View Full Version : HH Scott LK-72 biasing and owners manual


ltr317
10-11-2007, 12:46 AM
:music:Hi everyone. I just received a Scott LK-72 integrated amp that I won on ebay. It's an early model, as it has the brown faceplate. Other than the 4 12AX7 use in the preamp section, I'm not familiar with the rest of the tube complement: 4 7591A, 2 6U8/6GH8 (use as a split-load phase inverter), 1 5AR4/GZ34 rectifier. It looks like it has the original selenium rectifier. Anyway, I don't see a bias adjustment screw. Does anyone know if this amp is auto-biasing? I got a schematic, but where can I get an owners manual? I check the Scott website and the owners manual for the LK-72 doesn't seem right, as the controls in the manual don't match the controls on my amp. Thanks for any input.

It sounds pretty good right off the bat, though not as revealing as my Jolida integrated, or my modified Dynaco PAS preamp and McIntosh solid state amp combo. I'll listen long term to get a better idea of the sound. :music:

FalconEddy
10-11-2007, 10:05 AM
Congratulations.

There were several versions of schematics for the LK-72 series. Although I have a schematic that shows the LK-72B has both a set of balance pots and a set of bias pots for each pair of 7591 output tubes, I believe you have the LK-72A, which only has balance pots. It is NOT auto-biasing.

Has the amp been restored/rebuilt, or at least had the coupling caps changed? You should also have the electrolytics in the power supply section changed out, as well as that old selenium rectifier and it's voltage dropping resistor. Changing over to a more stable bias supply (silicon full wave bridge) is important to lower the risk to the output transformers and tubes.

Since you own other tube amps, I'm speculating that you already know about the high operational voltages, as well as the stored H.V. present in the caps.

When I restored my LK-72A 14 months ago, I also incorporated a pair of full biasing circuits and test points above the chassis. This allows me to properly dial in the recommended current draw for each of the four 7591's; or, back it down a bit to gain longer tube life. Something you should seriously consider whether you're planning on making some upgrades to the present components, or not. Plus, the four cathode resistors I installed also put a layer of protection between the output tubes and the output transformers if a 7591 shorts out.

. . Falcon

ltr317
10-11-2007, 11:47 AM
Thanks for the valuable info. You're right, it looks like I have an early LK-72 or "A" model, as the schematic I have doesn't show any biasing pots.

The amp looks to be completely original, including the tubes! I spoke to the guy who sold me the amp, and he said it was sitting on the shelf for a number of years, as well as several other Scott amps which he all sold on ebay.

My thought on refreshing the amp is the same as what you did to yours. I will be replacing all caps and resistors, especially the selenium rectifier. I was thinking a silicon bridging rectifier, but what value did you use for the voltage dropping resistor? Also, there are so many cap and resistor choices available, which ones did you get? Anything else I should replace while I'm at it?

"When I restored my LK-72A 14 months ago, I also incorporated a pair of full biasing circuits and test points above the chassis. This allows me to properly dial in the recommended current draw for each of the four 7591's; or, back it down a bit to gain longer tube life. Something you should seriously consider whether you're planning on making some upgrades to the present components, or not. Plus, the four cathode resistors I installed also put a layer of protection between the output tubes and the output transformers if a 7591 shorts out." Can you provide a schematic of the biasing circuits, cathode resistors and test points location above the chassis for installation? I wanted to reduce the bias by about 10% for longer tube life as you suggested. I hadn't thought about upgrades, which ones were you thinking?
Many thanks for your input.

Paul

FalconEddy
10-11-2007, 03:58 PM
I'll be able to get you more info this evening.

Does it have two standing ceramic resistors above the chassis, or only one?

What are your can cap sizes near the output tubes, and the one in the back right corner above the chassis?

Any photos?

Also would like to know what the size of 7591's screen grid resistor is inside the actual unit (not the schematic). 1.2K or 750ohm?

. . Falcon

ltr317
10-11-2007, 05:36 PM
I just took some photos. Starting from the top left photo: 1) left cap can near the output tubes (short), 2) right cap can near the output tubes (tall), 3) right rear corner cap, 4) top chasis view, 5) front panel, 6) cap can near preamp tubes. Looks like only one ceramic resistor. I haven't open the chasis yet.

FalconEddy
10-11-2007, 05:52 PM
Interesting.

WHAT the heck is wrapped around those 7591's???????

. . Falcon

ltr317
10-11-2007, 06:00 PM
Oh, plumber's tape. Thought I could keep the output tubes a little cooler, since they are so close to the output transformers.

Steve O
10-11-2007, 07:25 PM
Oh, plumber's tape. Thought I could keep the output tubes a little cooler, since they are so close to the output transformers.

"Plumber's Tape" = teflon pipe thread tape? I don't think that's a real good idea. Tubes are cooled by both convection and radiation and the tape is probably going to interfere with the radiation portion. If heat is an issue, a small fan is a better choice IMO. Let those 7591s breathe. They're a 6V6 size tube with 6L6 power capabilities.

ltr317
10-11-2007, 09:35 PM
Hey thanks. I took off the teflon tape and will get a small fan instead.

FalconEddy
10-11-2007, 09:42 PM
Oh, plumber's tape. Thought I could keep the output tubes a little cooler, since they are so close to the output transformers.

???

Do you mean teflon tape??

Those glass envelopes on the output tubes need to dissipate a LOT of heat. Covering them up isn't something I'd recommend. Unlike a Pearl Tube Cooler, there is nothing to capture and radiate heat away from the tube using a convection process.

Here's a link to my rebuild schematic (around 6MB):

http://www.falconeddystudios.com/images/HH-Scott-Schematic-LK72A-COLOR-Rebuild-July2006.jpg

. . Falcon

ltr317
10-12-2007, 12:44 PM
Thanks for the schematic. I'll look at it closely over the weekend. I took the teflon tape off already as Steve O suggested the same thing you did.

NOSValves
10-12-2007, 01:26 PM
That schematic will not be correct especially in the driver/phase splitter area. From the pictures his amp he posted his utilizes 7199 driver/phase splitters. Also his amp has no DC balance adjust pots. They made so many different versions of the LK-72/299C amps an accurate schematic is impossible task to find. I attached a schematic of a 299C that uses 7199 driver/phase splitters but it also has DC balance controls. I've yet to see a scat for a LK72/299C without DC balance controls.

Craig


PS darn the file size is to large. Email me at craigostby@comcast.net and I'll send the Schematic.

ltr317
10-12-2007, 02:43 PM
I just looked at the phase inverter tubes and they are 7199 instead of the 6U8/6GH8 listed on the HH Scott website. Craig, thanks for pointing that out. I just sent you an email for the 299C schematic

FalconEddy
10-12-2007, 02:50 PM
DAMN!!!

Nice catch Craig.

I had originally started typing up that reply around 6:15PM last night, but was waiting to send it since I still had to take a photo of the schematic (like his amp photos showed), and put it up on my website.

It's Scott's early version drawing number D-LK-72-C1 sub 1 from 10/18/60. The visible differences I noticed were that all five of the power supply caps were 20MFD, but the driver section was correct and it had no balance pots. So I thought that may be a good reference for him.

I got sidetracked after going to pick up my son from karate, and couldn't remember why I hadn't sent the reply yet. :scratch2:

If you'd like, I can send you a link later on tonight pointing to this particular scat.

Thanks,

. . Falcon

ltr317
10-12-2007, 04:24 PM
Yes, the link would be great. Thanks.

Falconeddy,
Craig just emailed me the schematic for the early LK-72 amp, so you don't need to. Thanks anyway.