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 Post subject: Re: First-time Restoration: Scott LK-72 and LT-110
PostPosted: Apr Mon 26, 2021 5:19 pm 
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I also have a color scan of the assembly manual for the LT-110 as well as some service bulletins (which you may already have).

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 Post subject: Re: First-time Restoration: Scott LK-72 and LT-110
PostPosted: Apr Mon 26, 2021 6:54 pm 
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BTW, on the LK-72 the larger knobs go on the bottom row, and the smaller ones go on the top row. I know, rather anal, but thought you might want to know.

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 Post subject: Re: First-time Restoration: Scott LK-72 and LT-110
PostPosted: Apr Tue 27, 2021 6:58 pm 
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Don Cavey wrote:
BTW, on the LK-72 the larger knobs go on the bottom row, and the smaller ones go on the top row. I know, rather anal, but thought you might want to know.


Oh, man - I will have to change this once it arrives in DE to get the cleanest look ;-). Thanks...

I'll check about the LT-110 once I see it in person again, my Dad said there are several manuals and diagrams, not sure if it's complete. Will get back to you if something is missing.


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 Post subject: Re: First-time Restoration: Scott LK-72 and LT-110
PostPosted: Apr Wed 28, 2021 12:23 am 
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one51 wrote:
Don Cavey wrote:
BTW, on the LK-72 the larger knobs go on the bottom row, and the smaller ones go on the top row. I know, rather anal, but thought you might want to know.


Oh, man - I will have to change this once it arrives in DE to get the cleanest look ;-). Thanks...
It is a bit counter-intuitive having one size knobs for the treble controls and a different size for the bass controls but I guess some will say it is more esthetically pleasing to have the smaller knobs on the top row and the larger knobs on the bottom row. I notice that Scott solved the problem with the "B" by having concentric tone controls.

Attachment:
File comment: Web Image (I don't have one)
Scott LK-72B.jpg
Scott LK-72B.jpg [ 93.75 KiB | Viewed 1528 times ]


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 Post subject: Re: First-time Restoration: Scott LK-72 and LT-110
PostPosted: Apr Wed 28, 2021 12:46 pm 
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Nice, Dave. I have one of them too. When I was younger, I was all about the look of the barrel shaped knobs and "B" style faceplate. But as I got older, I came to appreciate the brown faceplate with the original brown knobs. Today, they are my favorite. One can not have enough vintage HH Scott vacuum tube equipment. But as I have matured (READ: gotten older!), I realise that I must downsize. I am in that process, albeit slowly.

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 Post subject: Re: First-time Restoration: Scott LK-72 and LT-110
PostPosted: May Sun 02, 2021 3:26 pm 
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One question I've been searching the web for is the electrolytic cans. It looks like replacements in the "original" style are hard to come by, expensive, or not the exact needed values. If I order something new I might do so in the US and bring it here with the amp, unless I find a good EU source.

There are some good threads about how to put new electrolytics inside the can while preserving the original can + appearance (e.g. https://audiokarma.org/forums/index.php?threads/clean-can-capacitor-rebuild-scott-owners.750137/. This looks like a pretty good and long-lasting method that will last until the next person inherits this amp ;-) Thoughts?


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 Post subject: Re: First-time Restoration: Scott LK-72 and LT-110
PostPosted: May Sun 02, 2021 9:14 pm 
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I "restuff" original can capacitors, sometime referred to as "FP" capacitors, the designation that Mallory used. It takes me about an hour to stuff a capacitor. But if you don't care about originality, JJ makes several capacitors that you can use. But they are clamp mounted capacitors, not unlike what the Japanese did.

Hayseed will make just about any can electrolytic capacitor that you want. They are not cheap but they look a lot like the originals albeit a bit shiny. Link below; I have not used their product nor have I any affiliation with them.

https://hayseedhamfest.com/

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 Post subject: Re: First-time Restoration: Scott LK-72 and LT-110
PostPosted: May Sun 02, 2021 9:30 pm 
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Hello Dave,
What great unit and welcome aboard


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 Post subject: Re: First-time Restoration: Scott LK-72 and LT-110
PostPosted: May Sun 02, 2021 10:18 pm 
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Don Cavey wrote:
I "restuff" original can capacitors, sometime referred to as "FP" capacitors, the designation that Mallory used. It takes me about an hour to stuff a capacitor. But if you don't care about originality, JJ makes several capacitors that you can use. But they are clamp mounted capacitors, not unlike what the Japanese did.

