HH Scott LK72B, a dirty dog!
- Thread starter spxer
- Start date
spxer
Active Member
Schematic here.
ttps://www.thetubestore.com/lib/thetubestore/schematics/HH Scott/HH-Scott-lk72-B-Schematic.pdf
ttps://www.thetubestore.com/lib/thetubestore/schematics/HH Scott/HH-Scott-lk72-B-Schematic.pdf
The 5ar4 is the rectifier tube. I bought some really cheap ss subs from ebay and added a second pair of diodes inside to get proper piv rating but you can make your own or temp tack them on the socket directly. Cathodes to pin 8, anodes to 4 and 6. Only 2 diodes, no bridge.
Just watch the voltage doing this, it will have less drop than the tube and it will be possible to exceed ratings on the caps.
The bridge is the negative dc bias supply and phono tube filament. Positive to ground on that one.
Just watch the voltage doing this, it will have less drop than the tube and it will be possible to exceed ratings on the caps.
The bridge is the negative dc bias supply and phono tube filament. Positive to ground on that one.
Not usually, I just do it as a temp sub to try to re-form capacitors or for low voltage testing. A 5AR4 isn't going to do much until you get to at least half of the mains voltage in most cases, and at that point you're dealing with a lot of B+. You can pop a solid state thing in there and feed 12 volts in and confirm the outputs are approximately 10% of what they ought to be. I'm not much for leaving old electrolytics forever but I do tend to test things before making changes just to see if there are good signs of life or if I'm about to waste a lot of time. Last time I did this, I found I have a bad output transformer that arcs when it gets some voltage on it. I would have been pretty pissed off to have spent time and money on caps only to find out I have a rebuilt, useless amplifier.
The 5AR4 has a PIV rating of (from memory) 1500 volts? A pair of 1N4007 in series will get you 2kv of PIV which is more than adequate for the job. Maybe not the finest or fanciest diode available but for this its more than good enough and they are very cheap.
https://audiokarma.org/forums/index...heap-solid-state-rectifier-tube-subs.1008237/
has pics of what they came with. Apparently I didn't post pics of the after, but its just a second diode in series with each of the ones already installed. I have a bag of the things.
The 5AR4 has a PIV rating of (from memory) 1500 volts? A pair of 1N4007 in series will get you 2kv of PIV which is more than adequate for the job. Maybe not the finest or fanciest diode available but for this its more than good enough and they are very cheap.
https://audiokarma.org/forums/index...heap-solid-state-rectifier-tube-subs.1008237/
has pics of what they came with. Apparently I didn't post pics of the after, but its just a second diode in series with each of the ones already installed. I have a bag of the things.
Sounds2good
Member
I'll be following this one close. I have a 299D which is the factory version that I need to redo.
-Thanx, guys!
-Thanx, guys!
I've refreshed a few Scott amps that use 7591 output tubes.
I replaced:
1) The original electrolytic can caps
2) Coupling caps located between the phase inverter outputs and power tube control grids
3) Coupling caps between 12AX7's and phase inverter tubes
4) Revised 7591 bias adjust circuit to provide individual 10ohm cathode resistors to provide individual adjustment of each tube tubes idle current.
5) Resistors as required when originals were found to have drifted from schematic values.
6) Also replaced selenium bias circuit rectifier with a silicon bridge rectifier.
The replacement can caps came from Hayseed Hamfest, and most other components were from Antique Electronic Supply (aka Tubes and More) or Tube Depot.
If you do some searching on this forum using "Scott amplifier repair" or "Scott amplifier restoration" you will find lots of threads detailing how folks have gone about working on their projects. You can specify the specific model you are working on in your search, but all the 7591 amps are very similar and you can use the AK Database to find schematics, to see the similarities in each models circuits.
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you might want to add an inrush current limiter ala CL-80 or -90 while you're in there.
I just skimmed this thread so apologies if it was already mentioned, but did you already replace the bridge rectifier? I updated my 72B with a silicon version.
Great job cleaning up the Scott!
I just skimmed this thread so apologies if it was already mentioned, but did you already replace the bridge rectifier? I updated my 72B with a silicon version.
Great job cleaning up the Scott!
spxer
Active Member
sbcrockett, I have not replaced anything as yet. I am trying to get a handle of what all should be replaced. I have a list from the assembly manual of all the parts. I will use that as my guide. I did not know about inrush limiters. I have the amp on a variac now warming at 40v ac in.The tubes are getting slightly warm.
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gotcha. You won't need to replace EVERY component. As Pablodeux already listed above, the power supply caps, coupling and any electrolytics are a good place to start. the bridge rectifier as well plus any resistors that may have drifted out of spec. Then there are the safety mods which you should just do from the start, in my opinion.
When you get to installing the cathode resistors, you will want to do the mod I did on mine for adjusting bias, post #68: https://audiokarma.org/forums/index...capped-and-ready-for-the-variac.888059/page-4 Complete notes for the mod I found in another thread and you can read them in post #67
idk how often you order from mouser or digikey but finding the right part to order can take almost as long as replacing it! I have PDFs of everything I ordered (some of the parts I didn't use and bought mostly because I may have had a question about which resistor or cap to use between two types or manufacturer). let me know if you'd like me to PM them to you
or if you enjoy the process of digging for part numbers, you'll have a lot of fun
-Steve
When you get to installing the cathode resistors, you will want to do the mod I did on mine for adjusting bias, post #68: https://audiokarma.org/forums/index...capped-and-ready-for-the-variac.888059/page-4 Complete notes for the mod I found in another thread and you can read them in post #67
idk how often you order from mouser or digikey but finding the right part to order can take almost as long as replacing it! I have PDFs of everything I ordered (some of the parts I didn't use and bought mostly because I may have had a question about which resistor or cap to use between two types or manufacturer). let me know if you'd like me to PM them to you
or if you enjoy the process of digging for part numbers, you'll have a lot of fun
-Steve
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towards the top of this page you'll see your member name. you can either click on it and go to your Conversations or you can click on the Inbox next to your name. you will also get an email notification and the alert button (also at the top) will indicate you have a message. Standby
it does but take your time and you won't need to order everything I did
some of the most expensive things on those orders were the heatshrink and copper braid. the HH cans are the biggest expense but worth it IMO
**just did the quick maths and, all in, my restoration only cost 300 bucks which I think was a bargain. I probably had 40-50 bucks in sundry items and spare parts or duplicates that I didn't end up needing. now...that doesn't include the DBT, variac and kill-a-watt that I also bought for testing...
David_NC
Super Member
It's good to see another one of these 420's on AK. They are very nice sounding receiver that many used to harvest OT's for power amp builds, apparently. I used a set of very cheap, no-brand (printed on the glass) EL-84's. It's my first and only amp using '84s, and they do sound good. Did you replace the 'hidden cap' in the box protecting the power switch? That one got missed, when rebuilding mine, and later exploded, zapping my DAC and PC's USB port with 120 grounded to chassis. Still, it's a nice, somewhat uncommon model.Heck, that looks clean compared to the Pioneer ER-420 I just finished refurbing... the underside looked just fine btw. After cleaning tube pins/sockets and a full recap she sounds great!
Yup, got it! Nice to hear from another 420 owner.
