5 Best Stereo Tube Amps for 2-Channel Audio

For a long time, I was on the search for an affordable tube amplifier to use with my new Klipsch Heresy IV speakers. 

After quite a bit of online research I ended up with a list of 5 integrated stereo tube amps that seem to offer extraordinary value for money. 

I even got a bit carried away and ordered (no less than) three of them to test in my own system.

A Decware Zen Triode, a BoyuuRange A50 and a Reisong A12.

I guess I can always sell the two amps I don’t want to keep long term. 

So, in this article, we’ll take a deeper look look at these 5 affordable vacuum tube amps for 2-channel audio I found that get so much praise online. 

Update/Warning! The delicious sound from these tube amps can be quite addictive and my tube journey has ended up as a very expensive one. After having so much fun with these high-value tube amps for many months I began the (somewhat unnecessary) search for the ultimate end-game tube amp. My last amplifier if you will…. So, I am now the happy owner of a high-end Feliks Audio Arioso 300B tube amp and top-notch Western Electric 300B tubes, which for me is the ultimate end-game tube amp setup. Yeah, that broke the bank… You are hereby warned. 🙂

#1. BoyuuRange A50 MKII

Price: Click to check price on Amazon or China Hi-Fi Audio

Type: Single-Ended Class-A 

Input/Driver Tubes: 6SN7  

Output/Power Tubes: 300B 

Output Power: 2 x 7.6 Watts

Inputs: 3 analog RCA inputs

The BoyuuRange A50 MKII is one of the tube amplifiers I purchased to test with my own speakers.

It is one of the China made tube amplifiers that is highly acclaimed and reported to perform very well for the money. 

What I found especially intriguing about this amp is that it is a true no-feedback, single-ended, class-A 300B design. 

Single-Ended 300B is arguably the most legendary type of tube amplifier designs. 300B tubes are known to sound extremely sweet and musical, with an exceptionally open and clear midrange. They seem to have a special standing among tube enthusiasts. A cult status, if you will. 

The original Western Electric 300B tubes, that created the legendary status, are extremely expensive. The high price of new old stock (NOS) 300B tubes has been a barrier that has stopped many vinyl lovers from getting into 300B amplifiers.

That has, luckily, changed. Today we can purchase very good 300B tubes from PSVANE, Genalex and other manufacturers for quite sensible money. This makes it possible to enjoy the classic 300B sound without a huge budget for tubes. Don’t get me wrong, good 300B tube are still costly, but not nearly as costly as they used to be. 

My first impressions of the A50 is that it seems extremely well and solidly built. It is heavy as a rock. Based on perceived build quality, it could easily cost many times its selling price. 

It has three inputs and an input selector, which is nice. I can connect both my digital streamer and my turntable at the same time. The tubes glow slightly when it is powered up. (PS! The A50 does not have a built-in phono preamp.)

I was furthermore satisfied with its ability to play relatively loud, given that it only outputs a handful of Watts. This is my first ever low-power tube amp, so I didn’t really know what to expect. 

Surprisingly, it made my 88dB sensitivity B&W 705 S2 speakers play much higher than my normal listening volume with the volume knob at only 11 a clock. These are not high-sensitivity speakers. 

With these low-power tube amps, you quickly start to rethink your perception of how much amplification power that is actually needed to make speakers play fairly loud. These tube amps can really make the air move with only a handful of Watts. Quality over quantity, I guess. 

When I later hooked up my new 99dB sensitivity Klipsch Heresy IV speakers, I rarely needed to pass 9 a clock with the volume knob to reach my preferred listening volume. The A50 seems to output more power than you will probably ever need for these high-sensitivity speakers. 

Being pleased with the overall volume, I noticed that there was an unpleasant harsh sound in the higher midrange when there was a lot going on in the music. Complex parts of pop and rock tunes didn’t sound too good at times.  

I had read online that the stock 6SN7 input tubes are of poor quality and can cause this shrillness, so I had already ordered high-quality 6SN7 Tubes from PSVANE to quickly replace the stock input tubes. 

I didn’t want to enter the new old stock (NOS) tube jungle for my first tube rolling, so I went with PSVANE. 