Hayseed will make just about any can electrolytic capacitor that you want. They are not cheap but they look a lot like the originals albeit a bit shiny. Link below; I have not used their product nor have I any affiliation with them.

https://hayseedhamfest.com/


OK, so I'll probably look at restuffing it with this method from Audiokarma or a similar one. Hayseed looks interesting, but I guess that's the same result as if I were to restuff with high-quality caps, and it somewhat spoils the vintage look ;-). I suppose that will be the trickiest part, if nothing major is broken.

Thanks radiorich for the nice welcome!


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 Post subject: Re: First-time Restoration: Scott LK-72 and LT-110
PostPosted: May Sun 02, 2021 11:18 pm 
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I know my opinion below will not be widely shared here but let me give another sound:
I don't like "restuffing", it's a lot of work for dubious results at best... most of the "restuffing" I've seen were done with cheap small modern radial electrolytic caps which are not made (or rated for) the high ripple current found in HV filter circuits in tube amps. They will have a very shortened life (especially enclosed in a high temperature unventilated environment) and reliability will be poorer than the old 60+ years old caps currently in your amp ! Bigger (computer grade quality) caps won't usually fit into the original can. I don't like under chassis axial caps bridging either , or other butchery that some would call "restoration"
High quality vintage tube Hi-Fi gear (like your Scott's) desserves a good restoration job, both electrically and cosmetically. If you decide to do it, please do it right.

Since you're located in Germany why not use Authenticaps sold by AskJanFirst (Jan Wuesten, no affiliation). These are very high quality Twistlock FP type caps (manufactured in germany by F&T), mechanically and electrically fully compatible with the original parts used in most U.S made amps. No sawing, restuffing, glueing, additional holes in the chassis, etc...
Authenticaps are available in various configurations and rated up to 550V (600V surge !) which leaves you with a wide safety margin. Their type KTL9 and KTL14 will solve 90% of your restoration needs. They also have low voltage cans for DC heater/bias circuits, even some with positive can !
Of course they are more expensive than a few cheap asian (or unknow origin) radial "restuffing" caps.
But at the end it's up to you to decide the way to follow...


https://www.die-wuestens.de/eindex.htm


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 Post subject: Re: First-time Restoration: Scott LK-72 and LT-110
PostPosted: May Mon 03, 2021 10:23 am 
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Tubologic wrote:
I know my opinion below will not be widely shared here but let me give another sound:
I don't like "restuffing", it's a lot of work for dubious results at best... most of the "restuffing" I've seen were done with cheap small modern radial electrolytic caps which are not made (or rated for) the high ripple current found in HV filter circuits in tube amps. They will have a very shortened life (especially enclosed in a high temperature unventilated environment) and reliability will be poorer than the old 60+ years old caps currently in your amp ! Bigger (computer grade quality) caps won't usually fit into the original can. I don't like under chassis axial caps bridging either , or other butchery that some would call "restoration"...


Thanks for the compliment! Some of actually do look at ripple current and do not necessarily get "dubious results". Many of those "dubious" radial lead capacitors are rated 105°C with similar ripple current specifications. Electrolytic capacitors from Nichicon, Panasonic etc. have improved a little since 1961.

But again, everyone is entitled to his/her decisions.

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 Post subject: Re: First-time Restoration: Scott LK-72 and LT-110
PostPosted: May Mon 03, 2021 2:19 pm 
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The Authenticaps look interesting, except for the price, where probably I'd be looking at €200 just for 4 capacitors. And they seem to be can negative, whereas I read the Scott amps are can positive (?). I guess I will take a look into it when I finally get the amp here and see how things are looking. Anyway, I can always try re-stuffing with modern electrolytics and keep the expensive Authenticap or Hayseed options in my back pocket just in case.

Thinking of transport: I'll have to check if the vacuum tubes are OK to bring on the plane in a carry-on. This might be the safest way to get the original tubes here.


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 Post subject: Re: First-time Restoration: Scott LK-72 and LT-110
PostPosted: May Mon 03, 2021 5:17 pm 
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Most of the electrolytic filter capacitors are common negative (the can). But the one four section can, which is 75 µFD @ 75 VDC is the filter for the output bias supply and preamplifier tube filaments (4 X 12AX7), and is a positive can. You need a negative supply for the grid bias on the output tubes, therefore common positive.

