HH Scott LK-48 Restoration/Mods

I recently finished restoring an early model HH Scott 222d integrated amp, which is actually the circuit from the 222c, just with a different face plate. Along the way I decided to add the biasing circuit from the 299b. It turned out great and unseated my Ragnarok 2, not on technicalities, but on musical enjoyment. I've come to accept that I just like vintage stuff and I like tubes and I'm ok with that. Then I came across a cheap Scott LK-48 for sale locally and grabbed it. It's the same circuit as the 222c, but again with a different face plate, and uses 7199 tubes for the phase inverter instead of the 6UB tubes in the 222c/d I restored already, although some 222c's did use the 7199. Anyway I'm starting to restore/mod the LK-48 and will keep a log here.

I forgot to take before pictures, but the first order of business was to clean off the dust/grime and remove the output transformers and power transformer bell cap for a repaint. They looked similar, but a little worse, to the condition of the transformers on the 222c, which I failed repaint and will do later. Here's the unpainted transformers on the 222c/d


LK-48 with transformers and bell cap removed:


Face plate:


Repainted iron back on the chassis:


To be continued.

 

Next up was to remove the selenium bridge rectifier that supplies DC for the balance pots and preamp heater string. Selenium rectifiers age poorly and all of them eventually fail catastrophically usually taking out the transformers with them, so it's being replaced with a silicon bridge rectifier.
Here's the selenium rectifier de-soldered and unclipped:


I'm using the holes where the selenium rectifier was secured to put in the new bias pots, the holes just need to be widened with a step bit, and new small holes added for the anti-rotation tab on the pots:


New bias pots installed:


I moved the silicon rectifier to the terminal strip that held the dropping resistor for the rectifier voltage, which feeds the 12ax7 heater string and used to feed the DC balance pots, but those will now be fed with the individual bias pots.
Terminal strip with dropping resistor:


Removed and replaced with a larger one to accommodate the silicon bridge rectifier:


Bridge rectifier mounted on the strip with a little heatsink that probably doesn't do much for heat dissipation, but holds the rectifier securely.

 

Here are the shafts of the bias pots on the top of the chassis. I decided to use the locking type for this one because tiny adjustments are needed for dialing in the bias of the power tubes and the ones I used on the 222c/d turn a little too easily.


And the bias pots wired underneath to feed the dc balance pots:


I'm waiting on reproduction can caps from Hayseed Hamfest before I do anything else, since I need to fire up the amp to determine the values for the dropping resistor feeding the preamp heater string and the voltage feeding the bias pots. I'd prefer to do it with new electrolytics rather than 60 year old ones.

Edit: Bonus pic, NOS Soviet military K40Y-9 pio coupling caps just arrived from Ukraine:

 

Nice. I have a pair of HH Scott 99 mono integrated amps (refurbished by someone else) that I use - one per channel and I really enjoy them.

 

The reproduction multi section can caps arrived from Hayseed Hamfest and are now installed.

Buying reproduction multi section caps isn't the only option for these old amps. Lots of people cut open the old cans and re-stuff them with electrolytics of their choosing, but mine were beat up and didn't match visually and I didn't want the hassle, so I paid the $30ish premium just to get the drop in replacements. They're the same as the originals but with higher filter capacitance for the preamp stage, higher voltage ratings all around, and are made with modern electrolytics as opposed to 60's technology. Another alternative is to leave the old can caps in, but disconnect them and add new caps on the under side of the chassis, not a horrible option, but these tend to run pretty hot and I want to leave as much airflow as possible.

The last thing before installing the new caps was to widen the holes for the vents between the pairs of output tubes and install the test tip jacks that will be used for biasing. Here's 2 in with 2 ugly holes on the underside. Usually my trusty Irwin step bit does a better job, but I don't have a drill press, so it got a bit sloppy, but the ugly is covered by the jacks anyway.


And here's up top with all 4 installed showing how nicely the meter probes fit.


