300B Single-ended Amplifier Build Guide

Since last year, I have been focused on building my own tube amplifier, especially using 300B tube in the single-ended configuration. I got more enjoyable moment when listening music through the tube amplifier instead of solid-state amplifier, especially with the 300B tube amplifier. The sound is more musical, more engagement to the music, and human voice alike compare with the solid-state amplifier. Previously, I also had experience using others tube such as 2A3, KT88, 6L6 and EL34, but I still believe the 300B is a holy grail of all tubes, so I stick to this choice.

300B tube is direct-heated tube (DHT), meaning this tube heater also act as the cathode. In the single-ended amplifier, using DHT tube will potentially contribute the hum to the speaker. This is because the power supply voltage along with the noise feed into the heater tube and direct impact to the cathode itself. Annoying hum needs to take care. I took so many lesson when building several 300B amplifier before, making the list for do and don't that worth to share.

What is written in this blog is based on my knowledge and experience. This post might be more deep in the technical sharing, which most of the reader here should be familiar with. So, I hope this post might help someone to build their 300B amplifier, and if not, please always take this post as entertainment only.

Building concept.

Before starting to build this single-ended 300B amplifier, first thing needs to consider is the amplifier chassis. The chassis is where all the components are placed and connected each other, preferably by point-to-point (PTP) wiring. The chassis is required as the first step to finish the build, not others. This PTP method is far convenience for tube amplifier builder to use many different kind of components size in the circuit. The possibility for changing the design and the schematic become more flexible without stick to the initial design like what printed in the PCB.

I prefer to using full steel chassis in my build because it is more isolated from the outside noise. The steel chassis I got was bought from China manufacture and their quality is beyond my expectation. Seriously, I am not expecting this kinds quality with all precisely pre-holes cut suitable for the tube sockets and transformers. with about 0.8 mm chassis steel thickness, it built robust, without any flex in the material. Same with the transformer box, the thickness is 1 mm, thicker than the chassis with the same high-quality material.

If the fully steel chassis is not possible for you, then using wooden frame chassis with steel plate on top and bottom is also preferable. The wooden chassis frame is easy to build and the size can be tailored based on the builder concept. On top of the wooden frame is the steel plate where all components placed. Better material such as copper is become an upgrade, it is looking good as well. If compare with the steel, copper material is non-magnetic and theoretically better in the noise rejection. Although the wooden itself is non-isolated to outside noise, but the choice to use this kind of chassis is more about the vintage looking to the amplifier and more easy to build.

The accessories components

Input-output sockets such as the speaker binding post, RCA connector, and tube socket should be install to the chassis first. Using high quality components for this section is very important because we want to build amplifier that durable and reliable. Especially for the tube sockets, I can suggest to use high-quality ceramic base with pure copper connector. Ceramic base is heat proof, this to avoid the base deforming or melting because of the heat when the amplifier operates.

This 300B single-ended amplifier is operate in class-A, meaning current flow is constant when idle. At least 200-watt power consumption from the AC lines is constant every time you turn on this amplifier. Please keep the amplifier safe by using fuse at the AC line. The fuse rating can use at least 3 to 4 ampere for 220-volt or double when the AC voltage lines is 110-volt.

Using reliable and high quality power switch and AC socket is to avoid fail or burn on those components. The reliable components are not always using the boutique brand. I prefer to use reputable industrial brands for more affordable and proven reliability.

The transformers

For any tube amplifier, the transformer is the key factor for good sounding amplifier. Good transformers are not cheap, branded transformer such as Tango, ISO-tango, Hashimoto, Edcor and Lundahl is very expensive and probably beyond most of the builder budget.

Why it is so expensive?

Because good transformer using pure copper. The iron material for the transformer is also high-quality steel that effect to the end result, such as stability, vibration and temperature. Without judge to your budget in building this 300B amplifier, this transformer components are what you should spend most of your budget in it, not the tube.

In this amplifier build, I took an affordable solution by order to my local transformer maker. He is experienced in it for years. He is using import transformer material and rewinding it using high quality copper with the design he copies directly from the Hashimoto transformer that he dismantled few years ago. The build quality result is expected like the things he copied.

The power transformer secondary output for 300B amplifier when using 5U4 tube rectifier is 380v - 0 - 380v, at least 200 mA for the output current. Over 400v - 0 - 400v can be use as well, but you might have an issue with the power supply capacitor voltage rating should be above 500 volt, which is not quite popular for the most electrolytic capacitor.

