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KT120 Push-Pull Tube Amplifier Schematic

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

KT120 Push-Pull Tube Amplifier Schematic (Oddwatt Audio OBHO)

This KT120 tube amplifier is a follow on to the smaller Oddwatt power amplifier. It is about as large as that design can be pushed. Still most folks do not really need more than 40 to 45 watts of audio power. The basic amplifier design is the same as the smaller ones with upgraded components where needed. This KT120 push-pull amplifier is a class-A, ultra-linear design with a self-inverting output stage, much like the other ones posted in previous projects. The amplifier will deliver approximately 45 watts RMS with a bandwidth of 10 Hz to 25 kHz +/- 0.5 dB (with the negative feedback engaged). Distortion is under 1% at half power and gradually rises to the 2-4% range at full output. The variation is tube brand dependent, particularly for the driver tube brand. With Carefull construction the signal to noise ratio is about -90 dBv. The amplifier required approximately 2 VRMS of input to reach full output. The amplifier can run with either the minimal of negative feedback shown, or none. The feedback is used to ensure stability under possibly difficult loads as the amplifier has considerable high frequency gain without it. Therefore there is a possibility of exciting the 70-80kHz transformer resonance and causing ringing or oscillation. Use of alternate audio output transformers is not recommended for stability reasons.

KT120 Push-Pull Tube Amplifier Schematic (Oddwatt Audio OddBlocks)

KT120 Push-Pull Tube Amplifier Schematic

Anyone familiar with the earlier and smaller versions of the SIPP design will see that it is nearly the same. Naturally the amplfier does require a much larger power and audio output transformers. The audio output transformer seems like overkill, but in reality is not as the determining factor is the amount of idle DC current it has to handle (almost 300 mA). I used two LM317HV voltage regulators in the constant current source as the thermal dissipation was a bit more than I would like to have one handle. It proved not to be necessary to make any provision for balance of the load between the regulators. Un-matched pairs would track well within 10% of each other. No failures because of thermal issues have been reported in either the DIY amplifiers or the commercial Oddwatt Audio Assembled ones. The B+ going to the driver tube is regulated not so much for stability (although that does occur) but to insure the tube does not exceed its ratings. Modeling determined that it was possible for one section of the tube to heat up sufficiently faster than the other and thus see essentially the entire B+ supply voltage at least momentarily. With a B+ voltage of 450 volts available the tube would fail. A byproduct of the regulator is a slightly quieter output.

The power supply schematic for the KT120 push-pull monoblock amplifier is shown below (click schematic to enlarge).

Power Supply Schematic - KT120 Push-Pull (PP) Tube Amplifier

Power Supply Schematic - KT120 Push-Pull (PP) Tube Amplifier

Layout of the KT120 push-pull amplifier is uncritical and point to point wiring is fine. The circuit can be rewired for all 6 volt heaters if desired. An alternative that allows 6 volt driver tubes is to series a 20 ohm 10 watt resistor in series with the driver heater. This will allow use of 6SL7 tubes as drivers. The type 5751 tube can be used as well, other tube types are possible, but the results are unpredictable. They must have a similar Mu and anode impedance. Some of the lower gain 12AX7 variants may offer similar performance. AC heaters can be used in place of the DC heater supply shown. This will result in a reduction of about 2-3 dB in the signal to noise ratio and be unlikely to be audible except with extremely sensitive speakers. The constant current ICs (type LM317HV) need to be heat sinked as they dissipate about 6 watts each. Do not use the standard version of the LM317 voltage regulator as it will fail due to over voltage.

DIY KT120 Tube Push-Pull Monoblock Amplifier

DIY KT120 Push-Pull Monoblock Amplifier

A potential problem with any DIY build of a power amplifier that uses even a very small amount of negative feed back (NFB) is the phase of the output transformers. Since not all transformers are marked to show the phase, it can be a guessing game. If the feed back phase is correct then the output level will increase if the NFB is disabled. If the phase is wired backwards, the output level will decrease when NFB is disabled. With the incorrect phase the sound will tend to have too little bass and a sort of shrill treble. What I suggest as an easy fix is to make the two capacitors going to the output tube grids easy to connect to either the signal or the ground. Then with either jumpers or hard wires set them up in your best guess (or if using the Edcor output transformers as in the plans) and see if it is correct. If not just swap the connections and it should be correct. This fix has the same effect as if you swapped all 4 transformer leads without nearly as much hassle. BTW this will work on any push-pull amplifier. It just swaps the phase of the signals driving each of the tubes.

As always, grounding is important and here are some of my suggestions to follow regarding grounding and shielding for your DIY audio projects. For valve amplifier design and construction tips, see my blog entry about design and construction tips and suggestions for vacuum tube amplifiers.

KT120 Push-Pull Monoblock Amplifier with Cover

KT120 Push-Pull Monoblock Amplifier with Cover

How does the amplifier sound, Excellent IMO. It has some of the strongest, cleanest bass of any amplifier I have ever heard. It should because typical response puts the -3 dB point below 10 Hz. The midrange and top end are equally clean. The amplifiers are quiet as well. Typical signal to noise is better than -90 dB (wide band, input shorted). A recent professional review of a pair of the assembled amplifiers confirmed these observations. As a DIY project it still offers some areas for adjustment. Capacitor brands, resistor brands and driver tubes all highly influence the sound and can be tailored to your tastes.

Good listening

Additional photos and information is available on the forum in the KT120 Oddblocks thread. Questions and comments are also welcome in the KT120 Oddblocks thread.

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