With new PSVANE 6SN7 tubes installed and burned in for a few hours, the sound from the A50 transformed and became very satisfying. The harshness in the upper midrange was gone and the amplifier sounded sweet and natural throughout the whole frequency range with music from all the genres I tried. Pop, Rock, Alternative, Acoustic, Jazz, etc. 

The magic midrange of 300B tubes is definitely true. Listening to female vocals with the A50, after replacing the poor input tubes, is simply breathtaking. It draws me into the experience in a way that rarely happens with any other gear. 

When I use the A50 with my Klipsch speakers, it is honestly hard to stop listening to music, it is a very special and captivating experience.

There is really nothing I feel lacking with this amp. Besides shipping with better input tubes. For its price, it must be a gem.

The bass is sufficient, but not overwhelming. 

The highs are as round and sweet as I prefer them. 

And the midrange is pure magic. 

Pros 

• Unreal sweet, transparent and clear midrange (with upgraded input tubes) 
• Stock PSVANE 300B output tubes are of good quality and do not need to be replaced 
• Seems very solidly built
• Delivers real 300B magic for very sensible money
• Quiet, no hum/noise 

Cons 

• Stock 6SN7 input tubes don’t sound too good and need to be replaced with higher quality aftermarket tubes to get the amp to fully shine. 
• Rated at 7.6 Watts per channel, but as documented in Skunkie Designs Electronics videos, there is significant clipping and distortion starting around 3-4 Watts per channel. So usable output power is likely around 3-4 Watts per channel. 
• The meters don’t move much… 
• I am not a fan of the gold/orange looking plate they have put on the front. The A12, that we will look at shortly, comes without this plate and looks much better. 

Limitations 

No DAC. No digital inputs. No Bluetooth. No remote control. No subwoofer output. 

Traditional analog tube amplification. Without bells and whistles. 

Love it! 

The BoyuuRange A50 utilizes a JC Morrison / Sun Audio 300B circuit design. It is designed to run the 300B tubes very lightly to maximize tone, not power or bandwidth. It outputs about 3-4 Watts of useable power before heavy clipping sets in. 

So, this is not the most powerful or authoritative 300B amplifier out there. It has received some criticism for that. The 300B tubes can output up to 8 Watts of clean power if driven harder. 

Personally, I can live with a 300B amp that focus on tone over power. Heck, isn’t that why we buy 300B amps in the first place? For tone?  

Suitable speakers 

Having a 3-4 Watts of usable power per channel before heavy clipping sets in, 86dB sensitivity speakers are likely the lowest you will go for sufficient listening volume. It is probably ideal to use speakers with 91dB sensitivity or more. Even high 90s if you want to play very loud in a big room.  

These are only guidelines. It depends on the size of your room, your preference for listening volume and usage in general. 

Personally, I was positively surprised by how nicely it plays with my 88dB sensitivity Bowers and Wilkins speakers. More than loud enough for my listening level preference in a small to medium size room. My plan is, however, to primarily use this amp with my highly sensitive Klipsch Heresy IV speakers down the line. 300B amps are known to perform their absolute best with very sensitive speakers. 

Why buy

When upgraded with PSVANE CV181 T-II input tubes, this amp honestly got quite captivating to listen to. It really draws me into the experience, even when I used my low-medium sensitivity (88dB) Bowers & Wilkins 705 S2 bookshelf speakers. It sounds sweet, dimensional and amazing. For normal listening volumes, the low power output is not an issue, in my experience. 

Possibly your absolute best options for a relatively affordable single-ended 300B amplifier. 

It looks good and feels very solidly made. 

It can also be a good foundation for a project amplifier with endless options for mods and upgrades. Stephe has a brilliant playlist on YouTube where she upgrades the A50 in several stages to make it more powerful. Personally, I will leave my A50 stock, changing the inputs tubes was enough “modification” for me.

#2. Decware The Super Zen Triode (SE84UFO)

Price: Click to check price on Decware 

Type: Single-Ended Class-A

Input/Driver Tubes: 6N1P/6922

Output/Power Tubes: Russian 6P15P-EV or EL84

Output Power: 2 x 2.3 Watts

Inputs: 1 analog RCA input 

With the right speakers, this can quite possibly be the best amplifier in the world. 

