Forums :: NEW! Web Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives
Support This Site: Contributors :: Advertise


It is currently Oct Mon 25, 2021 7:37 am


All times are UTC [ DST ]





Post New Topic Post Reply  [ 66 posts ]  Go to page Previous  1, 2, 3, 4  Next
Author Message
 Post subject: Re: First-time Restoration: Scott LK-72 and LT-110
PostPosted: Jul Sat 24, 2021 10:52 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 13315
Location: Fernandina Beach, FL
Dave, you probably don't want to hear it but I don't modify the Scott. It worked in 1964 and worked well. I read with skepticism a lot of posts on AK. But you won't find Bovine Scat here on ARF.

As for the resistor, many Scott amplifiers already had a resistor there to measure the voltage and convert it to current. But if one wants to add a resistor, you need to lift the cathode off of ground and put a 10 Ī© or so in the place of the grounding strap. You can then read the current through each tube by doing the math.

_________________
Don


Top
 Profile  
 
 Post subject: Re: First-time Restoration: Scott LK-72 and LT-110
PostPosted: Jul Sun 25, 2021 10:43 am 
Member

Joined: Apr Mon 19, 2021 1:57 pm
Posts: 27
Hi Don, I also was originally of the mind to keep exactly as it was until I saw the output tubes:
Attachment:
7591-orig.jpg
7591-orig.jpg [ 1 MiB | Viewed 857 times ]

Probably they were not well balanced enough and burned out one by one; these replacements look quite overheated to my (granted, untrained) eye. The only original (H.H.Scott marked) tube was probably in the socket with lowest current.

My thought is: Scott made 3 main versions of this amp. 1) completely fixed (i.e. a tech could change out resistors to modify the bias) 2) with DC balance pots 3) with full bias adjustment. Obviously over time their thinking evolved and the 3) LK-72B was where they ended up; I have 2) and it still didn't save the tubes. The main thing I change is a simple addition to make the bias manually adjustable, closer to what the LK-72B did (though LK-72B has a much more involved implementation, with measurement possible from outside without opening the cover). Otherwise I'm only swapping the critical parts like electrolytics and the rectifier.

Great to hear they also had other amps with the bias measurement resistor - yes that's exactly what I will do, move GND from the cathode and put the resistor in series for measurement via Ohm's Law. As I understand this doesn't affect performance but only allows easy measurement. An option to switch the resistor in/out would be a bit more involved but seems unnecessary.


Top
 Profile  
 
 Post subject: Re: First-time Restoration: Scott LK-72 and LT-110
PostPosted: Jul Mon 26, 2021 11:07 pm 
Member

Joined: Apr Mon 19, 2021 1:57 pm
Posts: 27
Update: powered up with Variac, checked all voltages and there are a few things to tweak (e.g. resistor to get exactly -45V bias supply). Also need to wait for my new 7591A tubes in a few days as this mixed/damaged-looking set doesn't seem strong. I haven't hooked it up to speakers & an input source just yet. On one output pair I could adjust bias current to 35uA, on the other pair it didn't get quite that high.

Voltage comes up to a bit below what's in the "D-LK-72-C1 Sub 3" schematic at power on, and as it warms up voltage sinks so that most of the high DC voltages are 18-23V below intended / below schematic. 425V=>402V, 380V=>362V (Vscreen), 300V=>282V, 267V=>249V. Is that OK or do I need to search for an issue? E.g. GZ34 on its last legs, etc? Most big resistors test out as intended when disconnected. 8k 20W is 7.69k, 15k 25W is 15.27, 33 ohm 10W is 33.4.

The 750 ohm resistor in the schematic is actually a 1.2 kohm in my amp! Based on the Assembly Manual I downloaded, which shows D-LK-72-2 on the specs page, there WAS a revision with DC balance pots and 1.2 kohm for this resistor. Does anyone have this spec, I assume "D-LK-72-C1 Sub 2?" This might tell me if the voltages I see are the proper ones, or if Sub 2 is lower V than Sub 3.

