HH Scott LK-48 Restoration/Mods

Discussion in 'DIY' started by bobboxbody, Mar 29, 2023.

  1. Grattle

    Grattle Friend

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    Threads like this are so fun. Thanks for sharing all this. You've inspired me to start checking estate sales for vintage equipment to play with.
     
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  2. bobboxbody

    bobboxbody Friend

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    I found both amps insanely cheap on FB marketplace, I had to drive a bit, but so cheap. I also want to start looking at estate sales.
     
  3. bobboxbody

    bobboxbody Friend

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    Despite the success of the filter on the 7189 heater string, I'm still bothered by the buzzing/ringing from the power transformer, it's not likely to be anything good. I asked for advice on Audiokarma, which is a great resource for working on vintage stuff. The suggestions included:

    1. buying a new PT from Heyboer, who do custom transformers and happen to be a 20min drive from my house.
    2. removing the PT and submerging it in a bath of insulating varnish/epoxy under vacuum.
    3. getting some insulating varnish, thinning it with mineral spirits and painting it on the laminations in the hope that the thinned varnish will flow down in to any areas that have become uninsulated.

    Judging by the strength and sound of the buzz, I guessed that the de-lamination was close to the bell cap, and not wanting to buy a vacuum setup or pay as much for a new transformer as I did for the whole amp, I went with option 3. If this fails I'll likely just buy a new one since the PT's in these amps are kind of too small for the application anyway. First step was to get some insulating varnish, I went with red insulating varnish from MG Chemicals based on reviews. You should probably have some ventilation when using this stuff, but if you love the smell of VOC's like I do then it's great. I opened a window anyway just to save the few brain cells I have remaining.
    [​IMG]

    Next, with the bell cap removed I turned the amp on it's side and shoved a tube of solder under the edge to keep it from falling out. Then I mixed a 1:1 solution of varnish and mineral spirits and painted it on over and over again, letting it drip through as much as possible. Once that's cured I'll flip it over and repeat from the other direction, let cure, then put everything back together and fire it up with fingers crossed.

    [​IMG]

    Always keep old holey socks around for cleaning up chemical spills, they're OSHA approved.

    Edit: Bonus pic, I cleaned up and painted the transformers on the 222c/d that's now in use and added some silver capped Marshall amp replacement knobs instead of the old brown/brass cap knobs from Scott and I think it looks pretty sharp.

    [​IMG]
     
    Last edited: May 23, 2023
  4. InsanityOne

    InsanityOne Acquaintance

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    Have enjoyed following your progress on this. What was the result of the thinned insulating varnish application? Did it resolve the buzz?
     
  5. bobboxbody

    bobboxbody Friend

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    Sorry I forgot to update. I also forgot to take any additional pictures.
    After the thinned insulating varnish dried I spread on a healthy coat of un-thinned varnish everywhere I could reach with the brush. The result was about what I expected. The buzz is reduced, but still present. I can't hear it from about 4 feet away at my computer chair anymore, but it's still louder than the nearly silent PT on the 222C. The temperature of the transformer in the LK-48 is cooler than the 222C after 30 minutes of operation though, which is a nice side effect(This test was performed by touching hot things repeatedly and them comparing them in my memory, so high degree of confidence and scientific rigor here). Everything on these amps runs pretty hot except the conservatively biased 12ax7's. The 7189's are manually biased within spec, but on the lower side for longer life and because I couldn't hear a significant difference between running them cool vs hot, but "cool" still puts out a bunch of heat with 420V at the plate. As long as I'm on a heat rant, the worst offender is the "chalk stick" resistor, which acts as a voltage divider for the power supply, and dissipates a bunch of excess power as heat. You're probably wondering, "How does he know it's really hot? Common sense based on the amount of power dissipation? Infrared thermometer? Heat-seeking missiles? Bloodhounds?" No silly, that's stupid, I touched it to see how hot it was. The answer was blisters. I've found a modern replacement that should run much cooler, but would look even more weird and probably offend the people who believe in magic deteriorating components when I try to sell the amps, so the chalk stick remains. Despite the improvements to the LK-48 transformer with the varnish I plan to order a new PT and sell the old one on ebay. I'm pretty sure the new one will be lower noise, cleaner/more stable power, and hopefully let me take the extra filter off of the 7189 heater string, not because I've noticed any detrimental effect from the filter but because it's just simpler without. It'll probably be a while before any of that happens though, I'm going hiking with my wife for a while in Olympic national park, have just spent a ton of money on new audio toys, and am really enjoying just listening and not tinkering.
     
    Last edited: Jun 8, 2023
  6. bobboxbody

    bobboxbody Friend

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    Mini project.

