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Old 16th October 2006, 05:35 PM   #1
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Default Advice for NOOBie - Dad's Scott LK-72

Let me first say I have simply a basic understanding of electronics, have dabbled with passive crossovers, never touched an amp before - I don't have many tools, only an lcr, dmm, soldering gun...

Just yesterday I was thinking about building a kit tube amp and was checking out some threads here and looking other places on the internet. Later in the day I stopped at my brother's house and mentioned that I had been playing with loudspeaker design, had gotten back into listening and was thinking about tube amps.

"Well I've got Dad's amplifier in the basement" he says...
"Really? hmmmm" - my eyebrows raised
"You can have it if you want, It has been sitting down there for 20 years"

20 years, uncovered, in my brother's basement isn't good for anything - but I brushed the 1/8" layer of debris off of it and threw it in the trunk of my car. Got the assembly book and also the tuner Dad built to go with it in the early 60's.

Overall, I have to say it looks to be in pretty good shape, front panel is nice, everything appears to be intact. - but I understand appearances don't matter much.

Of course, what is the first thing a genius like me does with a newfound 45 year old tube amp? Yup - A nasty hot red glow from one of the 7591's convinced me to shut it down after only 20 or 30 seconds, two did the nifty blue thing, one sat quietly dark.

So after doing some research I find that a bunch of stuff should really be replaced right from the start - all of the tubular caps (20 of them), the selenium rectifier, the large can caps and according to my measurements some of the .5 and 1 watt resistors are really pretty far off. Add all that up including new 7591's and maybe the rectifier tube and I'm probably approaching $200?

Finally, my questions:
1. Is it practical to rebuild an LK-72?
2. Is it practical for me to try and do it or am I probably just going to run into real trouble?
3. Can anyone comment on specific components that should be replaced due to age, or at least checked? Resistors change with age?
4. Should I quietly go back to looking at kits?

Thanks, Ed
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Old 16th October 2006, 06:16 PM   #2
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Default Get you a new kit

The Scott is all 12ax7s and and maybe one or two 6GH8 (horrible tube) and those 7591 are Ok but the tube world has gone single ended Class A and high efficiency speaker. Sell the Scott on ebone to some young buck that doesn't know any better and wants to learn how to solder. Sure you can restore it and it will sound decent. Stay away from designer parts. Use those SBC716P capacitors at Antique Electronic Supply. Use Caddocks or Holco for those big out of spec resistors or the original Allen Bradley Carbon comp. You may have to replace tube sockets that have loose pins in which case you'll have to drill out the rivits holding them in and don't let a screwdriver slip through those one of a kind selector switches. Trying to replace them would be like searching for hen's teeth. Switchcraft still makes replacement slider switches. Just my two cents. Good luck! Ray
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Old 16th October 2006, 08:05 PM   #3
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Scott made some decent sounding amps. For a given power rating Scott amps typically had bigger transformers than the other kit amps of the era.

Yes, single ended amps and high efficiency speakers are the current rage. I have built a few dozen in the last 3 years. That doesn't mean that a well designed push pull amp is useless.

You should replace the Selenium rectifier (with a silicon diode and 100 ohm resistor in series), coupling caps, and any resistors that are seriously off value. The "can" caps may be OK, but might need replacing. If they get warm, change them.

I have a Scott amp from the 1960's that works fine with most of its original components. I replaced two ceramic resistors and one output tube that were physically broken, and the amp worked. It still does.
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Old 16th October 2006, 08:38 PM   #4
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Default 6gh8's

Ray, yeah two 6gh8's and four 12ax7's... Thanks for all of the tips.
I'm on the fence with this thing.

Ed
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Old 16th October 2006, 08:56 PM   #5
pixpop is offline pixpop  United States
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I don't think this is worth doing for practical reasons. By practical reasons, I mean doing it simply in order to get a nice working amp. There are much easier ways to do that.

But there are plenty of other reasons to do it. If you want to learn, you'll learn a lot with a project like this. My first tube project was to build a Bolltehead SEX amp. I built it, and it sounds fine, but I didn't really learn anything. If it broke, I would be able to fix it, but that's mainly because it's just so simple.