Attachment:
LK-72_1.2k_Resistor.jpg
LK-72_1.2k_Resistor.jpg [ 944.56 KiB | Viewed 838 times ]


Most likely I'll order a couple of additional tubes: 6GH8A and a (new manufacture, unfortunately) GZ34 just to have replacements on hand. But the low voltages are really puzzling me. Thanks for any assistance,

-Dave


Top
 Profile  
 
 Post subject: Re: First-time Restoration: Scott LK-72 and LT-110
PostPosted: Jul Tue 27, 2021 12:29 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 13315
Location: Fernandina Beach, FL
I forget if you have replaced filter capacitors yet. But if they are weak, voltage may be lower than the schematic. I have found voltages to be off from the schematics frequently, but not by a large percentage.

_________________
Don


Top
 Profile  
 
 Post subject: Re: First-time Restoration: Scott LK-72 and LT-110
PostPosted: Jul Tue 27, 2021 6:37 am 
Member

Joined: Apr Mon 19, 2021 1:57 pm
Posts: 27
Don Cavey wrote:
I forget if you have replaced filter capacitors yet. But if they are weak, voltage may be lower than the schematic. I have found voltages to be off from the schematics frequently, but not by a large percentage.


Hi Don, I have replaced all PIO, ceracaps, Electrolytics already: Hayseed Hamfest and Orange Drops. Is this V percentage ~5% below spec acceptable or too much below schematic? I'm still looking at the GZ34 as a possible cause, or having an intermediate amp rev with different voltage (which I haven't found a schematic for).


Top
 Profile  
 
 Post subject: Re: First-time Restoration: Scott LK-72 and LT-110
PostPosted: Jul Sat 31, 2021 7:22 pm 
Member

Joined: Apr Mon 19, 2021 1:57 pm
Posts: 27
So I'm all finished for now, thanks for all the great help & tips from forum members to get me started! At the end it went very smoothly after component replacement, as I didn't encounter ANY issues after powering on (for the first time in 25 years). I was a bit uncertain about the low B+ voltage, but as the sound is great, that's not a concern at the end.

Here are some pics, and a list of the components I replaced.
Attachment:
Amp-FinalTopView.jpg
Amp-FinalTopView.jpg [ 922.75 KiB | Viewed 782 times ]

Attachment:
Amp-Inside-almost-done.jpg
Amp-Inside-almost-done.jpg [ 1.11 MiB | Viewed 782 times ]

Attachment:
Amp-FinalNewTubes.jpg
Amp-FinalNewTubes.jpg [ 528.47 KiB | Viewed 782 times ]


  • Replaced can caps with Hayseed Hamfest LK-72 re-cap kit
  • Replaced all internal electrolytic/PIO caps with 630V Orange Drops
  • Cleaned tube sockets, RCA plugs, potentiometers, switches with Deoxit D5
  • Replaced selenium rectifier with silicon; exchanged resistor for a 20ohm to maintain correct -45V
  • Exchanged weak/damaged-looking power tubes (3 different brands) for new (Russian production) Tung-Sol 7591A
  • Exchanged inverter tubes (2 different brands of replacement tubes themselves) for EI/NIS ECF802, 80's manufacture
  • Exchanged DC Balance Pot with an Individual Bias Adjustment Mod / IBAM (preserving exact same grid-to-ground resistance)
  • Lifted cathode from ground, added 10 ohm resistors there to measure bias idle current
  • Replaced 330k 10% grid resistors with new 1% ones, as some originals were indeed 10% off
  • To achieve same grid-to-ground as originally (about 200k ohm), the 270k resistor after DC Bal pot was replaced with a 470k 1%
  • - Considering each tube had 270k resistors in series with ~250k from approx. half of the 500k DC Bal pot, parallel to the 330k resistor, this calculates to about 200k
  • Added 180 ohm screen stopper resistors to minimize risk of screen over-current

Comments:
  • Bias adjustment is very exact, easy to adjust to below 35mA as recommended for the output tubes with 25-turn 10k potentiometer.
  • I was able to keep the original Telefunken 12AX7's and the rectifier tube (Realistic Lifetime / marked Great Britain, possibly a vintage replacement itself).
  • It seems likely this is the D-LK-72-C1 Sub 2, whereas schematics findable online are D-LK-72-C1 Sub 1 and D-LK-72-C1 Sub 3.
  • - My (surely original) resistor between 425V and 380V (screen voltage) is 1.2kohm where the schematic for Sub 3 shows 750 ohm. This matches Sub 2 assembly manual (1.2kohm)
  • Internal high DC voltages are about 15-20V below schematic; but the amp sounds fine so most likely this is just fine. No need to change anything.
  • Sound is excellent, and the soundstage is fantastic especially on live recordings. I can really pinpoint everything, especially vocals. Great results!