I am not joking.

Here is what a few of the most watched Hi-Fi reviewers on YouTube have to say about the Decware Super Zen Triode (SE84UFO/SE84UFO2): 

“Flawless. My favorite amplifier of all time.” 
    – Andrew Robinson

“So, even if you already have a system that is pretty good, and you want something that is just that much more special. Then, this is it!” 
    – Steve Guttenberg

“To make something at this level, for this price, in the USA, is quite Amazing.” 
    – Thomas & Stereo 

I could go on. 

I rarely (never) find so much positivity around a product when I am researching Hi-Fi gear. Or any other type of gear, for that matter. 

I haven’t been able to test The Super Zen Triode yet because there is a very long waiting list to get one. I have however paid the 10% deposit and earned my spot on the waiting list. When will it arrive? I don’t know. But I am very much looking forward to that day! 

I will update this review with my own experiences when I have received and tested the amp. 

The Super Zen Triode is a no-feedback, single-ended, class-A amplifier that has been tweaked and improved into pure perfection over the last 25 years. 

It is not designed to look flashy or to produce impressive measurements or spec sheets. It is designed to sound as good as an amplifier can possibly sound. Without costing tens of thousands of dollars. 

It ships with Russian 6P15P-EV output tubes but can also use EL84. The 6P15P-EV is slightly faster than EL84 and Decware’s first choice. It will sound slightly more relaxed with EL84 tubes. 

It is known to be a fast tube amp with good dynamics throughout the frequency range, including the bass. Engaging and detailed, but still sweet and pleasant to listen to. Big soundstage with three dimensions. And pretty much flawless when driving speakers with decent sensitivity…. 

From all the research I have done on this amp, I have a strange feeling that this will end up as my long-term Klipsch Heresy IV amplifier. But I can’t say for sure until I have got one and tested it. 

I will come back and update this part when I have tested The Super Zen Triode for some time. For now, and solely based on what others have said, I believe this is the best affordable low-power tube amplifier that money can buy. 

If you a serious about finding your first and last low-power tube amplifier, I recommend that you check out Decware’s website. And the reviews by the Youtubers I have quoted above. 

Pros 

• US made with mostly US sourced parts
• Lifetime guarantee (original owner) 
• Sound might be impossible to critique… 
• With efficient speakers, maybe the last amp you will ever buy 

Cons 

• If your wife is an interior designer you might have to buy her something nice (expensive) to put this in your living room. The design might not be for everyone. The design is however much nicer on the more expensive models than on the entry level SE84UFO.
• Long waiting list!!!

Limitations 

No DAC. No digital inputs. No Bluetooth. No remote control. No subwoofer output. 

Get over it.

Suitable speakers 

At Decware’s website, it is recommended to use speakers with more than 89dB sensitivity. Ideally 94dB and up. 

So, if you run efficient speakers from Klipsch or similar, you should be in very a good place to get the most out of this amp. 

Why buy

I buy it because it appears to be the best sounding low-power amp in the world, independent of price. 

It is possible to buy in multiple configurations, but the lowest priced SE84UFO will probably give you the full Decware sound experience. You don’t really need to opt for the more expensive variants. I think…

#3. BoyuuRange Reisong A12

Price: Click to check price on Amazon or China Hi-Fi Audio

Type: Single-Ended Class-A

Input/Driver Tubes: 12AX7 (ECC83)  

Output/Power Tubes: EL34

Output Power: 2 x 6 Watts 

Inputs: 2 analog RCA inputs

I ordered the Boyuurange Raising A12 together with the Boyuurange A50 to test and compare against each other. 

To get personal experience with single-ended 300B vs. single-ended EL34. 

The A12 is an upgrade of the A10, where the most essential change is the input tubes. The A12 uses 12AX7 input tubes that are easier to tube roll than the 6N2J tubes in the A10. 

In the product description on Amazon, it says that you must make sure that your speakers are bookshelf speakers to use this amp. That does not make much sense to me. 