Using single power transformer for all the voltage need is preferable for its simplicity, not else. We need to include the others secondary voltage tap for the heater supply in the same transformer. This is the recommendation for the power transformer that use AC to supply the heater in the tube amplifier: 0 - 5v /4 ampere is for 5U4 tube rectifier, 0 - 5v/ 3 ampere, times two for stereo 300B tube amplifier, 0 - 6.3v /2 ampere for two 6SN7 driver tube.

Now we discuss for the output transformer. The common impedance value for 300B amplifier output transformer is 5k-ohm at the primary. For the secondary is based to output driver, which is suitable to drive 4 or 8-ohm load for most home speaker. If we want to use different load, let's say for headphones, the secondary output can be tailored to higher impedance such as 150-ohm or 600-ohm. This higher impedance suitable to make this 300B amplifier as a pre-amplifier as well.

It is easy for me to ask the specific impedance value with the transformer builder to match with the amplifier that I build. But in the condition if buying the transformer available in the market, you need to check their various output as an option for different load option in the future.

The last transformer needed is the inductor transformer. Popular name is called choke. This inductor transformer function is to reduce the power supply voltage ripple after the rectifier. More voltage ripple meaning more hum. I suggest to use at least 10mH with 250 mA current capability. This inductor transformer cannot be replaced by a simple resistor to get the same result, otherwise using resistor before and after the inductor will improve the ripple rejection and rectifier stability. We will continue discuss this specific section on the power supply section.

Power supply section

This power supply section is the heart of every audio system. Taking good care about it will result in operating stability and lower noise. In this 300B amplifier build, I am using 5U4 tube rectifier because it is better than using diodes. 5U4 tube rectifier is full-wave rectifier DC output from the pin-8, normally this output goes directly to the first capacitor. As per datasheet suggestion, using maximum 50uf for the first capacitor (C1) is the mandatory for tube stability. But we can do more to improve it. 5U4 tube rectifier require minimum series resistor before goes to C1, it is written is some manufacture datasheet, in this case we can improve from normal schematic from C1-L-C2 to R-C1-L-C2. The recommended value for the series resistor before C1 is 50 ohm, 5-watt power resistor. In this build, I am using 49 ohm, 10-watt Dale wire-wound resistor which still within the 10% tolerance.

The inductor (L) is 10mH as describe before, and my recipe for the C1 value cannot go above 22uF. Again, this small value to improve 5U4 tube stability and its life-span. The DC voltage measured after rectifier and series resistor on the C1 is about 430 volt. This condition is when all tubes inserted. If without any load from the other tubes, the voltage can go up to 500 volt. So, please be careful not to install 5U4 without any other tubes inserted otherwise it will be damaging the C1.

Epcos 22uF 500v electrolytic capacitor do pretty well for C1. Later on, I am using film capacitor for better performance, it is the 5.6uF 600v Miflex MKP capacitor. While I think 10uF would be a better choice, but I didn't find any issue to use this smaller value capacitor instead.

The C2 after the inductor playing important role to reduce hum even more. A 47uF capacitor value is too small for the capacitor to reduce the ripple voltage. I recommend to use 2x 47uF as minimal with 450-volt rating capacitor as minimal as well. My golden formula for C2 is using single 220uF or dual 100uF in parallel for lower ESR. I didn't find any improvement using C2 larger than that value.

AC or DC voltage for 300B heater

To determine which is better when using AC or DC voltage supply for 300B heater is back to the plus and minus between them. I personally prefer AC voltage because it is simple schematic with reasonable hum level that I can bear when listen to the music with my speaker. The hum is listenable within a meter range from my ears to the speaker, moving further away, the hum is unheard. This condition is acceptable for many listeners.

Use AC voltage supply for 300B tube heater will create hum. The solution is using hum-pot as mandatory but it will never bring down the hum to become null or zero. The hum-pot basically a wire-wound potentiometer with minimum 2 watt rating, preferable 5 watt for less heat and resistance stability. The hum-pot is connected to pin-1 and pin-4 in 300B tube, it is in parallel with the heater supply. I recommend to use 50 ohm potentiometer for less hum result, but using 100 ohm also can do, but with two pieces of 47 ohm 2 watt resistor in parallel with the potentiometer pin-1 to pin-2 and pin-2 to pin-3. The pin-2 which is the center pin of the potentiometer is the cathode pinout of the 300B tube.