Attachment:
Amp-FinalFrontView.jpg
Amp-FinalFrontView.jpg [ 786.01 KiB | Viewed 782 times ]


Top
 Profile  
 
 Post subject: Re: First-time Restoration: Scott LK-72 and LT-110
PostPosted: Jul Sat 31, 2021 10:42 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 4791
Location: Boston, MA USA
It's great that the amplifier turned out well and is functioning properly. However, soundstage isn't really a figure of merit for an amplifier. All a stereo amplifier has to do is make sure both channels exhibit equal phase shift at each frequency across the frequency range. Not at all difficult -- just use the same circuit design and components for each channel. Soundstage is captured on the original source material and is reproduced according to speaker placement. Get these two right and you'll have a good soundstage -- the amplifier has very little or nothing to do with it.

-David


Top
 Profile  
 
 Post subject: Re: First-time Restoration: Scott LK-72 and LT-110
PostPosted: Aug Sun 01, 2021 9:09 am 
Member

Joined: Apr Mon 19, 2021 1:57 pm
Posts: 27
dberman51 wrote:
It's great that the amplifier turned out well and is functioning properly. However, soundstage isn't really a figure of merit for an amplifier. All a stereo amplifier has to do is make sure both channels exhibit equal phase shift at each frequency across the frequency range. Not at all difficult -- just use the same circuit design and components for each channel. Soundstage is captured on the original source material and is reproduced according to speaker placement. Get these two right and you'll have a good soundstage -- the amplifier has very little or nothing to do with it.

-David


Sure, a modern $50 kit transistor amplifier can probably do that too, one can discuss all day which amp "sounds" better. But in this case it means after replacing 50+ components on a work of art entering its 7th decade, I didn't make any glaring mistakes AND my Grandfather was a pretty good electronic hobbyist (for having been a chemist). ;-)


Top
 Profile  
 
 Post subject: Re: First-time Restoration: Scott LK-72 and LT-110
PostPosted: Aug Tue 03, 2021 4:01 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 13315
Location: Fernandina Beach, FL
one51 wrote:
dberman51 wrote:
It's great that the amplifier turned out well and is functioning properly. However, soundstage isn't really a figure of merit for an amplifier. All a stereo amplifier has to do is make sure both channels exhibit equal phase shift at each frequency across the frequency range. Not at all difficult -- just use the same circuit design and components for each channel. Soundstage is captured on the original source material and is reproduced according to speaker placement. Get these two right and you'll have a good soundstage -- the amplifier has very little or nothing to do with it.

-David


Sure, a modern $50 kit transistor amplifier can probably do that too, one can discuss all day which amp "sounds" better. But in this case it means after replacing 50+ components on a work of art entering its 7th decade, I didn't make any glaring mistakes AND my Grandfather was a pretty good electronic hobbyist (for having been a chemist). ;-)

Looks great. I am glad to see yet another Scott amplifier brought back to life.

_________________
Don


Top
 Profile  
 
 Post subject: Re: First-time Restoration: Scott LK-72 and LT-110
PostPosted: Aug Wed 04, 2021 2:42 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 10761
Location: Beaver Falls, PA. USA
Nice work! I can vouch for the Hayseed Hamfest capacitors; I have been using them for several years. Not cheap, but they fit perfectly.

_________________
Tim KA3JRT


Top
 Profile  
 
 Post subject: Re: First-time Restoration: Scott LK-72 and LT-110
PostPosted: Aug Wed 04, 2021 1:37 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 13315
Location: Fernandina Beach, FL
At the risk of hijacking this thread, my choice has been Scott tube amplifiers for that last couple of decades. I started with an FET transistor Scott LT-112B that I built right after High School. I worked at E. J. Korvette selling audio equipment 1966-67 and the trend was transistors at the time. I didn't even know about the Scott tube equipment.