What you want to make sure is that your speakers have decent sensitivity (efficiency). As the case is with every other low-power tube amp. Floorstanding speakers often (usually) have higher sensitivity than bookshelf speakers. This amp can drive enormous Klipsch Cornwall or La Scala speakers with ease. In fact, that would probably be two of the best speakers on earth to use with this amp. 

The A12 comes with both input tubes and output tubes from PSVANE classic series, which is PSVANE’s most affordable series. That is a step up from the A50 where only the output tubes were from PSVANE. I guess they had to make a compromise on the A50 to make up for the expensive 300B output tubes and still hit the desired price point. 

I have tested the A12 with both my 88dB sensitivity Bowers & Wilkins 705 S2 and my 99dB sensitivity Klipsch Heresy IV speakers, and it works well with both of them, in my opinion. I will do more testing with the Klipsch Heresy IVs when I have a bigger room that is more suited to them, so most of my testing has been with my 705s so far. 

It is definitely louder with the Klipsch Heresy speakers, but it drives the Bowers & Wilkins speakers to decent volumes to. More than enough for my normal listening volume. 

When it comes to sound to the sound, I don’t think it can fully compare with the A50s super clear and open midrange, but it is not far behind. When it comes to bottom-end punch, I do however think that the A12 offers a bit more authority. The A50s design philosophy of driving the tubes softly might cause it to lose some authority in the bass. 

One benefit with the A12 is that it sounds nice with stock tubes whereas the A50 needs to be purchased together with a set of better input tubes, in my opinion. 

I spent a night comparing the A12 to my A50 (upgraded with PSVANE input tubes) listening to my favorite songs on Roger Waters in the Flesh – Live on Tidal over and over again. 

And, if I am honest, I felt more drawn into the experience with the A50. There is certainly something special about those 300B tubes. 

There are a lot of beautiful female vocals on that record, and they sounded breathtaking on the A50. Strings from acoustics guitars sound more present and real through the 300B tubes on the A50 compared to the A12. 

I did however notice a fraction more bottom-end grunt with the A12. So that is a small win for the A12. 

Pros 

• Ships with quite decent PSVANE input and output tubes 
• Good bass for a 6 Watts amplifier 
• Looks very nice. Doesn’t have that gold/orange looking plate on the front like the A50, which I think is nicer. 

Cons 

• Not as clear and open in the midrange as the A50 (with PSVANE input tubes)
• Not much else to critique at this price point. 

Limitations 

No DAC. No digital inputs. No Bluetooth. No remote control. No subwoofer output. 

The usual stuff… Traditional and minimalistic tube amp design. 

Suitable speakers

With 6 Watts per channel, I believe it is recommended to use speakers with more than 85dB sensitivity. Ideally 90dB and up. 

(I base that on the recommendations on Decware’s website of minimum 89dB and ideally 94dB for a 2.3 Watts per channel amp. And do an approximate calculation to find what the numbers are for 6 Watts per channel.) 

Why buy

A highly affordable tube amplifier that looks awesome, comes with decent input and output tubes. It uses tubes that are cheap and easy to tube-roll. It also has slightly more usable power than the other two single-ended amps on this list. 

#4. Willsenton R8 

Price: Click to check price on Amazon or China Hi-Fi Audio

Type: Push-Pull Class-A/B

Input/Driver Tubes: 6SL7 and 6SN7  

Output/Power Tubes: KT88/6550/EL34

Output Power: 
2 X 25 Watts triode mode 
2 X 45 Watts ultra-linear mode with KT88/6550
2 X 40 Watts ultra-linear mode with EL34

Inputs: 3 Analog RCA inputs + 1 RCA preamplifier input

Alongside Decware’s The Super Zen Triode, the Willsenton R8 is the tube amp on my list of five that seems to receive the most praise from owners and reviewers. These two amps really stand out in that regard. The positivity that surrounds them is close to insane. 

Is the Willsenton R8 good? Yes. From what I have read and watched; I believe that is obvious. I can’t see how this amp isn’t very good. For the price, it is likely a rare bargain.  

