October 1, 1999

Welcome to my Bias Monitor Modpage for Scott D-Series Integrated Amps. This is a simple idea, which Scott should have implemented themselves, in my opinion. Basically, what this mod does is allow you to measure individual cathode current on each output tube from the top of the chassis, allowing adjustment without a complete disassembly. Some modification is required, like ripping out ALL of the original Scott bias setting curcuitry, which is just ridiculous (your opinion may vary). This mod is, in a way, analagous to the EICO ST-40/ST-70 Loudness Switch Mod, in that it doesn't change anything intrinsically about the amp--although the EICO mod provides a dramatic sonic improvement at low volume control settings--but rather makes living with it and using it a LOT easier (and more fun). This mod is applicable to other Scott amps as well, primarily the LK-48B and the LK-72B. It may apply to others I haven't seen as well (233?). The main thing is you need the above-chassis layout of the D-series (as shown above) to do it.

This is what you do:

1. Take out the common 10-ohm cathode resistor in each channel, and replace each one with individual 10-ohm 1/2 watt carbon films, one per tube (Radio Shack has nice ones).  You'll have trouble reaching the original ground point, so you can extend it by running the ground out to the original plastic test points and using them as ground points. You won't need them anymore anyway (less burned hands).

2. Get some 4-conductor wire--telephone cable or whatever. Solid is better, but I found some multi-colored stranded in a protective sheath (unshielded). Cut a length that reaches from the DPDT slide switch up in the center of the chassis to the output tube section (maybe a foot max). Strip the ends, tin them, route the cable, and tie it in place with some tie wraps.

3. Rip out ALL of the resistors and caps connected to the DPDT slide switch and dual RCA jacks next to it (leave the grounds intact on the RCA strip).

4. Notice that the dual RCA jack is labelled Left and Right, so label it under the chassis as well to make referring to it easier than flipping the chassis over every time you forget.  Also label the output tube sections under there as well.

5. Connect the bottom center tab of the DPDT switch to the bottom RCA jack center pin (looking at it from the bottom, with the back closest to you), and the top center tab of the SPDT switch to the top RCA jack center pin. Note which section of the switch now refers to what channel, per the RCA jack designations above (and now below) chassis.

6. The slide switch will provide switching between tubes in each push-pull combination of each channel, so associate each side of the switch with the tube of that side, if you get what I mean.  And start wiring--ground is already provided, so all you have to do is connect a wire from each outer pin of each section of the slide switch to each associated tube's cathode pin for each channel. This then allows measurement of the voltage across the individual cathode resistor on each tube at the RCA jacks (instead of from under the chassis), depending on which jack you're plugged into and what position the switch is in.

The measurement drill then goes like this:

1. Connect a DMM to a male RCA plug, plug it into the chassis RCA jack for the channel you want to measure and slide
the switch to tube you want to select and measure and set the bias to read .22 vdc (in the case of the 222D and LK-48B--I'd set it top about .30vdc per tube inthe LK-72B and 299D).

2. Move the switch, and adjust the bias balance to read the same voltage on the other tube.

3. Repeat this procedure, back and forth, a couple of times until you have it balanced, side to side (on one channel).

4. Plug the meter into the other RCA jack and repeat the procedure in steps 1 through 3 for the other channel.

5. Repeat the above steps a few times (they all interact), until you're done.

Believe me, with the channel designations all there in front of you, it goes a lot faster than you might imagine, and you make a lot fewer mistakes than by working above and below the chassis at the same time--take my word for it, I know.

What's cool about this mod?

After it's installed, everything happens above the chassis, and it uses existing hardware.  No new holes. No drilling rivets. I love it. I'm doing it to my 299D when I get it downstairs again.
 

There it is--see what you think. You might prefer leaving these things stock, but I say the heck with it--this allows exact tube balancing and push-pull amps just love that.

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