300B Amplifier Hum Problem Solved

My previous post about the 300B tube amplifier build was a breakthrough. At least for myself. That was my first 300B amplifier build which if I remember, a decade ago, it will never across my mind to build such this expensive amplifier.

I learn a lot from that build and the 300B tube by its nature is direct-heated triode which I read everywhere saying that DHT will always have small hum present at the output, especially when pairing with the speaker sensitivity above 90 db.

My 300B amplifier built intended to drive a headphone. It is also can act as pre-amplifier. Small hum might be a problem for the listening enjoyment. Probably I should accept it.

But when the amplifier is done, the hum is worse than my expectation. It is loud and not at the acceptable level at all. The hum always presents at all time, even with the volume control fully closed.

I think more than a month and I tried to figure it out how to solve this issue. With the help 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 about 100 Hz noise, start about 30 second after the power is ON, when the tubes heater is 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. The RCA input jack is not isolated from the steel panel. I check to short it to the steel panel and the hum increase.

I also touch with my fingers the top and bottom steel plate 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 the signal ground. It will act as the output from the 6SN7 driver side, then feed to the input of my other amplifier for test the hum.

The result of this method is the 6SN7 driver contribute for the hum issue.

I am pretty sure the problem is with the AC supply for 6SN7 heater. I use several methods to make sure this driver circuit is free from hum. First is using 2 pieces of 100-ohm resistor, I create a virtual ground from one of 6SN7 heater pins to the ground. This method reduces the hum level but still considerably loud. So, I remove the resistors, then built a LM317 regulator power supply from 9-volt AC to 6.3 volt regulated DC output. The result with this DC regulated power supply is remarkably quiet. Then, I can continue to check the amplifier again.

I put back the 300B to the socket, turn it ON, but the same hum is still present.

It is not done yet.

The next test is removing the 6SN7 tubes and only using the 300B tubes.

The hum still present, but it is quite strange. The hum applies on both channels even when only one 300B tube is insert. So, I suspect the output transformer is place too close each other so the hum radiated. The worse layout is they are also side-by-side with the choke transformer.

I rework this transformer layout by separate the output transformer far away from each other's and also with the choke transformer are move further away. The result is hum still present at the same level like before, but only the channel when the 300B tube is insert.

I tried to cover the output transformer and choke transformer winding with copper sheet. This method to cover the inductor from other interfere. Unfortunately, this method doesn't make the hum away.

I also checked all the ground plane and re-work it, but it not gives any good result.

I suspect the 5-volt DC unregulated power supply for 300B direct-heated is not clean enough. Then, I using the pre-made PCB for DC regulated power supply, same design like Rod Coleman DHT voltage regulator, but in my PCB, it using 2 pieces TIP122 and a 2N222 transistor.

The result is remarkable. The hum is now gone!

Although if using certain headphones with higher sensitivity and with the volume control fully open, the hum is not completely gone.

Honestly, I can stop here and satisfy with the result.

But I keep continue.

I checked again for the high-voltage power supply circuit is using 5U4 rectifier tube, goes to C1 = 47uF then 5 H choke and C2 = 220uF.

Then I modify this circuit by adding another step of ripple reduction using R = 47-ohm after 5U4 rectifier tube, goes to C1 = 47uF, then 6 H choke, then C2 = 47uF, then again 5 H choke and last C3 = 220uF.

This new circuit brings the hum level to completely silent. There is no hum at all even when the volume control is fully open.

This amplifier hum problem is completely solved.

To clean all this thing up, the power transformer is rewind again to rise the AC voltage required for the DC regulator. Previously 0-5, 0-5, and 0-6.3 volt are change to 0-7, 0-7, and 0-9 volt.

Also, for the high-voltage circuit previously using 350-0-350 volt, now change to 400-0-400 because added many steps above 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.

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.