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HH Scott LK-72 rebuild log

As already said by someone, find some place to test your tubes. The 12AX7’s are probably still healthy but the 7591’s, even if looking good visually, could be on their last leg (or not...).

Great amp BTW!
 
carlman14 said:
Another question for y'all: Is a 2W enough for the first resistor after the bridge rectifier? I got the bridge rectifier and the 75/75/75/75 can cap replaced today, and am trying out different first resistors to get -45V. However, that first resistor is getting extremely hot, even after a minute or two of operation. It's so hot I can't touch it. Is that ok? Or should I get a resistor with a higher watt rating?
Click to expand...
I put in a 33 ohm-5w resistor in mine as recommended. I also keep the amp plugged into my Variac set at 117v. Probably not necessary, just my anal brain thinks it’s safer.
 
This is shaping up to be a really great integrated amp. Once you have the caps replaced and the fixed bias supply working, you'll bd able to tell pretty quickly if your output tubes are working. My guess is they probably are working good enough to give you 25 or so watts per channel, which is most likely all they really did when brand new.
You are going to love that little amp.
 
LK-72 is about the best one that Scott had (other than the LK-150, 272 and 296 which are scarce and expensive!). For the most part, from the beginning, they were the more powerful Stereo Integrated amplifiers utilizing the 7591 outputs.

I am sure that you will be pleased.
 
kvflyer said:
LK-72 is about the best one that Scott had (other than the LK-150, 272 and 296 which are scarce and expensive!). For the most part, from the beginning, they were the more powerful Stereo Integrated amplifiers utilizing the 7591 outputs.

Cant say for sure what sounds better but i personally hold the 280 mono amps to be the totl for HH scott.
That being said, my 299d isn't for sale...

I am sure that you will be pleased.
Click to expand...
 
Patrice B said:
As already said by someone, find some place to test your tubes. The 12AX7’s are probably still healthy but the 7591’s, even if looking good visually, could be on their last leg (or not...).
Click to expand...

I found this site that does tube testing: https://tctubes.com/tube-testing-service.aspx. I might send them there if I don't have any better options.

GD70 said:
I put in a 33 ohm-5w resistor in mine as recommended. I also keep the amp plugged into my Variac set at 117v. Probably not necessary, just my anal brain thinks it’s safer.
Click to expand...

I ordered some 5W resistors a couple days ago. That 2W was just too hot for my liking. I also happen to have a CL-80 inrush current limiter on-hand, which I heard can help reduce the line voltage a little bit. Though I'm not 100% sure how to hook that up.
 
Bradbrews said:
It's been said to me many times that the absolute best test for tubes is the device that your using them in!
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It’s ok but it’s fine to know, even with a basic emission tester, when an output tube is completely dead or shorted...

Just tested two weeks ago two 7591’s (seller included the old replaced tubes) and they were barely moving the needle on my Jackson 648, under 5 percent. So, they won’t even be tried on any of my amps.
 
Making progress! Most of the orange drops are in, as well as the new bridge rectifier and larger first resistor to get -45V. Next is all the caps around those 7199 tubes. I am not looking forward to replacing those parallel 0.05uF disc caps... Those are going to be a pain.

IMG_20201103_145725.jpg

IMG_20201103_145833.jpg
 
Aaaaaand Done! I'm going to triple check everything is wired up correctly, then turn it on and check voltages. Kinda nervous to turn it on...

IMG_20201104_211701.jpg
 
I turned it on... and it didn't catch fire! So that's something! I measured the important voltages. Here's the results:

12ax7
Expected: -45V
Actual: -45.5V

7591 plate (pin 3)
Expected: 450V
Actual: 466V

7591 grid 3/cathode (pin 5)
Expected: 0V
Actual: 0V

7591 grid 1 (pin 6)
Expected: -20V
Actual: -23V

7591 grid 2 (pin 8)
Expected: 430V
Actual 450V

The 7591 plate and grid 2 are both a bit higher than the expected values. Is this still within spec for 7591 tubes?
 
What is your wall voltage? The unit was designed in an era where the expected voltage was 117 or so volts. I used a couple of thermistors in series to drop mine a couple of volts to get the voltages down. You would then likely need to adjust the bias string resistor also if you go that route. Those voltages aren’t dangerously high though, but it is a good idea to get them closer to the schematic values.
Good job!
 
LinderK said:
What is your wall voltage? The unit was designed in an era where the expected voltage was 117 or so volts. I used a couple of thermistors in series to drop mine a couple of volts to get the voltages down. You would then likely need to adjust the bias string resistor also if you go that route. Those voltages aren’t dangerously high though, but it is a good idea to get them closer to the schematic values.
Good job!
Click to expand...
This is why I keep mine plugged into my Variac, set at 117v.
 
Those voltages all look great. The plates and screens are bit high, but you have 3v extra bias at the grids to compensate. Do you have a way to measure plate/cathode current?
Thats about the last meaningful check to make before you play some music.
My guess, is that you will find the bias current to be within spec, and you are about to find out that an HH Scott can sound really good!
 
LinderK said:
What is your wall voltage? The unit was designed in an era where the expected voltage was 117 or so volts. I used a couple of thermistors in series to drop mine a couple of volts to get the voltages down. You would then likely need to adjust the bias string resistor also if you go that route. Those voltages aren’t dangerously high though, but it is a good idea to get them closer to the schematic values.
Good job!
Click to expand...

My wall voltage is 123.5V... so definitely higher than the average back then... and even high for the average now. I have some CL-80's on hand. How would I wire those up?

trainbuftony said:
Those voltages all look great. The plates and screens are bit high, but you have 3v extra bias at the grids to compensate. Do you have a way to measure plate/cathode current?
Thats about the last meaningful check to make before you play some music.
My guess, is that you will find the bias current to be within spec, and you are about to find out that an HH Scott can sound really good!
Click to expand...

I have some panel meters I can measure current with. I'll measure that tonight!
 
I had the same problem with my 299C running from the wall. I get between 121-125v here in the summer. I built a bucking transformer and with input voltage at 115 my plate voltage is within 2v. Really can't tell a difference in sound with the lower voltage though, just makes me feel better.
 
carlman14 said:
My wall voltage is 123.5V... so definitely higher than the average back then... and even high for the average now. I have some CL-80's on hand. How would I wire those up?

You wire them in series with the hot line before the power transformer.
Click to expand...
 
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