I lied. Before I moved on to the cans, I also first desoldered the cathode ground and bias resistor for the 7189 output tubes so I could test the resistor values and get ready for the new star grounding and bias sense layout. While I was at it I re-twisted all the wire pairs for the heaters since they were sloppy and one side runs close to the preamp section. The resistors were all out of spec and will be replaced. Here's the first socket:


And tightly twisted brown and grey heater string:


Another side note, desoldering kind of sucks and solder gets flung around a bit so I keep the shop vac handy to suck up stray solder bits when I finish with each tube socket:


Here's the underside of the first multi section cap, it's good to have a picture of all the components before you start since they'll have to go back in the same place on the new cap and an easier visual reference than the schematic.


I also like to take a second pic for the wire connections after all the resistors are out of the way.


Old cap removed on the right and replacement on the left.


New cap in, then wired back up and resistors installed, then top view.




It's a good opportunity to clean formerly blocked portions of the chassis while the cans are out, so I did that along the way. I won't show pics for the other 3 since it's the same process and this already has a million pics. One thing to note when removing the old caps is that the fiberboard or whatever the insulating base riveted to the chassis that they slot into is very brittle. The tabs that you twist to secure the caps have wings that spread as the tab is twisted, so care must be used to get everything out smoothly without shattering the insulator. The part I'm talking about is shown in the top left here with the can removed:


Finally, here's all three new caps installed and the chassis pretty well cleaned up:


Coupling caps and bias sense/star grounding layout up next.

 

if you wanted to retain that NOS cap appearance - some folk remove the old cap from the sleeve and then use the sleeve to cover the new caps. I personally like the new green , but offer this up as a trick some restorers use..dB

 

That is exactly what the tech did on the restoration of one of my Scott amps (without me asking)......

 

It is the cheaper option for diy, depending on which caps you choose to put inside, and maybe a cheaper alternative for the tech, depending how long it takes him and how much he charges per hour. One of the problems is that the cans are secured by tabs that you twist when you install them, and usually little wings spread out to further secure them in place. It can be tricky to get them out with the tabs intact and without damaging the brittle phenolic/fiberglass/whatever it is socket that the cap slots in to. I'll probably go that route on the next project but this was the easiest option while I'm still a relative novice.

 

Next thing was to set up the group of current sense resistors that will connect to the test tip jacks for measuring bias on the output tubes. A bus wire running from the cathode pin to a 10ohm 1/2W resistor allows the bias current to be read on a DMM as millivolts and acts as a fuse if the tube starts to draw too much current. One end of the resistor goes to the test tip jack and the other to common ground. I installed a 5 lug terminal strip with the center lug going to ground and serving as the star grounding point for all of the other ground bus wires, which is supposed to reduce noise in the circuit. I'm sure most folks here know more about how circuits actually work so I'm not going into detail (a) because I don't really know any more detail, and (b) because I just read things and then follow instructions.

Here's the terminal strip installed with the first resistor and wires from the first pair of output tubes attached.

Now both resistors installed and wired up to the test tip jacks for the pair.


Here I combined some ground bus wires to run to the central grounding point on the terminal strip.


Finally all resistors, ground wires, and jacks connected. It's starting to look like a spaghetti monster. You can also see the output coupling caps at the back there, I failed to take progress pics for those.


Next was to start the majority of the recap. The K40Y-9 caps have a conductive metal case which probably wouldn't cause any problems, but I saw that the Abbas man wrapped his in clear heat shrink on the el84 phono stage, so I decided to do the same. Here are some of the small coupling caps for the phono stage next to one of the originals.


And the re-capped phono section.


Before replacing the phase inverter coupling and grounding caps I needed to replace one of the small terminal strips with a larger one so there would be a convenient place for the filter caps and resistors for the bias pots. Here's the small one to be replaced.


And the larger one installed, along with the first coupling cap. The two empty lugs on the left will be for the bias pots.