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 Post subject: Re: First-time Restoration: Scott LK-72 and LT-110
PostPosted: May Tue 04, 2021 8:50 am 
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Don Cavey wrote:
Most of the electrolytic filter capacitors are common negative (the can). But the one four section can, which is 75 µFD @ 75 VDC is the filter for the output bias supply and preamplifier tube filaments (4 X 12AX7), and is a positive can. You need a negative supply for the grid bias on the output tubes, therefore common positive.


Aha, that is good to know, had not seen details about this difference. Thanks!


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 Post subject: Re: First-time Restoration: Scott LK-72 and LT-110
PostPosted: May Thu 06, 2021 1:54 am 
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You came to right place for real advice on restoration without hype! (here hype gets sniped)
I have some schematics I can scan if Don doesn't have all you need I've acquired for my store-bought 222C amplifier, (Came from a local logging baron's estate) which as posted previously is the same amp as the kit built LK-72 assembled by your Grandfather.
Send me a PM with your email if you need further info other than what Don can provide and definitely save all your original tubes as likely the quality cannot be replaced today,
unless you own a gold mine and have a generally benevolent attitude and tolerance level.
Removing the front panel for transit to Germany on the amp is probably a great idea as well.
Best of luck with the project!

EDIT: Sorry I posted about materials without realizing I have probably the store bought version of the HH Scott kit LK-48? , not LK-72,
as I just noticed the 7591 (or similar) octal power tubes in your photo, which are just as difficult to find as the 7189 tubes.

Attachment:
H.H. Scott 222C 3 10 21 2.JPG
H.H. Scott 222C 3 10 21 2.JPG [ 646.09 KiB | Viewed 1394 times ]

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 Post subject: Re: First-time Restoration: Scott LK-72 and LT-110
PostPosted: May Thu 06, 2021 10:06 am 
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ampstamp88 wrote:
You came to right place for real advice on restoration without hype! (here hype gets sniped)
I have some schematics I can scan if Don doesn't have all you need I've acquired for my store-bought 222C amplifier, (Came from a local logging baron's estate) which as posted previously is the same amp as the kit built LK-72 assembled by your Grandfather.
Send me a PM with your email if you need further info other than what Don can provide and definitely save all your original tubes as likely the quality cannot be replaced today,
unless you own a gold mine and have a generally benevolent attitude and tolerance level.
Removing the front panel for transit to Germany on the amp is probably a great idea as well.
Best of luck with the project!

EDIT: Sorry I posted about materials without realizing I have probably the store bought version of the HH Scott kit LK-48? , not LK-72,
as I just noticed the 7591 (or similar) octal power tubes in your photo, which are just as difficult to find as the 7189 tubes.


Very nice looking amp! Thanks for the schematic offer. I have some that I found on a website (not HHScott as that always seems to crash for me). Once I get the unit here from the USA I'll have to compare a bit and see if the schematic I found is the right one.

I might visit family at home in the summer, and then bring back the amp + tuner as my luggage, with all tubes in carry-on. If that works out I'll be shopping for soft foam to make a luggage-filling cutout to hold the two chassis ;-)


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 Post subject: Re: First-time Restoration: Scott LK-72 and LT-110
PostPosted: Jun Wed 16, 2021 1:41 pm 
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Update: I'll be picking up the amp + tuner in the US in early July. My plan is to remove the tubes and carry them on the plane, while checking the two chassis in a large box with nice cut-out soft foam as packing material. TSA doesn't seem to have an issue with vacuum tubes in a carry-on.

At the end I went with a Hayseed Hamfest cap set for the LK-72, will pick that up in the US as well. Since I only plan to do this pair of amps, I figured it's not worth the learning curve of re-stuffing the electrolytics and risking a mistake (or ugly-looking cans afterwards).

I think the selenium rectifier on the LK-72 (assuming that's what it has) needs to be replaced with a silicon one? Any suggestion on what model to use and other tweaks (I read the resistor value should be adjusted)? I'll buy all the remaining parts (cleaners, rectifier, smaller caps to replace the paper ones) after getting the units back here to Germany.


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 Post subject: Re: First-time Restoration: Scott LK-72 and LT-110
PostPosted: Jul Fri 09, 2021 9:27 pm 
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UPDATE

The tubes and Hamfest caps are back in Germany (carry-on). The luggage (a large box) with tuner/amp packed well with foam+styrofoam should arrive shortly as well, after some detours Newark-Boston-Newark before Germany (should have landed today in Deutschland).