Fast forward, eBay changed that. I have way too many pieces of tube equipment but I am working to thin the herd. I still thoroughly enjoy pulling an amplifier off of the shelf (usually has had the tubes pilfered by a previous owner) and getting it going. It takes about a day (as long as the three transformers are OK) and they are just working.

That is why I was very interested in this thread. I have a couple LK-72 and LT-110 pieces in my collection. They are simple, conventional in design yet work very well

_________________
Don


Top
 Profile  
 
 Post subject: Re: First-time Restoration: Scott LK-72 and LT-110
PostPosted: Aug Sat 07, 2021 9:30 pm 
Member

Joined: Apr Mon 19, 2021 1:57 pm
Posts: 27
I probably won't get a whole bunch of vintage tube pieces on Ebay, though now I'm dreaming about a bass guitar amp with all tubes at some point ;-). Probably there aren't cheap ones to be found on Ebay although I might have to check now. I looked for kits and didn't find anything bass-specific that caught my eye. Being used to a 400W Hartke, I can't play my bass through something with 15-50W.

The LT-110 is next on the list. I replaced all the necessary caps and powered it up; with only the rectifier tube the voltages at the big electrolytics look good. But when adding all the other tubes, the rectifier output voltage drops to 50-60V. I measured the current on the first big resistor, it was only something like 25mA so I can't imagine it's an overcurrent drain from the rest of the amp. Probably I need a new rectifier tube to start with...? That could be the failure that drove my Grandfather to shelve the H.H.Scott pieces for an Onkyo receiver, I think sometime in the 80's.


Top
 Profile  
 
 Post subject: Re: First-time Restoration: Scott LK-72 and LT-110
PostPosted: Aug Sat 07, 2021 10:28 pm 
Moderator

Joined: Jan Thu 01, 1970 1:00 am
Posts: 18803
Location: Utica, NY 13502 (USA)
Since Don has now officially hijacked this thread, let me add that my first HH Scott is this model 310-DR FM tuner found in a pile of dump-bound, unused equipment at the transmitter site of WSGO-1440 in Oswego, NY and their co-owned FM station.

Attachment:
HH Scott 310-DR.jpg
HH Scott 310-DR.jpg [ 244.54 KiB | Viewed 674 times ]


The long-time owner of the station (who was retiring) told me that the tuner was used to receive and relay FM broadcasts from the Northeast Radio Network (formerly the Grange Federation League's Rural Radio Network) to participate in their state-wide "Weather Roundup" program. This model, which is not listed on the HH Scott tube audio website (although an earlier model 310-CR is listed) was chosen for its ability to receive weak, distant FM stations in the presence of very strong, local adjacent stations. I find this to be entirely true. The not-so-pretty Scott 310-DR rack-mounted tuner is what sparked my interest in other HH Scott products.

Here is an audio clip of the last Weather Roundup program broadcast on the network in 1968.

http://northeastairchecks.com/checks/nern68.ram

BTW - My LT-110 is essentially equal to my 310-DR in RF performance since the front-end sub-chassis and circuits are the same.

Dave


Top
 Profile  
 
 Post subject: Re: First-time Restoration: Scott LK-72 and LT-110
PostPosted: Aug Mon 09, 2021 8:11 am 
Member

Joined: Apr Mon 19, 2021 1:57 pm
Posts: 27
Dave Doughty wrote:
Since Don has now officially hijacked this thread, let me add that my first HH Scott is this model 310-DR FM tuner found in a pile of dump-bound, unused equipment at the transmitter site of WSGO-1440 in Oswego, NY and their co-owned FM station.
...

BTW - MY LT-110 is essentially equal to my 310-DR in RF performance since the front-end sub-chassis and circuits are the same.


That is quite cool, I hope that I can get the LT-110 working again with a new rectifier tube, otherwise I'll have to find some other possible reasons for low DC voltage once the other tubes are plugged in. What's interesting is that just 4 months after my LT-110 model, they have replaced the rectifier tube with a silicon one (if I read the online-available schematic correctly). My original schematic has the tube shown. I wish I had my Grandfather's original LK-72 schematic as well, but that seems to have been lost in time.


