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
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
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
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
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.
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.
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.
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.
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"
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"
Vespasian said:
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.
Hi l cant help but buy into this discourse as l have an old fisher x100 a amp that originally ran 7591's and is on el 84's now though l dont know how long they'll last 
as far as l can make out the russian equivilants of the 7591's are ok so if its any consolation you can do it this way use el 84's till you can pick up some nos 7591s somewhere such as ebay and restore the amp to its orig wkng cond l think is a worthwhile project cheers TC
Thanks Everyone
I've decided to go for it and try a rebuild. I want to thank everyone for taking the time to give opinions and pointers. I've got a lot of reading to do now (before I bother you with any additional questions), will start with the perm thread on "Online Tube Learning".
Cheers, Ed
I've decided to go for it and try a rebuild. I want to thank everyone for taking the time to give opinions and pointers. I've got a lot of reading to do now (before I bother you with any additional questions), will start with the perm thread on "Online Tube Learning".
Cheers, Ed
You want to be careful if you replace the 7591A output tubes with modern versions. Apparently the EH7591As are a bit larger and have a fit problem in some designs. The JJ 7591As are supposed to be the same size as the NOS parts. Of course, you could re-tube with NOS devices, but that would cost big-time.
Since it was your father's, restoration is the way to go. If it came from eBay or a swap meet, anything goes...
I don't know if you have the manuals, but if not, the schematic is at http://www.hhscott.com/database/vhhs_00011.html
I don't know if you have the manuals, but if not, the schematic is at http://www.hhscott.com/database/vhhs_00011.html
Yes, I found the JJ's and thought that would be the way to go if my "resource" (he's not 85, he's only 83!) doesn't have the nos for me. Thanks for the heads-up wrenchone. I've got the request going out to him, I should hear by Monday if he has some tubes for me.
Sorenj07- I would really be interested in looking at some schematics / documentation for what you think would be worth building - if you could post a url or two it would be greatly appreciated.
Thanks for the link Tom, I've got the original manuals, a tad moldy but complete. Dad was surprised that my brother still had the amp and was well aware of problems that develope with age so he knows the thing is going to take a bit of work. I don't think he really cares if I do a restoration or build based on a different design - It was just a kit that you assemble like a paint by numbers painting. If he had designed it, that would be different. Actually, I think he would get a real kick out of it if his son did something different with it esp. if performance / quality were improved.
I can't resist but to ask a couple other questions:
Should I try to get a used scope or can I get by fine without one?
What is the purpose of the metal shields around the 12ax7's?
Thanks again guys, you've been great.
Cheers, Ed
Sorenj07- I would really be interested in looking at some schematics / documentation for what you think would be worth building - if you could post a url or two it would be greatly appreciated.
Thanks for the link Tom, I've got the original manuals, a tad moldy but complete. Dad was surprised that my brother still had the amp and was well aware of problems that develope with age so he knows the thing is going to take a bit of work. I don't think he really cares if I do a restoration or build based on a different design - It was just a kit that you assemble like a paint by numbers painting. If he had designed it, that would be different. Actually, I think he would get a real kick out of it if his son did something different with it esp. if performance / quality were improved.
I can't resist but to ask a couple other questions:
Should I try to get a used scope or can I get by fine without one?
What is the purpose of the metal shields around the 12ax7's?
Thanks again guys, you've been great.
Cheers, Ed
The shields help keep noise from getting into the low level stages., like in the phono preamp.
There are some shields (IERC made them, for one) that double as heat dissipators.
If I were going to do anything to the power amp stage, I'd change the input stage to the power amp a bit and use another tube. I've got the iron from an old Fisher receiver that also used 7591As, and I'm thinking of using a 6CM7 front end/splitter using a folded cascode with the first stage (dreaded sand state hybrid) or a 5965 for both input and splitter.
For the record, I had a Scott LK72-B some time around the 1972-73 time frame, and remember it sounding quite fine - I never should have sold it.
There are some shields (IERC made them, for one) that double as heat dissipators.
If I were going to do anything to the power amp stage, I'd change the input stage to the power amp a bit and use another tube. I've got the iron from an old Fisher receiver that also used 7591As, and I'm thinking of using a 6CM7 front end/splitter using a folded cascode with the first stage (dreaded sand state hybrid) or a 5965 for both input and splitter.
For the record, I had a Scott LK72-B some time around the 1972-73 time frame, and remember it sounding quite fine - I never should have sold it.
I'm with Shifty on this one, I'd restore it. The LK-72 when working properly is a pretty decent sounding integrated amplifier.
Note that the selenium rectifier in all Scott amplifiers I have encountered is a siemens bridge and runs the phono stage filaments as well as providing the bias for the output stage. The bias supply electrolytics are generally floating off the chassis on brown phenolic mountings. (IIRC can usually negative) Series resistance should be limited to a few ohms typically. The filaments are usually wired in series and you are in the ball park when the voltage across each filament is in the 11 - 12V range, Scott ran them a little cool, was supposed to reduce schott noise, etc., in the phono stage.