So my current opens are the selenium bridge,
>>I think the selenium rectifier on the LK-72 (assuming that's what it has) needs to be replaced with a silicon one? Any suggestion on what model to use and other tweaks (I read the resistor value should be adjusted)?

and the strange tube complement, especially in the power tubes. Is it OK to mix 6GH8 and 6GH8A? More concerning is the 7591/A mix from 3 different brands as they can't be well matched (which may have caused the death of the originals in the first place). Should I look for a matched quad to avoid bias issues, as I think it's not adjustable on this unit?

V1-4 Telefunken 12AX7 (whew)
V5 Realistic 6GH8A
V6 Telefunken 6GH8 (?) - markings rubbed off
V7 GE 7591 A
V8 GE 7591 A
V9 HH Scott 7591 -- I guess the only original power tube, and only one with no dark marks besides the square holes in the outer screen
V10 Sylvania 7591 A
V11 Realistic 5AR4/GZ34

Cheers,
Dave


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 Post subject: Re: First-time Restoration: Scott LK-72 and LT-110
PostPosted: Jul Tue 13, 2021 9:55 pm 
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New components & lots of tools ordered. Will try to get a matched set of new Tung-Sol 7591A's as well.

Attachment:
DE-Amp_and_Tuner.jpg
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Tuner-Wiring.jpg
Tuner-Wiring.jpg [ 1.16 MiB | Viewed 804 times ]


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 Post subject: Re: First-time Restoration: Scott LK-72 and LT-110
PostPosted: Jul Thu 22, 2021 10:26 pm 
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Help! I've made quite some progress on the LK-72 and now am down to the tricky part: re-wiring the output stage to include an IBAM circuit (found on Audiokarma, https://audiokarma.org/forums/index.php ... 692/page-5 post #98) and adding sensing resistors. The challenge is this guy had an earlier version of the amp, whereas I have the added DC balance pots (which are apparently not needed anymore with independent bias adjustment).
Attachment:
Amp-072221-Status.JPG
Amp-072221-Status.JPG [ 1.11 MiB | Viewed 670 times ]


Below is the current status:
  • Most caps replaced: orange drops & hayseed hamfest; just a few left that are tricky to access
  • Rectifier replaced with Si including an additional resistor to compensate Silicon vs Selenium efficiency (let's see what voltage it gives me)
  • IBAM circuit built, not yet soldered in
  • Balance pots removed, to be replaced with IBAM (new terminal strips added for easy access), Independent Bias Adjustment Modification - tiny PCB in upper right corner
  • Broken fuse holder replaced, dent in rear panel where it got hit (probably in luggage "throwing") gently pounded back flat
  • New production Tung-Sol 7591A matched set is arriving next week with a buddy who's coming back from the US

Questions from the mechanical engineer, who can do the fine soldering & measurement but doesn't understand the circuit 100%:
  • This IBAM circuit has a different resistance (around 12-22k per output tube) than the DC Bal pot it replaces (avg 250k per output tube if I understand it right). Do I just wire it directly into the 270k resistor, and add an additional resistor in series like 220k? Or, do I remove the 270k resistor and reduce the value of the 330k grid resistor to 180k or 220k?
  • I plan to add 10 ohm resistors from Pin 5 to GND to measure idle bias current. As I understand Pin 5 is suppressor/cathode (combined), I have to rewire the ground connections a bit and will use my new terminal strips for that. Does that make sense?
  • I have a recommendation to add "screen stopper resistors" 220-275 ohm at each 7591 pin 8 (which is shorted to pin4, and all the screens are connected with the orange wire in the picture) - as I understand this protects the screen at high drive levels. Does that sound good and what is the other connection point of this resistor x4 -- do I then need to rewire so all pins (4x pin4 and pin8) are not shorted in a row, but 4 parallel strings of pin4-pin8-resistor-Vscreen?
  • I plan to set a bias current of 32-35mA per tube with this modified configuration. High voltage Kleps delivered yesterday, variac tested & working fine, 240-120 voltage transformer is standing by ;-).

Thanks for any help to get the last steps of my soldering done without too much rework/experimenting ;-) BTW below is a close up of one of the power tube sockets,

Attachment:
Amp-072221-PowerTube.JPG
Amp-072221-PowerTube.JPG [ 789.07 KiB | Viewed 670 times ]


Cheers,
Dave


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