You’ve seen various tube amp descriptions that read Class A or Class AB amplifier, but what does any of that mean? And why should you care?
What’s the difference between class A and class AB tube amps?
Check out this break down of the differences for the quick answer. The detailed explanation and some examples follow further on….
Class A Amplifiers
- The tube always warmed up and ready to amplify the signal.
- Tube does not have to “wake up” from a “ready” state.
- Class “A” amps have greater output than Class “AB” amps. If two EL84 output tubes in a Class A design may produce only 10 watts of output power, then the same two tubes in a Class AB design might produce 50 watts.
- Smooth compression.
- Less headroom. (NOTE: This is a disadvantage to some musicians who are looking for good, clean tone at higher volumes).
- More responsive to the touch – i.e.: feels good to play.
- Class A amps with EL84 tubes in push-pull design emphasize high order harmonics. This emphasis makes the amp “sing”.
- Keeping the tubes warmed ready requires more current at all times. This constant current leads to shorter tube life.
- Lower power rating than a Class “AB” amp with the same tube configuration.
Class “AB” Amplifiers
- Longer tube life.
- More headroom – cleaner cleans at higher volume.
- Tighter bass response – less prone to “muddy” tone.
- More efficient use of power.
- Not as responsive as a Class “A” amp.
- Tone won’t “break up” as easy.
Class A vs Class AB, the Detailed Explanation.
The three major parts that make an amplifier tube are: the Cathode, the Grid and the Plate. The Cathode heats up when voltage hits the tube, causing a cloud of electrons to form. The Plate has a positive charge, which causes the negatively charged electrons to flow toward it. The Grid controls this flow of electrons. It is also the audio signal input for the tube.
An audio signal entering the tube causes a change in voltage at the Grid. This change in voltage changes the flow of electrons and causes amplification.
The behavior of electrons described above is an example of a Class A amplifier. These amps apply a positive voltage to the Grid. Class AB amplifiers apply a negative “bias” voltage to the grid. This bias causes the electrons in the cloud to avoid the Plate. This is the standby mode of the tube.
The voltage of the audio signal entering the Grid causes the voltage on the Plate to change from negative to positive. This attracts the electrons in the cloud and causes them to flow to the plate. The tube in the Class AB amp then behaves like a Class A described above.
The need to change the charge of the plate from positive to negative causes the Class AB amplifier to feel less responsive than a Class A amp. But it also means the tube components aren’t in full use even when a signal is not passing through. This means the tubes generally last longer.
Class A amplifiers
Bugera V5 Infinium
Looking for a low watt amp that won’t blast a hole in your eardrums or your budget? Bugera is for you.
The Bugera V5 Infinium is a 5 watt all tube amp for less than $200.
Let that sink in for a moment.
For the price of a really good solid state amp, you can get an all tube amp. But that’s not all.
- The Bugera V5 is a Class A amp driven by a single EL84 tube with a single 12AX7 tube in the preamp.
- Specially designed 8 inch Turbosound speaker. (This is a classic combination for the British crunch and buttery blues tone)
- Power attenuator, which effectively lets you decrease the headroom even more and get killer overdrive or distortion at a lower volume
- Digital reverb capable of dialing in everything from subtle to downright cavernous.
Where the Bugera V5 Infinium really makes its mark is with it’s Tube Life Multiplier technology. According to Bugera, this tech ensures:
.. greater reliability and consistent tone over the lifespan of your tubes by monitoring the performance of each of the amplifier’s output tubes and maintaining them at their ideal operating point for an evenly distributed load.
There’s also an LED next to each tube that lets you know when it’s about to die.
This stylish little beast is another micro amp, 5 watts and an 8-inch speaker… and a built-in Tubescreamer circuit!
With a retro look of a 1950’s television it harkens back to a simpler age when televisions were the furniture centerpiece in a room; meant to attract attention and start a conversion. The Ibanez TSA5TVR is an attention-getter as well.
- 8″ Jensen C8R speaker.
- Ruby 6V6 and 12AX7 tubes.
- Lush Accutronics spring reverb
- Built-in Tubescreamer – complete with overdrive, tone, and level knobs
The TSA5TVR gives you everything from slight, dirty gain for blues and rock all the way up to high gain for lightly compressed distortion. It’s a small package that sounds much bigger.
Peavey ValveKing Royal 8
The ValveKing gives you punchy dynamics and classic rock tones that sound much larger than its small package.
- 5 watts
- One EL84 output tube
- One 12AX7 preamp tube
- Single-channel design
- Dual ¼” inputs
- Master volume and gain controls
Rollback the gain and crank up the master volume to get that great class A “breakup” power-amp tone. Or crank the gain and dial back the master volume and you’ve got raging preamp distortion.
It’s that easy.
Orange Tiny Terror TT15C-12
This loud and proud Orange Terror owes it’s punch to a special reissue of the Celestion G12H 12 inch speaker. The G12H is a mix of the Vintage 30 and Greenback speakers and produces thick, warm and sugary sweet tones.
- 15 watts, Switchable to 7 watts
- Single Celestion G12H 12″ Speaker
- Gain, tone, volume controls
- 2 x EL84; 2 x ECC83/12AX7 tubes
The cool thing about the TT15C is that it’s switchable from 15w to 7w, which makes it great for break up or recording.
Class AB amplifiers
Vox Night Train NT15C1-CL
The Night Train is a big sounding Class AB amp covers all the Vox tones from clean to crunch and all the way to high-gain. The classic Vox look with suitcase style handles makes the package complete.
- 3 – 12AX7 tubes in the preamp section
- 2 – EL84 tubes in the power section
- Two-channel BRIGHT/GIRTH
- The THICK switch provides an additional gain boost on the BRIGHT channel
- 15 Watts
- 12 inch Celestion G12M Greenback speaker
- Inputs/Outputs: Guitar Input, FX Send, FX Return, Foot switch
The DSL15C is Marshall’s 15w, all tube combo amp. It’s compact, but packs a punch with classic EL84 tube tone. The DSL15C has two 6L6 tubes in the power section, but the preamp is all EL84s.. three of them to be exact.
15 watts may not sound like a lot, but for a tube amp it’s plenty loud for practice or small gigs. In fact, one of the things that makes the DSL15C so cool is that it has a pentode/triode switch for selecting the full 15w or a half 7.5w of output. That’s a great bit of versatility, and long time readers of this blog know that versatility is a big deal here.
The DSL15C also features a foot switchable gain channel that lets you switch between classic gain and ultra gain. Also present is a “deep switch” that gives your tone a bit of a low end boost.
With all these features and a 12 inch Celestial G12E speaker and built in reverb, the DSL15C captures the classic EL84 British sound in a tight little tone package.
Laney Cub 12
The Laney Cub 12 has a 15w and <1w input. Jack into the 15w input and you get the full throated roar, jack into the <1w and you get a more manageable volume for practice, with substantially less headroom. This means that you can get those dreamy, creamy British overdrive tones that EL84s are known for at a much lower volume.
- 15 watt all tube class A/B amplifier
- Three ECC83 tubes in the pre amp and two EL84s in the power amp stage
- Digital reverb (Tone and volume (as with the Cubs 8 and 10) and bass, middle treble EQ and gain control
- FX loop
- 12 inch Celestion speaker
The tube configuration gives the Cub 12 a much bigger voice than the Cub 8 and 10. Also, if 12 inches of Celestion isn’t enough for you, there’s also an external speaker out jack.