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 Post subject: H.H. Scott LK-72B recap and bias modification
PostPosted: Oct Sun 27, 2019 4:15 pm 
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Joined: Oct Sun 27, 2019 3:40 pm
Posts: 2
Hi guys,,

Just joined and this is my first post. I've recently finished re-capping my LK-72B, replaced the selenium rectifier and electolytics and have just started to adjust balance and bias and had a few questions. I really didn't want to just rely on the manuals method of balance/biasing with the hum and click method and instead wanted to make it an actual measurement.

I was directed to a modification that some have done which allows adjustment by looking at each individual tubes cathode instead of left/right bias. These are the instructions I followed from an older thread:

[i]This is what you do:

1. Take out the 10-ohm common cathode resistor in each channel and replace with individual 10-ohm 1/2W on each tube running between the cathode and ground. (two removed, four installed)
2. Rip out ALL of the resistors and caps connected to the slide switch and dual RCA jack next to it. But leave the grounds intact on the RCA jacks.
3. Connect the bottom center tab of the DPDT switch to the bottom RCA jack center pin, and the top center tab of the DPDT switch to the top RCA jack center pin.
4. Either get some four conductor wire or run individual wires like I did. Ground is already provided to the test points via the old wiring. Get to it – connect a wire from each outer pin of each section of the switch to the cathode of the tube you want to measure. This allows you to measure the voltage drop across the cathode resistor of the tube you want to measure, dependant on what RCA jack you’re plugged into and what position the switch is in.

The measurement drill then goes like this:

1. Connect a DMM to an RCA plug and plug it into one of the jacks on the chassis.
2. You’re now looking at one pair of tubes, and flipping the switch lets you look at one tube or the other.
3. Flip the switch back and forth while messing with the bias and balance pots to get both to read .22VDC (for 7189’s). Your amp should want to see .30VDC, but don’t quote me on that.
4. Go to the other RCA jack for the other channel and do it again.
5. Rinse and repeat till everybody is cruising along at the same idle.

IMO, this is the way Scott should have set these things up in the first place, SOOO much simpler and easier.
[/i]

Here's a drawing of what I did:
Attachment:
IMG_20191026_161837371_HDR.jpg
IMG_20191026_161837371_HDR.jpg [ 419.33 KiB | Viewed 243 times ]


The instructions said to remove all components associated with the switch which were R214 and C208 in the schematic below. But what about R215? It was connected between a buss/terminal strip and the can of one of the electrolytics:

Attachment:
schematic.PNG
schematic.PNG [ 128.31 KiB | Viewed 243 times ]


There was an orange wire going from the center pole of the switch to that same buss that this resistor was on so I just removed the wire. Should I leave the resistor in place?

Attachment:
IMG_20191026_183423664.jpg
IMG_20191026_183423664.jpg [ 624.98 KiB | Viewed 243 times ]


Since this is my first time working with a tube amp, I was hoping someone more experienced could confirm that what I've done is safe and will actually work before I power the amp back on. Thanks for your time and I look forward to learning more here on the forum.

-Steve


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 Post subject: Re: H.H. Scott LK-72B recap and bias modification
PostPosted: Oct Mon 28, 2019 2:40 am 
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Joined: Oct Sun 27, 2019 3:40 pm
Posts: 2
It worked :)

Got the DC balanced and the bias adjusted. Been listening for several hours today while I clean up my work area. I even fixed an intermittent active subwoofer and hooked it up with my little JBL's running with the Scott. Wow :)


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 Post subject: Re: H.H. Scott LK-72B recap and bias modification
PostPosted: Oct Mon 28, 2019 5:46 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 14903
Location: Fernandina Beach, FL
Good to hear Steve!

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Don


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