    The Scotts, and many vintage tube amps have no built in capability to run and select between multiple sets of speakers. Since I now have floor standers and desktop monitors that I want to run from the same amp, I bought a cheap speaker selector box from Amazon. I went with the Solupeak SP2 because it has individual switches for each pair of speakers. That way I can switch a pair on before switching the other pair off while the amp is running, to make sure it's always being presented with a load. The other function of the dual switch is that I can plug my headphones into the built in jack on the 222 and then turn off both sets of speakers for HP listening, another function missing in the design of the amp(it was added in the later revision of the 222D, but not on mine with the 222C circuit).
    Initially I wanted to build my own switch, but it was going to be more expensive even to order the cheapest possible parts for my DIY project that it was to just buy a finished product on amazon. Turns out the reason is because the finished product on amazon sounds like butthole(spellcheck is telling me butthole is supposed to be 2 words, but it's wrong). One of the switches had an intermittent connection and everything sounded tinny, lacking in body and dynamics, so I took it apart:
    [​IMG]
    Heavier gauge wire than I thought would be in there, but still tinned mystery metal for the conductor, clips for the binding post connections, and a PCB for the switch connections with a bad solder joint on one. I decided to get rid of the PCB and rewire everything point to point with some 12 gauge OFC speaker wire that I have on a spool. I hate working with stranded wire, but sometimes it's better, as is the case here.
    With the PCB gone, I needed a small solder lug terminal strip to route the new wires, so I drilled a small hole in the bottom of the case and mounted it:
    [​IMG]
    Then I ruined it trying to widen the holes in the solder lugs with a drill and attached a new one with holes widened by dremel. I cut all the wire to length, removed the circuit boards and then wired the new amp to switch connections on the terminal strip:
    [​IMG]
    Then I soldered the red wires to the bottom sides of the switch, which I thought was for the right channel, but was in fact for the left channel. Rather than de-solder and re-solder everything I just left it. I left the PCB on the bottom row of binding posts as-is because each connection there just grounds to the board itself, and I read somewhere that it's bad to have a common chassis ground for multiple speaker sets in a switch like this, I don't know if that's true or makes sense, but it's working fine for now.
    All wired up point to point:
    [​IMG]
    And back on the desk:
    [​IMG]

    It sounds so much better. No more intermittent drops, sound is just as rich and full as when connected directly to the amp. I might eventually replace the switches with something of higher quality for longevity's sake, but this will work for now. I'll update again, probably in a couple of months when I have the new power transformer for the LK-48.
     
    Last edited: Jun 9, 2023
  7. bobboxbody

    bobboxbody Friend

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    Turns out after 1 day of use that the switches are also worthless, ordered new switches from digikey, probably at the point where I could have ordered all the parts and built from scratch. As long as I'm throwing cash into this pit I might as well order some nice binding posts, but at least I got an enclosure out of the deal! muttered curses, long sighs, tears...
     
  8. bobboxbody

    bobboxbody Friend

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    There was some discussion earlier in the thread about alternative ways to replace old multi-section electrolytic can capacitors instead of using reproductions. I just did one of the methods on a tube tuner that I got with one of the amps. I didn't do the greatest job taking pictures of the process, but enough I think to get the point across.

    Here's a shot of the can being replaced:
    [​IMG]

    It's an easy example because all of the values are the same. Notice there is no insulating material between the body of the can and the chassis. Since multi-section caps like this share a common negative lead, that means all the negative leads and the body of the can are grounded to the chassis. Here's a shot of the underside:

    [​IMG]

    The 4 twisted metal tabs around the outside of the square cutout are securing the can to the chassis, and since there is an additional conductive/shielding plate on the underside of the chassis, they doubly confirm that it's grounded. The symbols etched into the phenolic plate on the underside of the can correspond to the printed values with the same symbols on the outside of the can from the previous picture.
    This is the power supply filter and for those I usually use Nichinon PW series. They're low ESR, long rated life with a wide range of operating temperatures and a much higher voltage rating that the original, 450 vs 250V. The closest common modern capacitance value is 47uf vs the 40uf originals, shouldn't be a problem but in general you want to stick as close to the original capacitance value as possible unless you're making intentional circuit modifications. A higher voltage rating is always fine, and probably a good idea since common household wall voltage is much higher now than it was 60 years ago when the circuit was designed. Here they are with all the negative leads tied together and one left sticking out to make the ground connection:

    [​IMG]

    Next I soldered those negative leads together. Then I removed all of the connections from the old can cap, drilled small holes in the chassis to mount a couple of solder lug terminal strips for mounting the new caps, and re-connected everything to the new filter caps. I failed to take any pics of the process, but here it is completed:

    [​IMG]

    The old cap with connections removed is circled in blue, with the new terminal strips and connections outlined in the green rectangle. The 2nd from the right solder lug is bolted to chassis ground and wired to the previous ground just to make sure. I replaced most of the hookup wire as well, just because the old stuff kind of sucks. You can see from the relative sizes of the new caps and the cutout for the old one, there's no way these would have fit inside the old can for the restuffing technique, and with plenty of room under the chassis this was the much easier option. I probably could have found smaller, lower voltage caps to stuff inside the old can but it wasn't worth it.