The new caps are, in some cases, much bigger than the crappy old ones, so I'm using a ruler to imitate the bottom of the case to make sure everything has clearance.


I'm also replacing all of the bias resistors for the output tubes as I go. Some people think the old carbon comp resistors have magic juice that make the amp sound better. I do leave all of the resistors that are still within spec, but these are all out of spec, most were too close to the heat of the tube base, and I don't believe in magic juice. Shiny pretty new blue Vishay 1% metal film resistors for the first output tube.


Two completed sets of resistors and caps, plus giant grounding caps installed.


I messed up and got 400V rated for the grounding caps instead of the specified 200V, I don't think it will cause any problems, they're almost too big to fit, but they passed the ruler test after some rearranging and smashing. Here's a comparison with the original.


Finally, everything completed for the phase inverter and output stage, all that remains are the coupling caps for the preamp line stage and then testing to determine values for the rectifier dropping resistor and low limit resistors for the bias pots, which I may even finish today.

 

Welp, I finally have the recap done and it's time to do some voltage tests to determine the correct value for the bridge rectifier dropping resistor and the low limit grounding resistors for the bias pots. So, I hook up the dummy load for the speakers and install all the tubes minus the GZ34 rectifier(since I'm just checking bias voltage I don't need it and if something's wrong having it out should save the tubes from being damaged) I turn it on and after a few seconds the dropping resistor starts smoking. I thought that was odd since the 3w resistor in the other amp has been doing fine for months now, so I check the bridge rectifier and it's wired in reverse polarity.



Luckily nothing got destroyed but the resistor.

Anyhoo, here's some recap pics.

Before recapping the line stage I remembered that I wanted to install some CL-80 current limiters on the AC line in. Since I increase the capacitance of the filter bank the CL-80's will act as current inrush protectors and they should drop the incoming voltage by 2-2.5 volts each, bringing the mains 122VAC down to 117-118 which is more appropriate for the original design.
The CL-80's are thermistors, so they get dissipate the extra voltage as heat which means they need some decent clearance from other components.

Here's the first one getting ready to be soldered to the AC line and then have the leads insulated.


And installed.


And one on the other side.


Next up,removing the old caps and corresponding resistors from the underside of the tube sockets.


I always try to pull the resistors first so they're exposed to as little heat as possible since I'll be reusing them as long as they're still within spec. Once everything is pulled out, I clean up the solder lugs with some braided solder wick.

So clean.


Here's a pile of crusty caps and out of spec resistors, a small portion of what's been removed from the amp so far.


Everything cleaned up and out of the way.


Here's a couple of the coupling caps to go in, one with the heat shrink tubing on and one yet to be heat shrunken, in case anyone's interested in the difference.


New caps all in.


The bias pots have 3 lugs each, the first lug is connected to the negative voltage coming from the bridge rectifier. The middle lug is connected to the wiper of the pots and goes to the balance pots, which then distribute the bias voltage to the output tubes, I'm using a 50uf 100V electrolytic capacitor as a ripple filter for the bias voltage, so that runs from the center lug to ground. Since it's negative voltage, the negative lead of the cap attaches to the lug and the positive lead goes to ground. The final lug determines the low end of the pot's adjustment range with a resistor going to ground. The goal is to have a range of about -10 to -23V per tube, I'm starting with a 15k ohm resistor and will test to find the correct value. Since I don't want to solder anything in yet and I need room to change out the resistors it makes for a janky and ugly setup while testing.
Here's a pic of the ugly.


Now I'm off to re-wire the bridge rectifier correctly and start voltage checks, I'll probably report back later from my phone while standing outside the smoldering ruins of my house.

 

Welp, so I did finish up everything I had planned the next day, and no homes were burned, but there was an awful buzz making it's way from the power transformer to the speakers that I've been trying to track down and eliminate ever since. I haven't done a great job of taking pictures while making changes to try to get rid of the buzz, but here's some of the process.

