| 40KG6A Triode Connection PP Amplifier |
40KG6A Hybrid SEPP OTL DC amplifier was modified to enjoy the clean sound of the all-stage differential push-pull amplifier with lower power consumption. However, since all-stage differential 1626 push-pull amplifier with lowest power consumption was completed, it became fewer turns, so I reconfigured it as a trans-driven triode connection push-pull amplifier. For a while, I enjoyed the amazing sound of transformer coupled amplifier. However, due to the dissatisfaction of lack of bass sound, I reconfigured the amplifier into a normal CR coupled push-pull.
The basic policy of remodeling is as follows.
1. Two driving transformer holes are closed with aluminum tape.
2. In order to earn output power, the bias of 40KG6A is changed from self to fixed.
3. In order to improve the low frequency characteristics, the DC balance of 40KG6A is adjusted with volume.
4. The plate current of 40KG6A is not changed much, that is, 65 to 70 mA per tube.
5. Negative feedback is not applied, because I want to hear natural sound of vacuum tube.
It's the front view of amplifier. An open hole was just blocked up by an aluminum tape for a driver
transformer.
If you put the mouse over the image, it will be enlarged. (Same below)
It is inside the amplifier. Although it has become increasingly dirty because of repetition remodeling
many times, since the self-bias resistance (820 Ω 15 W) was removed, it was refreshing only a little.
The direction of the parts is also uneven and it is not a substitute that can be shown to the very
person very much.
It is the circuit diagram of the amplifier. For fixing bias of 40KG6A, a power supply of -100 V or more
is necessary, so we have to examine. After all, a bias voltage of about -75 V is necessary in triode
connection, and in order to be able to adjust DC balance at the same time. In the push-pull amplifier,
taking low DC balance firmly improves low frequency characteristics. Adjustment was made with the
oscilloscope so that the square wave response waveform of 100 Hz was most beautiful. In this push-pull
amplifier, it is very important to prevent DC magnetization from occurring because the output
transformer is small.
It is the input / output characteristic of the amplifier at 1 kHz. Data could only be taken up to
about 6 W output, but if you estimate from the graph, it seems likely to go to about 10 W. The amplifier
gain is 3.0 times, the residual noise is 0.3 mV, and the damping factor is 3.2. Amplifier gain is very
low, but since we are using preamplifier with voltage gain 2.7 times at all times there is no problem
in practical use.
It is a square wave response waveform at 2 V p-p of the push-pull amplifier of 40KG6A triode connection.
It is 100 Hz, 1 kHz, 10 kHz from the top. Compared with transformer coupling, the sag of 100 Hz is
smaller, both rising and ringing at 10 kHz are improved. Impression of the sound after remodeling again,
but the vocal gloss did not change, the bass became rich, the maximum output also increased.
