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S-5 Electronics K-12M Tube Amplifier Kit

Gio Militano    Canada Flag     To email Gio, type out the email address.

Page 1: K-12M Tube Amp Kit, Kit Review, Enclosure Construction
Page 2: Upgrades and Modifications


Tube Amp Upgrades and Modifications

The modifications as proposed by VoltSecond are well thought out and have received positive reviews by others who have implemented them. I have completed bridge rectifier and snubber capacitor upgrades as described by VoltSecond. For more specific information about these mods, see the VoltSecond web page.

Bridge Rectifier

The stock rectifier bridge was replaced with four UF4007 diodes. To fit all four diodes on the board, I placed two of the diodes through the top of the PCB and the remaining two through the underside of the PCB. You can see two of the UF4007 diodes in the bottom left of the Photograph 6 below. VoltSecond shows that only one diode is required, but since I had the diodes and I did not feel like cutting a trace, I decided to replce the entire bridge. The UF4007 diodes helped reduce mid-range and tweeter noise.

Snubber Capacitors

I intalled the snubber capacitor across the HV and filament snubber capacitors on all four tubes. Across the HV I used a 0.01 uF Metalized-Film Capacitor from Radio Shack (272-1051) rated at 400VDC. For the filament snubber capacitors I used 1 uF Vishay Metallized Polyester Film Capacitors (MKT 1817) rated at 63V. You can see the installed snubbers in Photograph 6 below. The filament snubbers are the green rectangular capacitors. Changing the filament supply to DC would likely be a better fix, but would prove difficult to do given the space restrictions on the PCB.

The snubbers really helped clean up a lot the high frequency hiss. The midrange still has some minor hiss/hum, but the performance with these modifications is greatly improved over that of the stock amp.

K-12M Tube Amplifier Modifications
Photograph 6 - K-12M Tube Amp PCB with Modifications

Capacitor Upgrades

I have made several capacitor changes from the stock kit. The input DC blocking capacitors were removed and removing these capacitors from the signal path has made a big difference. Of all the mods I have tried, this one produced the most positive result. To close the circuit, I used 16 gauge solid copper in place of the capacitors. You can see the copper wire in Photograph 6.

I had some nice film/foil capacitors lying around, so I decided to upgrade the four metallized polyester coupling capacitors. A picture of an original coupling capacitor (orange) and the replacement film/foil is shown in the Photograph 7 below (Canadian Nickel used for scale).

Film / Foil Coupling Capacitors
Photograph 7 - Film/Foil Meatilized Coupling Capacitors

The capacitors I used seem to be very high quality film and foil capacitors with a dissipation below 0.0004. The new film/foil capacitors seem to have made the sound more dynamic.

To make the large film/foil capacitors fit on the board, you have to bend the leads. One of the capacitors gets a little close to the B+, so I covered the leads with some 18 gauge insulation I stripped from a wire. If you look closely on Photograph 6, you can see the white insulation on the far left coupling capacitor.

Tube Dampers

I have always read about tube dampers helping with the microphonics of tubes, so I thought I would give them a try and I purchased a set of tube dampers on eBay for about $8US. The dampers only provided some very very minor improvements in sound.

Finished K-12M Tube Amp with Tube Dampers
Photograph 8 - Front View of K-12M Tube Amp with Tube Dampers

S5 Electronics K-12M Tube Amp Kit with Tube Dampers
Photograph 9 - S5 Electronics K-12M Tube Amp Kit with Tube Dampers

Future Work

I have been so pleased with the performance of this little tube amplifier that I do not like to pull it out of my system to work on it. I do not plan to upgrade the resistors to metal film. I read on diyAudio that someone tried resistor upgrades but that the changes did not result in much of a difference. VoltSecond has a few more mods that I would like to try.

After Thoughts

If I were to build this tube amp kit again, there are a few things that I would do differently. First off, I would keep the inputs on the front on the front of the amplifier. Putting them in the rear like I have done increases the potential for picking up unwanted noise (hum) from the mains supply and the power transformer. Also, I would position the power transformer off to one side of the chassis and position the audio output transformers off to the other side with the core rotated 90 degrees. This would also help reduce the chances of getting unwantend noise.

For more information on these simple tube amplifier kits and a list of retailers, see the S-5 Electronics Tube Amplifier Kits page.


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