HH Scott claimed in marketing material back in the 60's that all of their amps are stable with any load, including no load, but since these are now 60 year old transformers, I decided to make some dummy loads anyway. I had a few spare cheap banana connectors from amazon and some 8ohm 30W resistors, so I mashed them together.



You may notice the blue painter's tape rolls under the amp in the above picture, the tops transformers and the edge of the faceplate are resting on those to give clearance for the tubes while voltages are being checked. Generally you shouldn't power up a tube amp without the tubes and a speaker load. As an alternative when I need access to the top of the amp for the bias pots and the underside for voltage readings, I also prop it on it's side, with a smaller roll of painter's tape under the side of the power transformer. Why rolls of painter's tape? Because we bought a house last summer and repainted everything.



Testing confirmed that, amazingly, the guesses for resistor values for the bias supply were correct, so I soldered them in.
Then I hooked it up to speakers and heard the 120hz buzz and a pretty bad channel imbalance. I decided to re-check resistor values in the line stage and volume/balance controls. I found a few that were within the loose 10% tolerance, but pretty far off at opposite ends of the 10%, so those were replaced.

Volume pot to balance pot:


And line stage, at the top of this image, which is also a reference shot for the sockets of the 7199 phase inverters:


The resistor changes didn't solve the channel imbalance, so my next guess was the 7199 tubes, which are pretty rare and expensive. Later models of this amp switched to an arguably better ECF82/6U8 tube which is also cheaper and more abundant, so I decided to rewire the sockets. Checking schematics for both 7199 and 6U8 versions of the amp I found out that it didn't require the value of any components to be changed, just switching pin connections. All pin 2 connections had to be moved to pin 6, pin 6 to pin 7, and pin 7 to pin 2. Musical chairs. Here's the first socket with everything switched up.



And then everything "complete" again:


This change fixed the channel imbalance, but did nothing for the hum. Internet searches, mostly on Audiokarma.org, told me that possible culprits were: Bad electrolytic filter caps(just replaced with new ones, so ruled that out), poorly twisted or poorly dressed leads for the heater wires(just redid all of those, so ruled that out), and then general poor quality of the original hookup wire in the power supply section. I also learned that 120hz hum typically is power supply related, and 60hz hum usually points to a grounding issue. Since I have 120hz noise I decided to rewire the power supply with new, better insulated, higher gauge solid core hookup wire.

Twisting up a new pair:


Really ugly old soldering:


Better twist for the PT taps going to GZ34 socket, better dressed leads for resistors, and all new power supply/filter wiring:


Hooked it back up and the buzz was as bad as ever. I recently found another post on AK for filtering the heater supply for the output tubes, I ordered the parts, so that's up next. To be continued.

Edit: VICTORY IS ACHIEVED!!
I had some parts lying around to test out the filter for the power tube heater supply, tacked it in, and hum is finally gone. When the real parts show up on Monday I'll solder them and be done? The power transformer still buzzes which is worrisome, but the noise no longer makes it to the speakers. There's a company not far from me that does custom transformers who gave me a quote of $160 for a replacement. It's probably worth it, idk.

Glorious buzz-free testing pics:

 

really beautiful work, wow. I have zero skills and just blow things up so to see someone with real skill is wonderful. Enjoy
best,
Josh

 

Thanks for the praise, but I really just search for guides and solutions to problems online and then follow directions. I would love to find a real guru to teach me to actually understand what I'm doing to a more significant degree. Multiple attempts at teaching myself any kind of circuit design/electrical theory have failed miserably. I have several partially read books on my shelf, but the information fails straight through my sieve-like brain.

