My previous post about the 300B tube amplifier build was a breakthrough for myself. That was my first 300B amplifier build that by its nature is direct-heated triode. I read everywhere saying that 300B amplifier will always have small hum present at the output. Mostly when pairing with the speaker sensitivity above 88 db. My 300B amplifier was built intended to drive a headphone. It is also can act as pre-amplifier. I only can imagine the result will be small hum present and I should accept it.
But the result is worse than my expectation. The hum is not at the acceptable level, even with the volume control fully closed.
I think more than a month I tried to figure it out, and with the help of one of my friends, now this 300B amplifier is completely free of hum.
Yes, you are read it correctly. No hum at all, it is dead silent. Only great music.
In this post, I will share every step that I takes when checking this 300B amplifier until it is now free of hum. But before that, let me explain the hum that I experienced before.
The hum is annoying. It is about 100 Hz noise, start about 30 second after the power is ON, when the tubes heater are start to glow. It is always present when the input is connected or not connected to the source. It is also present when the volume control fully closed or fully open.
As you can see from my 300B amplifier built, the case made by wood material and on top of it, where all the components is placed is using a thick steel plate. The bottom plate is the same as the top cover, it is steel material but thinner than the top. The grounding point is using star-grounding method but only at the top steel plate.
I touched the top and bottom steel plate by my hand to check if there is any ground issue but it is nothing happen. The hum still present.
The first things I did was remove the 300B tubes and connect a cable directly from the empty sockets of 300B pin-2 anode and signal ground act as the output, feed to the input of my other amplifier for testing the 6SN7 tubes. This method test for the gain stage from 6SN7 circuit.
It is still humming.
I am pretty sure the problem is with the AC supply for 6SN7 heater. With 2 pieces of 100-ohm resistor, I create a virtual ground from one of 6SN7 heater pins to ground. The hum reduced but still present quite loud. I remove the resistors, then built a LM317 regulator power supply from 9-volt AC to the DC output voltage of 6.3 volt require by the 6SN7.
the result with this DC regulated power supply is remarkably quiet.
Then I put back the 300B to the socket, the same hum still present.
It is not done yet.
The next test is I remove the 6SN7 tubes and only using the 300B tubes.
The hum still present.
But it is quite strange. It is still present on both channel when only one 300B tube is insert. So, I suspect the output transformer is place too close each other's, side-by-side with the choke transformer.
I rework the internal components layout and separate the output transformer far away from each other's with the choke transformer are move further, away from the output transformer. The hum still present but when only the specific 300B tube is insert.
Then, I cover the output transformer and choke transformer with copper sheet, covering the inductor and ground them. It doesn't make any effect to reduce the hum.
I checked the ground plane and re-work it, but it also not effects to reduce the hum at all.
I also suspect that the original 5-volt DC unregulated power supply for 300B direct-heated is not clean. Then, I using pre-made PCB regulated power supply for both polarity, the plus and minus using simple TIP122 transistor as regulator from 12 volt AC voltage to 5 volt DC.
It works like charm. The hum is now gone.
But not completely silent. There is very small hum only when the volume control is fully open. At this state, this 300B amplifier is completely acceptable.
Honestly, I can stop here and satisfy with the result. But I keep continue.
The original high-voltage power supply filter is using 5U4 rectifier tube, goes to C1 then 5 H choke and C2.
The new high-voltage power supply is changed to 60-ohm resistor, C1, 6 H choke, C2, 5 H choke and C3.
This new high-voltage filter brings the hum level to completely silent. There is no hum at all when the volume control is fully open.
To finish this, I need to clean this thing up. The power transformer is rewind again to rise the AC voltage for the DC regulator to keep up the regulation and for the high-voltage filter that added many steps causing the voltage drop more than before.
I am happy with the result.
Now what I hear is the magic sound of 300B amplifier. The popularity of 300B amplifier among audiophile is not a myth at all. It sound marvelous.
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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