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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

Oscilloscope Measurements - K-12M Tube Amp Kit

Back to the bench and "scope" to see what took place. Here is where stuff gets a bit interesting and controversial. With no load, the amp made an excellent oscillator. According to my scope I got full output at around 200 kHz. The output did vary with the volume control setting, max output at minimal volume setting, max output at maximum setting and very low output at some middle setting. It reminded me of some phase shift oscillators I built long ago. With an 8 ohm load, the level of oscillation was greatly diminished. With the load in place there was roughly 40 mV of it at the speaker terminals. This puzzled me a bit. Obliviously, it was a feedback problem. The logical place to check was at the output transformers. These have an unusual collection of parallel and series windings that allow for selection of 4-16 ohm loads. Another K-12 "DIYer" alerted me a similar problem that he solved by swapped the grounds and hots on the output side. This was the way I originally wired mine. I put the transformers back to the normal connection. It was essentially as bad. I finally tracked the oscillation down to the small (33pF) capacitors in the feedback loop (C10 and C11). Removing them solved the problem. This is exactly opposite to what I would expect. They should limit high frequency response and would inhibit high frequency oscillation. The only explanation I could come up with was that there was a certain amount of phase shift in the original design and replacing the output transformers and coupling capacitors changed it. Thus what would have been negative feedback through the 33pF became positive and caused oscillation. I used my scope in its dual trace mode (with a 1000 Hz square wave input) and disconnected the secondary windings from the amp. I determined that the proper phase orientation of the transformers was indeed the normal connection. This can be seen clearly in the scope photo with the feedback disconnected.

Output Transformer without Connection to Feedback or Ground
Photograph 03: Output Transformer without Connection to Feedback or Ground

After reconnecting the ground and feedback loop (with the capacitors removed), some 60 Hz hum remained. The scope setting in Photograph 04 is 5mV / division, so the actual hum and noise was around 20 mV.

60Hz Hum
Photograph 04: 60Hz Hum

With a one kilohertz square wave as an input signal, the output trace showed some overshoot and ringing. The lower trace in Photograph 05 is the output trace with an amplitude of about 1 volt PP.

Overshoot and Ringing
Photograph 05: Overshoot and Ringing

The ringing rapidly decayed and the top of the wave was quite flat. This is the sort of response that I would expect to see in a tube amp of this price range. If my memory serves me correctly, this is largely a function of the inductance and frequency response of the output transformer. The residual hum on the trace was about 20 mV at the output terminals. This isn't a lot, but it was annoying. If you are like me, it was something I couldn't ignore. So after a large amount of part swapping the best fix I found was to place a 10 uF Solen capacitor in parallel with C8. This removed some of the hum. I suspect that some of the hum was related in how the output tube screens were powered. In the original circuit they received power after resistor R18 (470 ohm). The B+ had one more section of filtering than the output transformers. Switching to UL mode bypassed the filter section. It appeared that nothing short of a major rebuild of the power supply would make the amp significantly quieter. As the hum was inaudible in my system unless you put your ear right to the speaker, I figured I would leave it alone. So back to the listening room.

How did it sound? Rather good in my opinion. It was quite a bit different from the original. Descriptions of sound are at best subjective, but I would characterize it as smooth, good overall balance, good detail. The sound stage is reasonable, depth and width are OK, height is better than expected. It seemed to have better bass than the original configuration, but I don't have long term calibrated ears and this might not be true. In either case the bass is sufficient. This is an easy amp to listen to. Strings and female vocals are its forte. Single instruments and singers stand out with great clarity. Massed instruments and vocals are not as well produced. But it is necessary to keep in focus that this is a "budget" amp. For around a $1000US more you can get a number of better tube amps, but I don't know of any in the K-12 price range.

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