[Vintage-Audio] Scott LK-72 biasing?

[Roy Morgan]

At 04:08 AM 6/23/02 +0000, you wrote:
>Hello
>I'm overhauling a Scott LK-72 for Duane.  I downloaded the owners manual and
>schematic from the Scott site but didn't see any procedures/specifications
>for adjusting the bias and DC balance pots.


Hi, Phil,

I mailed Duane copies of the manuals and maintenance instructions I have 
here for the LK-72 and LT-110.

The Amplifier intofmation tells what to do with the bias adjust pot: 
Essentially you adjust the relative bias on the two tubes till you get 
minimum HUM in the output.

This does not set the basic bias level, only the relative balance of bias 
between the two tubes.  Who knows what the actual standing plate current 
will be in the two tubes..

>This unit has the Bias select switch and two RCA jacks labled as Bias test
>points.  What Voltage am I looking for at these test ponts.

I do not remember these items in either the printed information I sent to 
Duane, or on the actual LK-72 unit I have here.

In my suggestions to Duane, I suggested that the bias on the final tubes be 
increased (that is more negative voltage at the grids) so as to reduce the 
standing plate current.

I can give more detailed suggestions once I see the schematic.  In the 
meantime, I suggest you find out where in the circuit those RCA jacks 
connect.  If I remember correctly, there is NO cathode resistor in the 
circuit. If that is so in your amplfier, then I would expect the RCA jacks 
to connect to the grid circuit some where, to allow you to set the negative 
bias voltage being applied to the grids.

IF on the other hand there are cathode resistors in the output tube 
cathodes, the RCA jacks are likely connecte  to those resistors and will 
indicate cathode current.  In that case, you need to apply ohms law to 
determine the correct or desired indication in volts at the jack.

I will be happy to hear from you on the phone or by email to help sort all 
this out.  I can dig out my amplifier but cannot put it into operation 
since the filter caps in mine are arcing POP POP SNAP!

Roy
- Roy Morgan, K1LKY since 1959 - Keep 'em Glowing!
7130 Panorama Drive, Derwood MD 20855
Home: 301-330-8828 Work: Voice: 301-975-3254,  Fax: 301-948-6213
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