    That's that. The tuner works well and I have no desire to listen to the radio so it's going on ebay. Here it is fully restored with new lamps, NOS K40Y-9 output caps, all new electrolytics, rewired power supply, and a glowing 6X4 rectifier:

    [​IMG]
     
  9. purr1n

    purr1n Desire for betterer is endless.

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    Notes from @bobboxbody (for future reference now that I have the 222):

    I also made a map that I thought might simplify the biasing layout, but now I'm not sure if it's just more confusing. It's pretty simple really. The 2 pots next to the power transformer control the voltage going to the balance pots for each pair of tubes. From this overhead view, the top pot corresponds to the balance pot for the left pair of power tubes and the lower pot to the right pair. Turning them clockwise will increase tube current draw, counter clockwise to decrease. The test points are between the 2 pairs of power tubes, the top left corresponds to the left tube in the left pair and the top right to the right tube in the left pair. Same for the bottom right and left to right and left of the pair on the right. Here is the illustration:

    [​IMG]

    The recommended idle current draw for each tube is between 22-27ma. This will show as 0.22-0.27V or 220-270mV when reading with a multimeter set to DC volts from the test points. I've been running the tubes in my amp at 23ma since I currently use it for my computer speakers as well as music listening, so I don't want to burn the tubes out too quick. If you're just using the amp for music only I think 24-25ma sounds a little better. The negative probe from the DMM can be clipped anywhere on the chassis and the positive probe in the test point that corresponds to the tube you're reading.

    This is the initial bias procedure I've been using:

    Once the amp warm up for a few minutes with speakers connected and the volume turned all the way down, then bias the tubes to your desired current draw. It will take some back and forth adjustments since it's not a perfect system and changes in the left pair bias can effect the right and vice versa. I usually bias the left pair first, then the right pair, then go back to check the left, and finally check the right one more time. Both the pots feeding the balance pots and the balance pots themselves are pretty sensitive, so minor turns with a flathead screwdriver are recommended, then give the tube/DMM 10 seconds or so to catch up to the adjustment. For the balance pots, turning clockwise will cause the left tube to draw more current and the right tube less. Once everything is at the desired level, then turn the volume up and listen to music for 30 minutes or so, stop the incoming signal, turn the volume back down to minimum and bias again. After you've done this once you can skip the initial bias and just check after music listening every few days as the tubes settle in and everything adjusts to the voltage at your house. Wall voltage fluctuations will effect bias, and it usually varies by time, so it's a good idea to bias the amp around the time you usually listen to music, or bias a little lower in the afternoon so they'll still be in a good range when wall voltage increases at night. Once everything is settled you should only have to check bias every couple of months. I do it weekly anyway because I'm paranoid and we live in an area with a very old power grid
     
  10. penguins

    penguins Friend, formerly known as fp627

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    bobboxbody - wow... a very well thought out restoration.

    and nice tea :)
     
  11. Erroneous

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    @purr1n How does it sound? This one made my Spidey senses all tingly. Was I right?
     
  12. bobboxbody

    bobboxbody Friend

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    Feel free to blast away if you have dislikes, I won't be offended. I'm an amateur who enjoys doing restorations, but have no deep knowledge of circuits.
     
  13. purr1n

    purr1n Desire for betterer is endless.

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    This is fantastic. It was already very good, until I cranked up the bias (@bobboxbody ran it conservatively). It was magical after I cranked the bias up. Will write a full review on it. @Josh Schor is selling me back the Fisher amp too that @bobboxbody is working on. I plan to have both of these together to do a comparo. They are both very good. I don't feel any today's PP amps are any good. They either cost thousands of dollars like the Leben (which last time I heard two of their models, sucked balls), are cut-down cheap imitations with inferior transformers, or horrid Chinese POS. It's the transformers from back the day. I used to HATE PP tube amps until I got a chance to hear the classic American stuff from the 60s just before the dawn on the transistor, which wrecked it all.
     
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  14. Josh Schor

    Josh Schor Friend

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    I agree whole heartedly, the 60's PP amp were fantastic. I am on the look out for Fisher mono blocks 50 watts each run with an autoformer pre, that will be beyond beyond
    with the bias yes I know it sounds better(same with the fisher) and it burns out the tubes much quicker. a trade off thats worth it
     
  15. purr1n

    purr1n Desire for betterer is endless.

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    LIght 'em up!

    Sure I hate spending so much on "wearable" items, but life is too short.
     
  16. dBel84

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    Just picked up a pair of original Dynaco ST35 transformers for this exact reason.
     
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