If you need more power than the 2-6 Watts the single-ended class A amplifiers reviewed above outputs, then this push-pull amplifier should probably the first amp to give serious consideration.  

I haven’t pulled the trigger on the Willsenton R8 yet. I wanted to first see if the simpler single-ended class-A amps I have already purchased are powerful enough to drive speakers. Which seems to be the case. Alt least for my room size and normal listening volume. 

To me, the simplistic no-feedback, single-ended, class-A designs are the most intriguing and desirable. So, single-ended amps are my first choice if I get away with their low-power outputs.

The Willsenton R8 are known to have a powerful and Rock & Roll type sound with a lot of bass if you opt for the KT88 tubes. Especially in ultra-linear mode. Its powerful bottom-end can however be tuned down by replacing the KT88 output tubes that are more linear and sweeter sounding EL34 tubes. 

If you order the Willsenton R8 directly from China Hi-Fi Audio, you can select between a variety of tube configurations. 

The R8 also has a switch where you can select between triode mode and ultra-linear mode. Triode mode gives sweet and musical sound, suitable for vocals, strings, classical music, and jazz. Ultra-linear mode gives more bass, stronger dynamics, and more power. Suitable for rock and more intense listening sessions. 

So, with a push on a button on the remote control, you can select seamlessly between two different sounding amplifiers. That is handy! 

I think the R8 looks awesome. To me, one of the best-looking tube amps on the market, full stop. It is probably easier to get the wife’s acceptance of this than any of the amp on this list.

You also get a little more in the features department if you go for the R8. It comes with a remote control, headphones output and preamplifier input. Handy stuff we don’t get with the other amps on this list.  

Pros 

• Powerful bass (Can be toned down by swapping KT88 tubes with EL34) 
• Stunning build-quality for the money
• Quality components internally  
• Remote control

Cons

• The most expensive amp on this list
• Some users have reported a strong strange smell from the amp when new. Smell of paint. I guess that is just temporary. 

Limitations 

With the Willsenton R8, you get enough power to drive pretty much every speaker out there. Save for extremely demining speakers, maybe. You also get volume control, headphones output and preamplifier inputs. 

What you don’t get is a subwoofer output and digital inputs. If you combine it with a DAC/Streamer like the Bluesound Node, it can take care of all that stuff for you. That way, the Willsenton R8 can remain a timeless analog design, which is probably what must of us want it to be. 

Suitable speakers  

You will probably find speakers that don’t pair well with the Willsenton R8. Just as you will with any amplifier on the market, even powerful solid-state ones. 

That said, the Willsenton R8 has the power to drive pretty much every speaker out there to very decent listening volumes with strong and tight bass. We are talking 2 x 45 high-quality Watts, and that makes a difference. 

Why buy

If the lack of a solid bottom-end and overall power and oomph keeps you away from tube amps, then the Willsenton R8 might be the one that convinces you to finally give tubes a try. This amp is rock and roll with a touch of tube magic. 

It can, however, be optioned with smother and more linear EL34 tubes from the factory, if strong bass is not your thing. 

A nicely designed tube amp with a bit more versatility than the other amps on this list. Meaning, it does at least come with a remote control, headphones output and preamplifier input. 

Universally regarded as a steal for the price. I have seen multiple forum posts claiming it outperform more expensive amps from Primaluna and other high-end manufacturers. That is, obviously, highly subjective. Primaluna owners would probably not agree to those claims that much. Nevertheless, the overall vibe is that this is an extremely good amp for the money. 

#5. BoyuuRange MT-34 MKII

Price: Click to check price on Amazon or China Hi-Fi Audio

Type: Push-Pull Class-A/B 

Input/Driver Tubes: 6N1J  

Output/Power Tubes: EL34

Output Power: 
2 x 25 Watts ultra-linear mode 
2 x 15 Watts triode mode 

Inputs: 3 analog RCA inputs

If you need more power than the single-ended amps can output and find the Willsenton R8 to be a little on the expensive side for a first tube amp, then the BoyuuRange MT-34 might be a great option. 

It lacks some of the features that the R8 has. It also lacks some of the hype. But it is about half the price, so that might be an acceptable compromise.  

