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This is the front view of 1626 all-stage differential push-pull amplifier. Since the hole of the socket can not be changed except by increasing the diameter, the spacing between the tubes is slightly narrow. Since the tube is not so tall, it is just at the current audio rack. Unnecessary holes in the chassis are open as it is the second modification.

It is the inside view of 1626 all-stage differential push-pull amplifier. The interior is very dirty as it is the second modification.

This is the circuit diagram of 1626 all-stage differential push-pull amplifier. Basically it is the same as 6CL6 PP amplifier except that the primary impedance of the output transformer is changed from 16 kΩ p-p to 8 kΩ p-p and the plate current per output tube is reduced from 35 mA to 25 mA.

It is the input / output characteristic of the amplifier at 1 kHz. It is almost linear up to about 1 W output, and to about 1.5 W if it accepts distortion. Although the amplifier gain is only 3.5 times, it always passes preamplifier (gain 2.7 times) so there is no problem. The residual noise is 0.5 mV.

It is square wave response waveform of 100 Hz at 2 V p-p of 1626 all-stage differential push-pull amplifier. There are a few sags, because the output transformer is so small.

It is square wave response waveform of 1 kHz at 2 V p-p of 1626 all-stage differential push-pull amplifier. It is straightforward and has no problem.

It is square wave response waveform of 10 kHz at 2 V p-p of 1626 all-stage differential push-pull amplifier. It is a beautiful wave without rampage.

It is 1626 all-stage differential push-pull amplifier housed in the amplifier rack. The height of 1626 seemed to be a problem but it was safe at last. Although it is vacuum tube amplifier under such a bad condition, generated heat is small and it can be used without worrying even in the summer. Although it is a push-pull amplifier, only 1.5 W is outputted because of all-stage differential, but it is enough in my narrow room from bass to treble.


Afterwards Remodeling

Actually, about a year after the completion of this amp, I converted it to the normal push-pull amplifier (DEPP). The sound of all-stage differential push-pull amplifier was honorable sound and there was no dissatisfaction, but since I originally liked unique sound, I feel that something is unsatisfactory. It seems that gorgeous sounds with much distortion rather than the clean sound. Therefore, the final stage was self-biased by resistance and electrolytic capacitor from the constant current. I can not believe it, but in my ears with poor frequency characteristic, the glossy vocal came out towards the front. Although it seems to be scolded by all-stage differential push-pull amplifier believers, I guess these results came from because 1626 had its own good characteristics as it is originally triode tube.

It is the final circuit diagram after remodeling. Although the first stage is a differential phase inversion circuit same as before, but since G1 on one side was dropped to earth, negative feedback is not applied. The final stage is the self-bias using a single resistance with 2 tubes gathered together. It is 410 Ω combined with the hand-held resistances, which is almost the same as the plate current at the all-stage differential. The easiest way to make ordinary push-pull from all-stage differential is to connect the electrolytic capacitor in parallel to the final stage constant current circuit, but this time the constant current circuit was also removed. Amplifier gain, residual noise hardly changed, the maximum output increased to 2.2 W. The newly measured damping factor value was 1.5.

It is square wave response waveform of 100 Hz at 2 V p-p of 1626 push-pull amplifier. Sag is improved more than in all-stage differential one, but bass sound is not changed.

It is square wave response waveform of 1 kHz at 2 V p-p of 1626 push-pull amplifier. It is the same waveform as in all-stage differential one, but I feel that the vocal sound rather comes out towards the front.

It is square wave response waveform of 10 kHz at 2 V p-p of 1626 push-pull amplifier. The response is slightly slower than in the case of all-stage differential one.