Using DC voltage supply for 300B tube heater require a clean regulated DC. This means the AC input to the regulated power supply need to set above 5 volt, preferable 8 volt, because the regulator needs a sink-down voltage for proper regulation to 5 volt as required from 300B tube. The meaning from this condition is when using DC regulator power supply will add more load to the transformer. This extra load is transfer to heat of the regulator before goes to the heater itself.

The 300B tube heater require at least 3 ampere current output. This is calculate from the maximum current needed from the heater itself, especially when it is on cold starting up. A simple 7805 regulator will not be working properly. I recommend to use LT1083 or LT1084 IC regulator. Discrete DC regulator circuit using Darlington NPN transistor such as TIP132 and Zener diode as voltage reference will also do the same job, might be better if it designed properly, but it is more complicated to do. The other option is to use separate switching power supply. This solution is the most cost effective and reliable for clean DC without add extra load to the main transformer.

DC regulated power supply for 300B tube heater doesn't need hum-pot. Good voltage regulator can null the hum, or at least almost zero, which is unheard from the speaker, or even from the sensitive headphones.

Other power supply variation for this 300B tube heater is using DC non-regulated power supply which consist of a diode-rectifier, a huge amount of capacitor and probably a series resistor is an option which I am always avoiding this method. The reason for this unregulated DC voltage supply is doesn't make sense to reduce the hum. This only make the layout more complicated, with the result not better than using AC power supply.

We are counting more components involved in this DC regulator circuitry which they are all solid-state. This might be the reason to avoid by the tube purist. And for the sound quality difference between AC or DC? I don't think it effecting too much and noticeable.

300B tube circuitry

Before we going to this discussion, I might share the schematic as a reference from this blog here. I strongly believe that the blog is create by the person who also passionate in this hobby.

The 300B as final amplifier circuity is simple and common. It only contains 4-pins which the anode, grid and cathode/heater. The anode goes from output transformer with the voltage against the ground which is about 420 volt. The grid is the signal input and by using 1K ohm resistor in series before the bias resistor took the place will reduce the oscillation possibility from the input signal circuitry. Then the bias resistor to ground is handle by 100K ohm resistor, not more or less. This bias resistor is to keep maintaining the negative bias within the 300B grid specification.

For the cathode circuitry is a fixed resistor connected from the center of the hum-pot to ground. Various value can be use in this position which effecting to its bias point, but mostly the range is from 750 ohm up to 1K ohm, minimum 20 watt resistor. This resistor produces heat, lot of heat, so, using higher than specification will improve the reliability of this amplifier. In this build I am using 820 ohm, 50 watt power resistor from Dale. The reason for this is because my trusted supplier have only this items.

Cathode bypass capacitor is major components that improve the sound quality of this 300B amplifier. It placed in parallel with the resistor from the cathode to the ground. I recommend to use high-quality capacitor in this section which I prefer to use audio-grade capacitor. You may try industrial standard capacitor in this position which turn out to be an exciting trial error on improving the sound quality. The capacitor value I choose in this position right now is 100uF 100v Nichicon KZ Muse capacitor. Previously using 220uF 100v Siemens capacitor which doesn't bloom the top-end sound. This might be using 220uF is too big in the capacitance value, which turn out to be affecting the top frequency response of the 300B tube amplifier, or maybe the audio-grade capacitor took the place for improving this frequency response, which I am not quite sure about this.

Driver section

This driver section is the most important to the whole 300B amplifier design. The 300B tube cannot produce good sound alone, which the driver section before it is the key factor on how well it drives the 300B. The driver section is basically a pre-amplifier circuity to increase gain and current that required for the 300B as the final amplifier circuit.

In this build, I only using 6SN7 tube to driver to 300B. 6SN7 is medium amplification, 8-pin, dual triode tube which is very popular to drive 300B. Why I am stick to it? Simply because the 6SN7 is physically bigger compare with 9-pin tube variant such as 6DJ8, 5687 or 12AU7. By this reason, using 6SN7 in this amplifier make it look better, probably sound better too.

You can substitute the 6SN7 circuitry directly to others tubes variant with similar specification, as long as you mention for the pin-out and the heater voltage rating. The 6SN7 is 6.3 volt heater voltage requirement and I am using AC power supply for this, which is good enough to bring almost zero hum coming from this driver stages as long as you make sure that one of the heater pin goes to the ground. Other method is to use central-tap in AC transformer (3.15v - 0 - 3.15v) that goes to ground also works well to null the hum. Using two 100-ohm resistor to create virtual ground point and goes to the ground can be done as well, but I prefer the previous two option mentioned. When AC power supply can do well, no need to consider using DC regulated power supply in this 6SN7 tube heater.

