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How do I adjust my Scott 299C Bias and DC Balance pots?


JDJohnson

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Guest thesloth

Do you have "current sensing" resistor installed in the cathode circuits? The schematic doesn't show any. I also only see a DC balance adjustment. If you have a voltage meter and are careful around high voltage I can show you how to do it without the "current sensing" resistors.

 

http://hhscott.com/pdf/299C.pdf

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Guest thesloth

You will want to first put your red meter probe in the cathode current test point and the black probe on ground. Adjust the bias adjust pot until you reach .22 volts. The balance is a bit trickier and I can explain how I do it. Refer to my image below. You will want to measure DC resistance with the amplifier off from point A to B and A to C. Lets use some hypothetical values here and say we measured 153 from A to B and 170 from A to C. We want equal current between the two so a little math will tell us what we want. We are looking for a total of 70mA per channel so that's 35mA a tube. What we are effectively doing is measuring the voltage drop across this DC resistance in the output transformer. For A to B we want .035*153=5.355 and for A to C we want .035*170=5.95. So being extremely careful with the amp on you will measure DC voltage from A to B and A to C adjusting the balance control until you get 5.3 across A to B and 5.9 across A to C.

 

PLEASE BE VERY CAREFUL!!!!!!!!!!!!

 

If you want measure the DC resistances first and check back with the data. You should have four measurements A to B and A to C for two channels.

 

ofaJvD8.jpg

Edited by thesloth
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Guest thesloth

thesloth

I measured DC resistances of:

 

Left channel:     A-B 155 ohms      A-C 170 ohms

 

Right channel:   A-B 155 ohms      A-C 167 ohms

 

 

 

Lol I know my Scott amps!!  My "hypothetical" values were pretty close eh :)

 

Left channel             A-B  155*.035=5.4v     A-C 170*170=5.9v

 

Right channel           A-B  155*.035=5.4v      A-C  167*.035=5.8v

 

 

So again BE VERY CAREFUL and what you are doing is measuring the DC voltage drop across those same points now with the amp on. Adjust the DC balance control until you get:

 

Left Channel             A-B 5.4v         A-C 5.9v

 

Right Channel           A-B 5.4v         A-C 5.8v

 

 

Keep me posted.

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thesloth

 

Yes, you know Scott amps!  Your "hypothetical" values were very close!

 

I'll adjust the DC balance controls to get the four DC voltage drops across the four points to your voltage calculations with amp on with old tubes in and then adjust again when I buy the news tubes soon and put them in.

 

I'll post the old tubes results soon.

 

I'll be very careful!

 

I'm wiring some test points up and placing them on top its chassis for future easier top accessible adjustments. What type of connector do people use for their test points?

 

Thank you!

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  • 6 months later...
Guest Dflip1
 
Here are the instructions that Craig sent me years ago.

 

You need a decent multimeter with varying scales for DC voltage and DC MA in order to properly adjust the bias. A digital meter is fine as long as it has varying scales.

 

Turn the amp on, but make sure the music source (CD, tape, tuner or turntable) is off. Set your meter to its lowest DC voltage setting, preferably 1 to 2 VDC. Connect the negative (black) probe to one of the test points for that channel (at the back of the amp) and the positive (red) probe to the other. It doesn’t matter which one goes to which. Adjust the DC balance for that channel to the lowest possible reading or zero. Then repeat the same procedure for the other channel.

 

Now move the negative lead to the ground test point, or any other part on the chassis. Put positive lead to either test point for the channel that is being balanced. Set the meter to 100 MA DC or above. Set the bias voltage pot for that channel to read 34 MA, then do the same for the other channel. When adjusting the voltage for one channel, it will effect the other channel, since they share the same voltage source. Keep repeating until both sides measure 34 MA. This is the maximum setting on your tubes, do not exceed this value. In fact, if you don’t listen to volumes above the half way point on the volume knob, set the bias adjust to 32 MA, your tubes will love you for it.

 

This procedure should be done every few months or when new output tubes (7591) are installed. This does not have to be done when you change preamp (12AX7 or 5751), phono (12AX7), phase splitter (6U8) or rectifier (5AR4) tubes. With brand new tubes, this should be done every two weeks for the first month, until they get burned in.

 

The Scott LK-72 was the kit version of the 299C.

 

Don

post-62751-0-79800000-1459357965_thumb.j

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Don,

 

       Not trying to be hard to get along with but please do not modify information I send you and then post it as what I sent you. Those instructions are modified and inaccurate version of my biasing instructions. I NEVER suggest using the DC mA function of a multi meter in my biasing instruction since they would be very in accurate. 

 

 Also as a side note those instruction are only valid for an amplifier I rebuilt and modified. The original poster does not have a NOS Valves rebuilt amplifier so those instructions will not help him. 

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