 

The LK48 is finally done. The extra little filter for the 7189 heater supply is super simple, just a 100 ohm resistor from each heater pin on the first 7189 socket filtered through a 0.1uf cap to ground. The rest of the sockets are daisy chained off of the first so everything gets clean power. It should have already been cleaned up by the main PS filter cap but I suspect my power transformer is not what it used to be 60 years ago. Despite all of that however, with the output tubes biased up to 23ma at idle, all voltages are within 5% of spec, which is pretty darn good. I decided to ditch the CL-80's on the AC line, just in case they were leaking AC back into the power transformer somehow. So I'm running the amp off of a variac set at 117V rather than subjecting it to the 122V at my house. It'll be a while before the new caps break in but the amp already sounds great. I have no testing equipment besides a DMM so no measurements, I might try to figure out ARTA and see if I can get any rudimentary measurements using the tape out on the amp and the line in on my motherboard, but I don't know if that will work. I also don't care how it measures as long as it sounds good blasting John Prine. The amp came with a full set of early-mid 60's Telefunken 7189's and 12AX7's, and I had Telefunken 6U8's from the 222c which will be living on the LK48 now, along with a NOS Mullard Blackburn GZ34, not a fat base, but also didn't cost $400. Here's the pics.

Heater supply filter:


Glowing fire bottles:




And celebrating with the last of my winter '22 Ali Shan "Secret Garden" high mountain oolong:


Thanks for tuning in!

 

I repainted the transformers on the 222C/D to get it ready for sale now that the LK48 is complete, then put it back in the system just to make sure everything is functioning properly and found to my chagrin that it kicks the shit out of the LK48 sonically. It even has "inferior" output tubes - NOS Reflecktor 6P14P-EV as opposed to the vintage Telefunken 7189 "Holy Grail" tubes. It's the exact same circuit with the same modifications, the only difference besides the tubes being that I recapped the tone control section on the 222 and left the original caps in the LK48 since they're not exposed to high heat/current and some folks claim that's where the "sweet" sound comes from. So I guess I'm selling the beautiful LK48 and keeping the ugly 222. Probably easier to sell the pretty one with magic tone control caps and mythical tubes I guess. Just sad because I wanted the best sound and the best eye candy, and a gold plated house, and a rocket car. I'm a simple man really.

 

Congrats!

Really really thought this was a recreational tincture of something else at first...

 

Hah! I do see the similarity, but I delved a bit too deep in all things recreational tincture related for a good long time, now I'm a boring old man who prefers regular old tea.

 

Great job! Lots of work there (of course not really work if you enjoy it.) Hope the results are worth it.

One thing about the heater filter: You said in the text that the resistors were 100k, but the stripes look like 100R.

I ask because I'm thinking of trying the same thing to cure a bit of hum in my DIY amp.

 

You are absolutely correct it was 100 ohm not 100k. Just a little bit of difference, whoops. I edited the post to fix it, thanks for catching that.
I did enjoy it for the most part and I learned a good bit along the way even though it was my second restoration of the same circuit. I've always liked making things and fixing things in addition to hifi so it's a good hobby for me. There's so much to learn that I think it'll remain challenging for a long time to come, which keeps it interesting. There's definitely a big change in sq with the recap on these. From what I understand the cericaps Scott used weren't all that great to begin with and 60 years of aging hasn't done them any good. The big surprise for me was the recent discovery of how much difference there is between the unit with the stock tone control caps vs the recap(not even fancy caps, just orange drops). Stock sounded great when I finished and started listening but very tubey in the old school rolled off, syrupy sort of way. The unit with recapped tone controls just sounds so much more lively and detailed in direct comparison that I feel like I'm losing too much with the stock caps. I'll decide which one to sell pretty soon and maybe give the buyer the option of having the recap for a little extra cash, it was a huge pain in the ass to get all those caps crammed inbetween pots and rotary switches, etc.

 

FWIW I ran back through a voltage check after installing the heater string filter and it didn't throw anything off, nor did I notice any sonic differences, besides the lack of 120hz buzz, so at least in this circuit it's a fairly innocuous mod. It's unlikely to do much if anything for 60hz hum though, that's most likely a grounding issue somewhere.