Tube Amps 101 - Guide for Dummies

A couple of years back, when I was trying to decide on a tube amp, I felt lost and overwhelmed. Back then I wished there was a guide that would give me a crash course on the types of tubes and tube amps. So I thought I’d put together a quick and dirty guide for folks, who are trying to get into tubes/tube amps.

Vacuum Tube Amplifier:
As we know an amplifier is a device that takes an input signal and puts out an amplified signal. This amplification can take place in a single stage or in multiple stages. In a tube amp, a tube is responsible for the amplification at each of these stages. For this post, we will see how the tubes are classified based on the number of electrodes, different roles of tubes in a tube amp, classification of tube amps based on the output coupling and topology.


Diode vs Triode vs Tetrode vs Pentode:
Diode - Diodes have an anode and a cathode and are used as AC to DC rectifier tubes.

Triode - Triodes have an anode, a cathode and a grid. These are the most common type of tubes you would come across in tube amps. Twin triode is a tube that has 2 x triode sections inside a single tube. Directly Heated Triode aka DHT is a type of triode where the cathode is directly heated.

Tetrode and Pentode - Tetrodes have an anode, a cathode, a grid and a screen. Pentodes have those and also a suppressor. The advantage of these tubes is higher output power relative to triodes. These tubes can also be run in a mode called ultra-linear.


Role of Tubes in a Tube Amp:
Rectifier Tube - The tube(s) that convert the AC power into DC. Some tube amps are solid state rectified, so you will not find rectifier tubes on these amps.

Input and Driver Tubes - An input tube is the first tube in the signal path. It takes the input signal and provides the voltage gain, aka amplifies the signal. A driver tube takes this amplified signal and adds some more gain before passing it on to the power tube. A driver tube should also be capable of driving the power tube. In 2 stage amps, the input tube also acts as the driver tube.

Power/Output Tube - These are the tubes that come in the final stage (output stage) of a tube amp and are the ones responsible for driving the transducers.


Output Transformer Coupled vs Output Transformer Less (OTL):
Output Transformer Coupled tube amps make use of an Output Transformer to couple the output tube to the transducer. In an OTC amp, the output transformers play a vital role in determining the sound quality/performance of the amp. 99% of the tube speaker amps are transformer coupled amps.

While the OTL amps do not have output transformers, most of the OTL amps are Output Capacitor Coupled. Meaning, a capacitor is used in place of a transformer, to couple the output tube to the transducer (or the following load). Many of these amps have relatively higher output impedance and are typically used in applications, where the high output impedance will not be detrimental to the performance. For example, many tube pre-amps are OTL designs. OTL amps with high output impedance can also work well for high-impedance, dynamic driver headphones.


Single Ended Triode (SET) vs Pentode vs Push-Pull:
As the name suggests, SET amps are single ended in topology and use triodes. SET amps are known for the tone, timbre and are often associated with the SET magic. The biggest disadvantage of a SET amp is the low output power. This is where pentodes come in. A pentode amp can offer considerably more power output than a SET amp. Many pentode amps give you the option to run it in Ultralinear mode and few have the option to run the tubes as triodes as well.

Push-Pull is kind of like a differential amplification and requires 2 x output tubes per channel. One tube drives the in-phase signal and the other drives the reverse phase signal. You get considerably more power output from a Push-Pull amp, especially when it uses pentodes. So when you see 50W or 100W tube amps, those most likely are push-pull, pentode amps. You can make push-pull with both triodes and pentodes.


What is a Hybrid amp? Is it considered a tube amp?
A hybrid amp has at least one tube stage and one solid state stage. The popular config is to use tubes for the initial gain stages, followed by one or more soild-state stages. This is almost as equivalent as using a tube pre-amp and a solid state power amp in the speaker realm. The logic here is that you get the best of both worlds; the tone of the tubes along with the high power output of the solid state. However, in the tube circle, a hybrid amp is not considered a tube amp.

On the other hand, there are some rare instances, where you have a solid state stage input/driver stage followed by a tube output stage. This can fly as a tube amp. However this usually is considered counter intuitive and some tube enthusiasts would argue that it shouldn’t be considered as a tube amp.


Let’s look at some amps so that we can relate to what we just learnt:
Bottlehead Crack - This is an OTL, SET amp. It is capacitor coupled and has high output impedance and works best with high impedance dynamic driver headphones

Schiit Valhalla, EC ZDS: These are just like the BH Crack, except, these have lower output impedance, giving users the flexibility to use low impedance HPs too

AmpsAndSound Kenzie, Dragon Inspire IHA-1 - These are transformer coupled, SET amps

DNA Starlett, SW51, AmpsAndSound Mogwai - These are transformer coupled, SET amps, which use pentodes strapped as triodes

DNA Stratus, Stellaris, Eddie Current Studio B - These are transformer coupled, SET amps which use DHT tubes. Stratus and Stellaris use the 2A3 and the Studio B uses the 300B

Eddie Current Studio T - This is a transformer coupled, SET amp that uses DHT tubes. It uses 2 x 2A3 tubes per channel, but in a parallel configuration. So this is a parallel, SET amp aka PSE

Woo Audio WA33 - This is a transformer coupled, push-pull amp. Unlike the Studio T which is a parallel SET, the WA33 uses 2 x 2A3s per channel in a push-pull configuration

Dynaco ST70, Leben CS300, CS600 - These are transformer coupled, pentode, push-pull amps

Elekit TU8200: This is a transformer coupled, single ended, pentode amp. You can run it in either pentode, UL or triode mode

Tubes4HiFi SP14, Don Sachs Line Stage, Elekit TU8500: These are OTL, SET preamplifiers

Schiit Vali 2, Lyr +, Garage 1217 Horizon, MCTH - These are hybrid headphone amps with a tube gain stage and an SS output stage

Schiit Saga + - This is a hybrid preamplifier with a tube gain stage and an SS output stage

 

Reserved.

 

Saga is a hybrid (like Vali), Freya is NOT a hybrid. Freya is switchable with either an all SS or all tube circuit

 

Thanks @Cspirou , corrected.