Top
 Profile  
 
 Post subject: Re: First-time Restoration: Scott LK-72 and LT-110
PostPosted: Aug Thu 12, 2021 9:02 pm 
Member

Joined: Aug Thu 27, 2015 5:07 am
Posts: 719
Location: Traverse City, MI 49670
Okay, another hijack but Scott pertinent. For the record I built new the 7199 version of the LK years ago. So much cleaner than the catastrophic Dynaco pc mount driver board that a small fortune has been made with subjective "solutions".
Pictured is all I have from customer seeking advice. Can anyone identify this Scott integrated and tuner ?
Attachment:
7D6310D1-2F22-48A0-A01B-332D4C366133.jpeg
7D6310D1-2F22-48A0-A01B-332D4C366133.jpeg [ 380.85 KiB | Viewed 582 times ]


Top
 Profile  
 
 Post subject: Re: First-time Restoration: Scott LK-72 and LT-110
PostPosted: Aug Fri 13, 2021 1:56 pm 
Moderator

Joined: Jan Thu 01, 1970 1:00 am
Posts: 18803
Location: Utica, NY 13502 (USA)
I don't think either of those components are Scotts. I don't recognize the logo on the amp (lower right).

Attachment:
Tuner and Amp.jpg
Tuner and Amp.jpg [ 80.13 KiB | Viewed 568 times ]


Dave


Top
 Profile  
 
 Post subject: Re: First-time Restoration: Scott LK-72 and LT-110
PostPosted: Aug Fri 13, 2021 3:13 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 13315
Location: Fernandina Beach, FL
There was another "Scott" company that came after H.H. Scott and was not associated with them.

_________________
Don


Top
 Profile  
 
 Post subject: Re: First-time Restoration: Scott LK-72 and LT-110
PostPosted: Aug Fri 13, 2021 3:55 pm 
Member
User avatar

Joined: Jul Mon 08, 2013 2:48 pm
Posts: 805
Don Cavey wrote:
There was another "Scott" company that came after H.H. Scott and was not associated with them.


Not "after", but before: It was E.H Scott, famous for their high end luxury very expensive chromed chassis radios. In their later period (after the merging with J.Meck) they tried to capture some of the Hi-Fi market but didn't succeeded, H.H Scott were already a well know leader and filed a lawsuit against Scott (Radio Laboratories) forcing them to stop using this name for Hi-Fi products. E.H Scott (Radio Labs) Hi-Fi products were of inferior quality (when compared to H.H Scott) and soon disappeared from the market. Some (but not all) E.H Scott Hi-Fi products were rebadged canadian Stromberg-Carlson's. The faceplate layout of your amp is very similar to a S-C ASR433 and the (partially hidden) tuner should be a S-C SR445.
The pictures you have posted are from a E.H Scott (Scott Radio Labs) Hi-Fi console, not H.H Scott.


Top
 Profile  
 
 Post subject: Re: First-time Restoration: Scott LK-72 and LT-110
PostPosted: Aug Fri 13, 2021 5:58 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 13315
Location: Fernandina Beach, FL
Tubologic wrote:
Don Cavey wrote:
There was another "Scott" company that came after H.H. Scott and was not associated with them.


Not "after", but before: It was E.H Scott, famous for their high end luxury very expensive chromed chassis radios....
I know about E.H. Scott and was NOT referring to them. There was another Scott company that made what looked like department store brand equipment. It was not E.H. or H.H. Scott. I have seen them on the auction site, but not lately.

_________________
Don


Top
 Profile  
 
 Post subject: Re: First-time Restoration: Scott LK-72 and LT-110
PostPosted: Aug Fri 13, 2021 6:00 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 13315
Location: Fernandina Beach, FL
Looking at the turntable, it appears to be a 50s or early 60s design.

_________________
Don


Top
 Profile  
 
Post New Topic Post Reply  [ 66 posts ]  Go to page Previous  1, 2, 3, 4  Next

All times are UTC [ DST ]


Who is online

Users browsing this forum: No registered users and 3 guests



Search for:
Jump to:  


































Privacy Policy :: Powered by phpBB