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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

Summary - K-12M Tube Amplifier Kit

Power supply modifications: Ultra fast rectifier, bigger filter cap, heater snubbers, noise filtering on the secondary of the power transformer. This is basic stuff to get rid of noise and hum. Cheap (under $15US) and a generally a must do.

Replacement output transformers: Better soundstage, more detail in mid and high ranges. Cost about $80US. My opinion is it is worthwhile.

Replace the inter-stage coupling capacitors with Auricaps: This makes a major improvement. Cost about $60US. This is a little tricky to do but absolutely a good thing.

Add a low ESR 10 uF capacitor to the power supply: Cheap (about $10US) and easy to do. There were minor improvements in hum and slightly better bass. I would recommend doing this as it is easy and inexpensive.

Remove 33pF feedback capacitors: No cost. Your amp might behave differently than mine, but in my case it was a must do.

Front View - K-12M Tube Amplifier
Photograph 06: Front View - K-12M Tube Amplifier

Front View - K-12M Tube Amplifier
Photograph 07: Front View - K-12M Tube Amplifier Without Cover

What would I do differently?

I would try the capacitor replacement before the transformer replacement. The transformers look great and changing to UL configuration should have benefits in lowering harmonic distortion, but since I can't measure the distortion I can't verify that it worked. The capacitor switch might make the transformer replacement optional.

What else might be worth doing?

I am considering replacing the remaining electrolytic capacitors with larger, higher quality ones. This has the potential for improving the bass and further reducing the residual hum. I might replace the volume control with a higher quality unit, or perhaps removing it entirely. The use of a constant current source in the output tube cathodes (like in VoltSecond's modifications) and employ a tube balancing circuit (same source) could be beneficial. Another possibility is to bypass the output tube cathode resistors. Most likely, I won't make any of these changes as I am well satisfied with the sound as it is.

What did it all cost?

The total cost of the amp, including the case, is no longer in the real el cheapo area but at about $400 is going to be very hard to beat. I hope I have inspired some of you to investigate ways to modify (and hopefully improve) your K-12s. If you have questions or suggestions for other improvements, drop me an email.

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Follow Up