were no rubber grommets installed on this chassis where the cables passed through it. This is VERY bad, as over time the wires will chafe and the insulation then becomes compromised - leading to these wires possibly shorting to the chassis. So, I first installed new grommets:

    [​IMG]

    [​IMG]

    Another idea I had was to make better use of that gaping rectifier hole in the chassis. I had a brand-new 16uf/16uf mini dual cap just like that old LCR in the photos, so I decided to mount this new cap to that hole vertically like in a Marshall. The other main filter cap I moved to the tag strip out of the way of everything else. Here’s how it turned out with the new caps and cleaned-up lead dress:

    [​IMG]

    Here’s a close-up of the caps. Note what WAS that old Sprague Atom has become a black F&T 16uf cap mounted to the tag strip at the extreme left side of the image.

    [​IMG]

    Here’s the top side of the chassis showing the new, vertically mounted dual filter cap:

    [​IMG]

    Guess what’s coming up next?!? YOU GUESSED IT! CAP FORMING SHOT!!

    [​IMG]

    Here are two quick shots of the upper chassis after cap replacement – the power cable has been replaced and it’s strain relief reinforced with hot glue, the death cap has been excised, and the old white electrolytic caps have been replaced with the silver Spragues:

    [​IMG]

    [​IMG]

    Finally, here’s the lower chassis installed in situ:

    [​IMG]

    To conclude, here’s a master shot of the back of the amp showing the cleaned up lead dress, re-glued Tolex, and new power cable:

    [​IMG]

    So how did she sound? With a bass, it was surprisingly clear and loud, but not very “tight” or “punchy,” as was to be expected for the aforementioned reasons. With a guitar, though, this amp CAME ALIVE! Especially cranked, she exhibited a smooth, creamy distortion that had a tonal signature all it’s own. Think warm overdrive with a hint of sharper-edged breakup. This amp would be PERFECT for indie rockers, punks, and blues aficionados.

    Overall, this amp was a JOY to work on. You simply don’t see a lot of them around, and I was thrilled to finally dissect one and find out just how she “ticked.” Studying schematics are one thing, but actually having an example in front of you really brings it home. Being as how I was lucky enough to land one of these in my shop last month, I simply couldn’t pass up the rare opportunity to share the repair experience with my friends here on the Marshall Forum!

    Thanks again for reading!

    -Lane
     
    CKCinMass and MM54 like this.
  19. Lowlife

    Lowlife Well-Known Member

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    nice read, you get to work on some pretty cool amps
     
  20. MM54

    MM54 New Member

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