The driver stage configuration and design will determine the characteristic of the sound of 300B amplifier. You might remember my first built for this 300B was parallel 6SN7 in anode follower design. This parallel triode is expected to aim for lower impedance and high current output to drive the 300B tube. This idea coming after reading the Luxman MQ-300 amplifier which when apply with only 1 driver stage, the result was bellow my expectation which turn out the sound-stage become shrink, less energy and less music engagement. This single stage driver required additional pre-amplifier to drive this 300B amplifier better. So, for the sake of the simplicity, I changed it.

On the next trial, I configure the driver stage as shunt-regulated push-pull (SRPP) circuitry. This configuration is popular to use especially in my previous DAC project using tube as the analog output. In this 300B amplifier, using SRPP tube configuration as driver stage is better than previous trial using single-parallel driver. The sound is open with good detail. The vocal is heavy and the sound-stage getting better than previous design. In this stages, I already satisfy. But I want more juice coming from this amplifier.

The latest is following the norm of 300B amplifier driver using two stages amplification in anode follower design, same schematic as the link I shared above. This schematic is proven and I also think the same way because my previous tube amplifier built use the same design like this as well. This design is high gain amplifier which more than enough room for the loudness. The 300B amplifier sound with this driver configuration is more open, better detail and sound layering, and also larger sound-stage.

Components selection

When the amplifier circuitry working in the high voltage, the components selection become the factor to the sound characteristic. We all know that capacitor in the signal line is the crucial components that effect to the sound quality. In this 300B amplifier design, the capacitor is placed between driver circuit to 300B grid input as an isolator to the negative bias of the 300B grid. I always got a good result with minimal cost spending when using Russian made paper-in-oil (PIO) capacitor. Previously I was using K40Y and in this build, I am using K75-10 PIO capacitor. The sound characteristic of this cheap capacitor is musical, sometimes I can say romantic. It is different when using MKP capacitor, this popular capacitor can be found from many brands which it sound more detail and neutral, sometimes a little bit thin in mid-range, but it is all depends from the manufacturer specification. Exotic capacitor that utilized gold, silver or pure copper, and especially when they covered in oil inside will sound marvelous. This exotic capacitor shines in this high voltage amplifier as well, but it is all depends on how much you willing to spend for this sound characteristic cause of the capacitor.

The capacitor that use in the power supply require large amount of capacitance and high voltage, mostly electrolytic type capacitor use in this section. Low ESR, high ripple rejection is recommended to use in this section. Rubycon product line up such as MXR is suitable in this section. Others brand such as Nichicon PW, Nippon Chemicon KZ, and others expensive audio-branded capacitor will also offer the same high level performance in this power supply ripple rejection. The choice in this section is widely available which nothing wrong to use what available in the market.

When talking about resistor, I want to deny about a resistor can change the sound in the tube amplifier circuitry. Resistor in high voltage amplifier like this 300B amplifier can affect the sound. I can tell the difference when replace Holco H-2 carbon resistor to TKD carbon resistor in the signal line resistor is the improvement in music transparency. Or the difference when replace Holco H-2 to Shinkoh metal film resistor in the 300B grid bias resistor is the improvement in musicality and depth. Am I hallucinating? Probably, but at the end I was eager to spend extra money to use better quality resistor which improved the sound quality as my expectation.

Cable and layout

Using high conductivity cable with high temperature resistance jacket for the amplifier internal connection will become the main consideration as the temperature inside tube amplifier is quite hot, which can up to 50 degrees at some point. My cable choice for this is using copper/silver plated cable with Teflon jacket. The price for this cable is affordable. The more exotic material is using pure silver wire with Teflon jacket, offering more glorious look without any claimed to the sound quality improvement. I saw many others tube amplifier builder using standard industrial copper cable with rubber jacket. Some of the cable melted to the chassis which become shorted-circuit and not safe at all.

Twisted cables are required for tubes heater supply. This to canceling the voltage noise to the tube heater. Those twisted cables to tubes heater can be combine together for better cable arrangement. For the high-voltage DC cable are not necessary to be twisted, they can lying individually without any issue to the hum. The signal cable is mandatory to use shielded cable because it is low signal level that easy to interfere with the noise around.