It uses two EL34 output tubes per channel in a push-pull configuration that gives considerably more power than the single-ended designs that only use one output tube per channel. 

Just as the Willsenton R8, it can be run in triode mode for the sweetest sound and in ultra-linear mode for more power and grunt when listening to Rock & Roll etc.  

The design is typical BoyuuRange. Well made. It weighs in at about 40 pounds, so there is a lot of iron in those transformers, which is generally a good sign. 

Pros 

• Affordable push-pull EL34 amplifier 
• Solid
• Nice design 

Cons 

• Nothing major at this price point 

Limitations 

No DAC. No digital inputs. No Bluetooth. No remote control. No subwoofer output. 

Timeless design, the way it should be! 

Suitable speakers  

The MT-34 drives most speakers without fuss, not only those with high sensitivity. The authority of 2 X 25 high-quality tube Watts might surprise you. 

That said, speakers that are well-known to be hard and demanding to drive should probably not be the first choice for the MT-34. Or any other tube amp, for that matter… 

Why buy

If you want to get into tube amps and don’t want to spend too much, then the MT-34 can be a great first push-pull tube amp. It is solidly built and appears to have quite decent output transformers which are a good foundation to start experimenting with tube rolling and other internal upgrades. 

Conclusion

This article essentially sums up the research, testing and decision process I went through when choosing an affordable tube amplifier for personal use. 

I am quite new to tube amps and no expert on this topic. But I still hope the article gave you some value if you are just starting out on your tube amp journey as well. 

If there is something I have totally messed up, feel free to write me an email and share your thoughts. 

As a write this, I am listening to Roger Waters In the Flesh Live on the BoyuuRange A50MKII with the PSVANE CV181-TII (6SN7) input tubes and it sounds magical. 

I will enjoy my BoyuuRange A50 MKII until the DECWARE Super Zen Triode shows up. And then, decide on which of them to keep long-term. 

My Testing 

Most of my testing is done on my 88dB sensitivity Bowers & Wilkins 705 S2, that cost around $3000 list.

My current room that measures 3m x 7m with 2.7m ceiling. That equals 10 feet x 21 feet with 9 feet ceiling. 

I have used 4 GIK Acoustic 244 acoustic panels to reduce direct reflections and trap some of the bass. In general, to improve the acoustic a bit. 

I have also done some testing on my Klipsch Heresy IV that cost around $3000 list. Because of logistical reasons and the fact that using the B&W speakers would put these amps to a bigger test, I have done most of the testing on the B&W speakers. I will do more testing with the Heresy IVs in the future and update this article with that experience. 

The two amps I have tested and compared so far are the BoyuuRange A12 and A50. I have ordered the Decware Super Zen Triode and will update this article when it is received and tested. I have not yet pulled the trigger on (or tested) the Willsenton R8 or the BoyuuRange MT-34. 

What are typical expensive alternatives? 

In this article, I have reviewed 5 affordable tube amps that are universally acclaimed and considered very well performing for their price. 

What are the more expensive alternatives you might ask? 

I don’t think I have a complete picture of all the models in the higher price points, but below are at least some of the highest regarded tube amp manufacturers independent of price. I have probably missed a bunch… 

• Primaluna 
• McIntosh 
• Audio Research 
• Audio Note 
• Quicksilver 
• Cary 
• Rogue Audio

Stereo Tube Amplifier FAQ

Tube Amps Power and Speaker Sensitivity 

I will try to explain why speaker sensitivity becomes so important when we talk tube amps, especially low-power single-ended models. 

The key to get these affordable tube amplifiers to play loud is to use efficient speakers with high sensitivity. These amps don’t output a lot of power, but with the right speakers, they can still play really loud. 

To get a good understanding of how speaker sensitivity affects their performance, I have swapped between my 88dB sensitivity Bowers & Wilkins and my 99dB sensitivity Klipsch Heresy IV speakers. The difference is huge! 

To give you a good idea of how crucial speaker sensitivity is, let me give you this example.

A typical bookshelf speaker with 87dB sensitivity produces 87dB of sound measured 1 meter from the speaker when 1 Watt of power is applied. 

