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Modifications for the K-12M Tube Amplifier Kit

Bruce Heran    USA Flag     To email Bruce, type out the email address.

Page 1: Introduction, Modifications
Page 2: Oscilloscope Measurements
Page 3: Summary
Page 4: Follow Up

Follow Up - K-12M Tube Amplifier Kit

I had to rewire the output transformers for a four ohm load as I have some new speakers coming shortly. The process was actually rather easy. Just remove everything but the power transformer from the case and try not to lose any of the nuts and bolts.

Top View of K-12 Tube Amplifier After Rewiring
Photograph 08: Top View of K-12 Tube Amplifier After Rewiring

There are a number of changes from Photograph 02. Notice the yellow wire. It is from the 8 ohm tap on the output transformer. The red one in Photograph is now tucked under the board in case I might have to change the configuration again. The solder joint looks bad in the photo but it is actually ok. Notice also that the power switch has been removed. Less hum (perhaps) and I didn't use it as the entire system is turned on centrally. The pattern of the output terminals is also altered from stock. The wiring from the new output transformers fit better in that configuration.

When I finished the rewiring I ran into a logistics problem. I was unsure weather to hook up the feedback loop to the 4 ohm output or to the 8 ohm (unused) tap. I tried both and the scope patterns for the 8 ohm connection were a bit flatter and cleaner.

4 Ohm Tap Feedback
Photograph 09: 4 Ohm Tap Feedback

The lower trace in Photographs 9 and 10 is the 1000 Hz output at approximately 0.4 volts PP.

8 Ohm Tap Feedback
Photograph 10: 8 Ohm Tap Feedback

The photos don't show it clearly, but the 8 ohm tap produced a cleaner output. I have two suggestions for anyone replacing the stock output transformers. First, run all output connections to a terminal strip. The European style with feed through connections (available from Radio Shack and other sources) is perfect. This will allow easy impedance and phase changing. Second, use a dual trace scope to verify the configuration. This will also pick up any odd stuff like the oscillations I discovered.

As a last note, the Dayton BR-1 speakers I am presently using seem to be slightly below a standard 8 ohm load and the K-12 does just fine with them even when set for 4 ohm output.

For more about the K-12 tube amplifier kit and for a list of retailers, see the S-5 Electronics K-12 and K-502 Tube Amplifier Kit page.

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