Reisong A10 Single Ended Tube Vacuum Amplifier (Part 3) High Bias 6L6

In this part 3 of the Reisong A10 single ended vacuum tube amplifier, I will share the upgrade progress to make this amplifier sound better by replace all the vacuum tubes and increase the bias on them.

On the previous upgrade, I have changed this amplifier configuration from ultra-linear to triode. The triode is well known for better sound but has lower power output compare to the ultra-linear. This lower power output will become an issue if the speakers connected to this amplifier is having low sensitivity. But because this amplifier is intended to use for balanced headphone amplifier only, so the power output is not become my concern.

This amplifier is having so much potential, so keep reading for this final progress of this amplifier upgrade.

This Reisong A10 amplifier vacuum tubes using all PSVANE brand. I use them quite a while during upgrade and testing of this amplifier but I always feel that the sound are not superior. Probably because of those low quality vacuum tubes that came in the amplifier.

Especially for the rectifier tube, once I replace to the vintage RCA brand, I can hear the sound improvement in better smoothness and dynamic.

The small vacuum tubes which are 6N1P also replaced with new-old-stock Russian made. This give the music more fuller and the sound-stage more deeper.

The original power tubes are using EL34. This is pin compatible with 6L6. So I change it with this new vacuum tube and test the sound.

With 6L6, the sound improvement is emphasis on smoother sound with darker background. This improvement is quite a lot and at this point I really like the sound quality from this amplifier.

I could stop this upgrade and get happy for this improvement, but I still continue.

The next improvement is to increase the bias for each of 6N1P and 6L6. Higher the bias will make all the vacuum tube running hotter. This bias adjustment will improve the sound quality but it has a drawback. Since I will push them to run hotter, then the lifespan of those vacuum tubes can be decrease. Perhaps from normal estimated lifespan from normal bias is about 2 year, then with higher bias, the lifespan will reduce to about a year only.

The guideline for this bias adjustment is this: the lower of the resistance, the higher is the bias, vice versa.

The bias on 6N1P are located in the R104 and R204. I change the resistor value from 2K ohm to 1K ohm. At the end, I lower the value again to 900 ohm and it is working fine.

The capacitors across this bias resistor is optional. I using Panasonic FC 220uF 63volt on each bias resistor. But please mind the negative polarity of the capacitor that should goes to the ground taken from each of the bias resistor connected to it.

The next step is increase the bias on 6L6.

This bias resistors are located on R108 and R208. I change to the lower value of those resistors from 500 ohm to 300 ohm. The resistor wattage is should be above 5 watt.

At this point, all of the vacuum tubes bias already increase a lot and I can feel they are gets hotter than before.

How does the sound from higher bias?

I can say this amplifier sound is not the same as when it comes on my desk.

The sound quality and characteristic of this amplifier is now far better, smoother and fuller, far from edgy and harsh sound.

I think this is all I can do to make this amplifier sound better. I can sleep well now:)

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