Arrange the cable as efficient as possible but without sacrificing the tidiness of the amplifier layout and especially for the safety. Do not cramped the cables to the chassis by any reason, or routing the cable near the open voltage terminals or the hot spot inside the chassis.

Grounding is the other major cause of the hum. using star grounding at the point where all signal ground goes into this point before going to the chassis ground. The ground point is preferable at the 300B cathode resistor as the star ground point. Power supply ground as the star ground point can be use as well.

Ground loop should be avoided because it will cause hum. This is the condition where two wires that goes to the ground are connected each other in a loop at the some points beside it. Let's say you have a star ground connection at the power supply ground, one ground goes from 300B cathode resistor ground and the other ground goes from 6SN7 driver cathode resistor ground. Then ground loop occurs when another cable is connect between 300B cathode resistor ground to 6SN7 driver cathode resistor ground, in most cases coming from shielded cable from the signal cable connection between them. This scenario will cause more hum.

Other ground issue that cause hum is the input signal ground at the RCA plugs cannot be connect to the ground or star ground. Let it connect to the driver stages ground, preferable at the cathode resistor to ground before it goes to the star ground. It is different case with the speaker output ground that should be connect together and goes to the star ground.

Although those are the best way to avoid hum, in some cases, the hum will still occurs when the connection is not tight enough, loose solder joint or bad connector between the joints.

Tubes selection

Tubes have various selection from affordable China or Russian made to the expensive American, European and Japanese made. The other variant is choosing between the new production or new-old stock unit (which I doubt it is a completely new stock anyway). I cannot deny that high performance tube, commonly comes with higher price tag will sound so much better than the cheap one. The layering of the sound, the smoothness and better individual music notes will feel more alive with the cost multiply than the normal one.

300B tube is an expensive tube. So, I can recommend to use Linlai 300B tube brand as the starting point. It is the most affordable tube with good sound and high reliability. If you have more money to spend, than the JJ tube will consider a better option.

Driver tube like 6SN7 is lot cheaper than the 300B tube, that is why choosing the American, European and Japanese made for this tube is more make sense. The brands like: RCA, General Electric, Sylvania, Haltron, Philips, or NEC will become a better choice compare with using the Chinese or Russian brand. Notes need to be taken is different brand for this driver tube will have slightly different internal structure which some of them can cause more hum to the amplifier. In my experience using 6SN7 General Electric will cause more hum compare with Haltron or RCA. So, I can suggest to have several brands in the driver tube as an option for testing the hum in amplifier and also for the sound quality improvement.

5U4 rectifier tube are easier to find with many substitute variant available. Although this rectifier tube function is basically the converter from AC to DC voltage, I was told that using GEC brand U52 type rectifier can improve the sound, which I got a chance to test it by lend from my friend. The sound improvement is real anyway, but with the price of five times from my currently Sylvania 5U4GB tube which in my opinion, it presenting superior sound quality as well.

The reason why I am not so excited with high level (and high price) tubes is because the tube itself have their own lifespan when operate. So, the spare tubes are wisely to have in advance. Although I saw many owners change the tube long before it reaches the end of the tube lifespan just because they are bored with the current setup, or they simply sold the amplifier entirely. The lifespan of the tube is various, depend by the condition in the circuitry, especially when operate within its working voltage tolerance. The other factor that determined the lifespan of the tube is on how to operate the tube amplifier, which pre-heat the tube at least 10 minutes before fully operate and turn-off the amplifier unit if not use. Having good ventilation for tube amplifier is also could prolong the lifespan of the tube itself.

In normal listening time, let's say 40 hours a week, a tube can operate on its prime for five years or more. It is different situation if the tube amplifier is use for commercial, like in the cafe that operate at least 12 hours a day. Probably two years, the tube needs to be replaced with the new one. The condition where the tube is reaching its lifetime can be determine by the gain of the amplifier become lower than usual and the sound become weak. This condition sometimes hard to tell until one of the tube fail to operate.

This post about 300B single-ended amplifier build guide probably cannot cover all various different method in building this amplifier. But in this post, I already put all of the main issue that need to have more attention, especially for the first time builder. This post also explains that building 300B amplifier is not difficult to do. The fear to build a 300B amplifier is not because of the complexity of the amplifier, but it is more on the initial cost that need to spend. The other consideration is deal with high voltage amplifier is also consider to be the no-go for starters.

We want to have this 300B amplifier because of its sound quality and the experience when listening the music with it. When we are talking about 300B tube amplifier, none of them will sound bad, if it was designed properly.

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.