Recently, someone gave me an old Fisher receiver (Uses 7591s like your Scott) with many problems. I had the blue flashing in the OP tubes as well. I decided to get it working as well as possible, purely out of curiosity. I'm taking it slowly, making every part work properly before I continue. I think it's a great project, and I'm having a lot of fun with it.

It's not something you want to hurry through, as if you were just fixing an appliance.

See my thread for more info.
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Old 16th October 2006, 10:46 PM   #6
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Vespasian,

Scott used GOOD "iron". That alone (IMO) justifies restoration or reworking.

Do you want to bring it back to the way (more or less) your dad had it, or do you want to improve on the Scott circuitry?

Do you already own or contemplate acquiring a turntable?
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Old 17th October 2006, 03:00 AM   #7
rickl is offline rickl  United States
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Ed,
I'd bring it back to life and ask (Eli) for some simple improvements.

If you don't want to restore the Scott, use the iron and build an El Cheapo. My hardest part of a project is the enclosure and iron. You have that!

rick
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Old 17th October 2006, 03:12 AM   #8
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Tubelab - yes the transformers are impressive looking and then when I picked it up I thought oooohhhh this thing is serious.
Thanks for the tip on the rectifier etc... So... I should check ALL the resistors 'eh? I think there are over 80 of 'm.... hehe...

Pixpop - My thinking is, I might get a decent sounding stereo tube amp, gain some knowledge and have quite a few hours of fun in the process. I wasn't sure if the expense in parts would outway the profits . Re 7591's, are the soviet(?)replacement tubes ok?

Eli - I wouldn't mind making improvements to the original design (that would be a kick) but I would probably require some straightforward guidance to get things right. I've got Dad's Miracord turntable also, I played some 78's on it the other day (Louis Armstrong), ran it through a Denon ss amp and some Mirage Om7 speakers but the records were in poor condition, well you know...

Funny, I just remembered a few things - A couple months back my boss told me that his father in law (who is about 85) has "tons" of electrical gadgetry in his basement including "hundreds if not thousands" of tubes.
"If you ever need a tube Ed, you should let me know"
And I'm thinking
"What the heck am I ever going to need a tube for???"
- yeah, I'll be checking that one out this week.

Seeing the Scott reminded me of a time when I was 15 or so, it was the only stereo amp in the house, -and I've got my 100w Ampeg V4 (6550's?) cranked up driving 4 12's. The ampeg is in my bedroom, on about 8 (only way at the time to get a real burning lead guitar sound), the bedroom door is closed, I'm in the living room with the scott cranked up pushing Dad's AR's - think I was spinnin' a Led Zepplin lp, I'm standing there trying to whip out some J Page licks and who walks in - yep...
"Ed, WTH are you doing???"
"Oh, Hi Dad"
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Old 17th October 2006, 03:02 PM   #9
pixpop is offline pixpop  United States
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Quote:
My thinking is, I might get a decent sounding stereo tube amp, gain some knowledge and have quite a few hours of fun in the process. I wasn't sure if the expense in parts would outway the profits . Re 7591's, are the soviet(?)replacement tubes ok?
For me, the profits have definitely outweighed the expense. Especially last night, when it really became listenable for the first time, and sounds great. The JJ Tesla tubes are fine. The others I've replaced so far are NOS.
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Old 17th October 2006, 03:40 PM   #10
EC8010 is offline EC8010  United Kingdom
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Quote:
Originally posted by Vespasian
Funny, I just remembered a few things - A couple months back my boss told me that his father in law (who is about 85) has "tons" of electrical gadgetry in his basement including "hundreds if not thousands" of tubes.
"If you ever need a tube Ed, you should let me know"
Get there quick before that becomes another one of those sad stories about widows, skips, and landfill.

Oh, and although I would prefer to build from scratch rather than refurbishing an old amplifier, look upon your old amplifier as a kit of transformers on a chassis with all the right holes drilled. For free, that's a really good deal.
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