JJ 7591 should fit fine and perform well. The originals may be ok, and even the "dead" one may having nothing more wrong with it than a cold solder joint in one of the filament pins. It is not that uncommon over long periods of storage to have a pin go open this way - it's happened to me several times.
The American Radionic caps that Scott frequently used are often intermittent, and in some cases go open, however they are not paper, but early films - and sound good. I have not seen any that leaked significantly, but YMMV. I would hold off on replacement unless you have something else (worse) or can measure the leakage.
The Scott driver circuit is not too great, but I'd leave it alone to start, the worst variant used the triode as an inverter driving one of the output tube grids while the other was driven by the pentode section directly. (AC coupled of course.) At first glance this looks like a paraphase but it's not. Later versions all seem to use a concertina/split load phase inverter like the dyna stuff.
Depending on the vintage the parts Scott used can vary greatly in quality, and 10% - 20% tolerance resistors were not that uncommon except in places where balanced resistors were required.
Yes, I've worked on a lot of Scott stuff over the years..
Note that the selenium rectifier in all Scott amplifiers I have encountered is a siemens bridge and runs the phono stage filaments as well as providing the bias for the output stage. The bias supply electrolytics are generally floating off the chassis on brown phenolic mountings. (IIRC can usually negative) Series resistance should be limited to a few ohms typically. The filaments are usually wired in series and you are in the ball park when the voltage across each filament is in the 11 - 12V range, Scott ran them a little cool, was supposed to reduce schott noise, etc., in the phono stage.
JJ 7591 should fit fine and perform well. The originals may be ok, and even the "dead" one may having nothing more wrong with it than a cold solder joint in one of the filament pins. It is not that uncommon over long periods of storage to have a pin go open this way - it's happened to me several times.
The American Radionic caps that Scott frequently used are often intermittent, and in some cases go open, however they are not paper, but early films - and sound good. I have not seen any that leaked significantly, but YMMV. I would hold off on replacement unless you have something else (worse) or can measure the leakage.
The Scott driver circuit is not too great, but I'd leave it alone to start, the worst variant used the triode as an inverter driving one of the output tube grids while the other was driven by the pentode section directly. (AC coupled of course.) At first glance this looks like a paraphase but it's not. Later versions all seem to use a concertina/split load phase inverter like the dyna stuff.
Depending on the vintage the parts Scott used can vary greatly in quality, and 10% - 20% tolerance resistors were not that uncommon except in places where balanced resistors were required.
Yes, I've worked on a lot of Scott stuff over the years..
LK-72
Looks like at least two of the 7591's need to be replaced, one is cracked and has the white deposit around the top, the other doesn't glow properly regardless of what socket it is in.
Re: the rectifier - perhaps one on this page would suit?
Bridge Rectifiers
Who is a good source for .5 and 1w resistors?
I can almost remember the characteristic smell this thing had after it warmed up... If I get this thing going and it puts out that scent again, it's going to through me right back to the 60's.
I might never come back.
Kevin, Shifty, Wrenchone - thanks again
Cheers, Ed
PS - Here are some PDF's on a Scott 222C Rebuild - I thought it was pretty interesting, ...
• Replace all the ceramic-tube paper American Ceracaps with modern polypropylene film caps.
• Replace the selenium bias supply rectifier with a silicon rectifier (mandatory!)
• Update the output tube grid bias pot circuits to the 222D model.
• Replace the aluminum can capacitors with modern versions made by CE Manufacturing. • Convert the output stages to ultralinear output transformers.
• Use modern speaker binding posts and phono jacks.
• Clean and polish the aluminum chassis.
(scroll down aways)
222C Rebuild
Looks like at least two of the 7591's need to be replaced, one is cracked and has the white deposit around the top, the other doesn't glow properly regardless of what socket it is in.
Re: the rectifier - perhaps one on this page would suit?
Bridge Rectifiers
Who is a good source for .5 and 1w resistors?
I can almost remember the characteristic smell this thing had after it warmed up... If I get this thing going and it puts out that scent again, it's going to through me right back to the 60's.
I might never come back.
Kevin, Shifty, Wrenchone - thanks again
Cheers, Ed
PS - Here are some PDF's on a Scott 222C Rebuild - I thought it was pretty interesting, ...
• Replace all the ceramic-tube paper American Ceracaps with modern polypropylene film caps.
• Replace the selenium bias supply rectifier with a silicon rectifier (mandatory!)
• Update the output tube grid bias pot circuits to the 222D model.
• Replace the aluminum can capacitors with modern versions made by CE Manufacturing. • Convert the output stages to ultralinear output transformers.
• Use modern speaker binding posts and phono jacks.
• Clean and polish the aluminum chassis.
(scroll down aways)
222C Rebuild
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