To increase the sound volume with 3dB from the speaker, we must double the power applied. It looks like this. 

1 Watt = 87db

2 Watts = 90dB

4 Watts = 93dB

8 Watts = 96dB

16 Watts = 99dB

So, it takes 16 Watts to produce 99dB of sound with this speaker. 

99dB of sound is also exactly what a speaker with 99dB sensitivity, like the Klipsch Heresy IV, produces with only 1 Watt applied. 

So, a speaker with 87dB sensitivity needs 16 times more power to play as loud as a speaker with 99dB sensitivity. 

16 times more power! 

That is how big the difference is between Low/medium sensitivity and high sensitivity speakers are. 

To play very loud, let’s say at 108dB volume, a typical bookshelf speaker with 87dB sensitivity needs a 128 Watts amplifier whereas the Klipsch Heresy IV rated at 99dB sensitivity only needs an 8 Watts amplifier. 

Input tubes vs. output tubes 

Power is a product of voltage and current, and tube amp amplifies voltage and current in two separate amplification stages. 

The input/driver tubes form the first stage of amplification in a tube amp and take care of the voltage amplification. They amplify the input signal from the music source, usually around 1 Volt in size, to a signal with a much higher voltage swing. 

The output/power tubes form the second stage of amplification in a tube amp and take care of the current amplification. They take the already voltage amplified, but still low-current, music signal from the input tubes and transform it into a high-voltage and high current-signal that is applied to the output transformers. 

Tubes brake-in time 

Whether tube break-in time is real or not, I don’t know. 

Some people argue that tubes gradually improve in sound quality of the first 50-300 hours, depending on the type of tubes. 

Others state that this is primarily you adopting to the new sound and that the sound from the tubes really doesn’t change that much over this period. 

Nevertheless, what everyone seems to agree upon is that it is ideal to burn in the tubes for a few hours to get rid of any noise that brand new tubes might produce before you put them to the test. 

Single-Ended vs. Push-Pull

In a single-ended class-A amplifier, the whole signal is amplified by only one tube (per channel, per amplification stage). 

In a push-pull class-A/B amplifier, the top half of the signal is amplified by one tube and the bottom half of the signal is amplified by another tube. The signal is split at the zero point. 

A push-pull amplifier might cause some distortion and un-linearity around the center of the signal, where the first tube hands over the signal to the other tube. This is not a problem in a single-ended design, where we use the linear area of only one tube to amplify the whole signal. 

For a stupidly simple, and likely stupid, analogy, think of an electric car without a gearbox as single-ended. The acceleration is smooth and linear from 0 Mph to 60 Mph, using the same gear all the time. 

A car with a traditional combustion engine and gearbox is the push-pull of car design. There is a bit of un-smoothness every time the gearbox shifts gear. When one gear takes over for the other. Just as it might be a bit of distortion when one tube takes over for the other in a push-pull amplifier design. 

Because a single-ended amp uses the most linear part of the tube and never shut off and hand over the signal to another tube, this design is regarded as more special and sweeter sounding than push-pull by many tube amp enthusiasts. Including me. 

The downside with single-ended design is that they usually output only a handful of Watts, whereas a push-pull amp can output several tens of Watts. Single-ended class-A designs also produce more heat because of the high idle current that runs through the tubes. 

Triode Mode vs. Ultra-Linear

The difference between triode and ultra-linear is down to how the grids/screens are wired on the output tubes. This internal wiring can be seamlessly swapped back and forth with the mode switch on many push-pull tube amplifiers. 

Triode mode sounds sweeter, softer and laid-back than ultra-linear mode, which sounds more detailed and powerful. 

Main Sources

Steve Guttenberg Audiophiliac YouTube channel 

Skunkie Designs Electronics YouTube channel

Thomas & Stereo YouTube channel 

Andrew Robinson YouTube Channel 

https://forums.stevehoffman.tv (Multiple threads) 

https://Audiokarma.org (Multiple threads) 

https://community.klipsch.com/ (Multiple threads) 

https://www.diyaudio.com/ (Multiple threads)

https://www.china-hifi-audio.com/en

https://www.decware.com/newsite/homepage.html