This 300B amplifier build is coming far earlier than expected.
This amplifier build was initiated around 6 months ago. Then it starting from sourcing the major components such as the amplifier case, tube sockets, resistors, capacitors and the tubes itself. All of them are mostly imported and with a long waited, they've just arrived two weeks ago. The other key components such as output transformer, power transformer and choke required a week to custom made based on this 300B amplifier schematic and idea.
It is very common when building an amplifier from a scratch like this, there is an issue appears after it complete. Most common issue is the hum. This annoying hum can ruin my time to troubleshoot it. Especially when building 300B amplifier, hum is mostly present at the speaker output. But this project is different. This 300B amplifier is finished without the annoying hum issue. This amplifier working like a charm since the first time it powering ON.
And the sound is really good.
In this build, I searching and study many kinds of schematic for the reference. My searching is end up to the design of Luxman MQ-300 amplifier. This is a simple design, which is consist of 2 stages of common schematic of low gain driver and continue to 300B power stages.
The design has an interesting part. It is the idea for paralleling each driver stages. Later on, I know the benefit about this method is for lowering the output impedance and improve the current output require by 300B grid input. This parallel driver is also adopted in the Audio Note Meishu amplifier as well. So, I think I will keep this idea for my 300B amplifier build.
Let's check the schematic on the photos bellow.
The first one is Audio Note Meishu 300B schematic which I got it by searching online.
The second diagram is Luxman MQ300 took from their official website.
See how much similar on their design.
Both of those designs shared the same principal. For my 300B amplifier build, I only use one gain stage, instead of two. The result probably more simple design with adequate gain that I set to ten times. Pretty low actually.
Another difference with my 300B amplifier build is the filament schematic, or the tube heater voltage.
Both of those design use DC regulated power supply to the 300B tubes filament. DC power supply for filament is hum free but more complicated due to more components involved in it. Since my 300B amplifier build idea is simplicity, so I will use AC power supply for the 300B tubes filament.
This AC power supply known for best sounding result compare to DC, but I cannot say it now. For me using AC is pure simplicity.
Now, let's start this build.
The casing I am using here is pre-made thick steel chassis. The weight for the casing is around 5 kilos. It is very solid, great finished with pre-cut for input output mounting holes.
I install all accessories such as speaker terminals, RCA input terminals, power switch and 300B tubes mounting.
I invest the tubes socket so much on this amplifier build.
The tubes socket I am using here is all come from CMC. The quality is outstanding with fine ceramic housing finish and pure copper terminals.
I am using PCB pin terminals for the component mounting on each tube sockets. This pin terminal quality actually very good, although it looking like a cheap PCB terminal.
Other than that, the first resistor I put into this casing is the 300B cathode high wattage resistor.
I am using 820-ohm 50-watt resistor for this job, while normally 20-watt will do just fine.
The next components is the high voltage filter capacitor for the power supply. I am using Rubycon SV type, meaning surge-voltage up to 570 volts. I calculate this amplifier will running on 430 volts.
The next components is very important to reduce the hum. It is the hum-pot, basically a potentiometer for adjusting AC phase feeding to the 300B tube filament.
The next step is installing power supply cable from IEC power socket to the power switch using high quality Cardas cable.
To securing the cables from hanging around, I tie up to the plastic bracket that bolt on to the chassis.
The transformers are the big parts of this amplifier.
I custom made all the transformers. The output transformers and choke transformer are put inside the box and fill with resin. This method is very effective to reduce the vibration and noise from the transformers.
The power transformer is placed with different orientation and close the top using thick steel cover to reduce the noise as well.
Well, it is common from many of the readers in this blog asking for the schematic. I rarely provide the schematic because I never draw it. All of my build from a DAC, amplifier, power supply, or everything, I always using the components datasheet as my reference.
It is the same like in this 300B amplifier build.
All of the schematic is on my head already.
The installation process starts from 5U4GB power supply section, continue to 300B power amplifier section, then the parallel 6SN7 as the driver section.
I estimate this amplifier finished on the next weekend, but somehow it is finished now.
This amplifier is finished and now is time for testing.
5U4GB is rectifier tube is the first tube to put in and test.
I check the DC output without load is 480 volts. Pretty good.
All AC voltage for tubes filament are reading as per specification.
With all of the tubes are installed, now I turn this amplifier for the first time.
No smoke, no overheating issue, no hum, all normal.
After several minutes this amplifier warms up, now I start to adjust the hum-pot to reduce the hum at the speaker output.
Without adjustment, at the hum-pot position in the center, the hum is present in very low level. Which is already acceptable.
Adjusting the hum-pot a little bit makes the hum start to lower again. At certain level, the hum cannot go any lower.
But without this adjustment, the hum is acceptable already.
The amplifier sound really good. I listen few albums for 3 hours straight without any issue.
This build is finished faster than expected and become my last project in this year.
Cheers!
Disclaimer: Any statement and photos in this article are not allowed to copy or publish without written permission from the writer. Any injury or loss from following tips in this article is not under writer responsibility.
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