Here we have gathered some information about tube amps and some modern technologies used in Bluetone amps.

Biasing a tube amp in modern way

Many people avoid tube amps because power tube biasing is way too complicated. Many tube amps have such a power tube biasing system, that it demands a skilled person to do it. In this case amp chassis has to be opened to to get tubes biased properly. Inside tube amp chassis there are dangerous voltages.

To avoid that kind of problems, we developed Bluetone Bias Rate bias adjustment system, which makes power tube biasing safe, fast and easy. This system has been installed successfully to all latest fixed bias Bluetone tube amps. However we wanted to go further and developed even better Bluetone Auto Bias Control biasing system.

Bluetone ABC is based on a software application with semiconductors. This system adjusts and controls power tube biases automatically. Bias level is set with a user friendly ABC-meter in a safety voltage level (max. 5 V). This meter menu contains all common power tube types. After targeted bias level has been set, system automatically controls it and adjusts it to properly level if needed. This check and adjustment is done individually to each tube every time when amp has been turned on. With ABC you can also use unmatched power tubes and save money.

There is also a small led indicator nearby each power tube. When led is burning, it shows that tube is in working condition. If led turns off, it means that power tube is possibly broken. With these led lights it is fast and easy to find a broken tube and replace it. After tube change, ABC inside amp chassis adjusts bias to the targeted level.

ABC will be an option to all fixed bias Bluetone amps. This option contains ABC system and a ABC meter with detailed biasing instructions.

Our ABC biasing system has been developed further to a wlan based application. This means that there is no need for a separate bias meter. Power tubes biasing can be done wirelessly either with a normal pc or with a smart phone.

Point-to-point- or printed circuit (PCB) boards?

Many players wonder if there is any difference if an amp is based on p-to-p or pcb-technology. Here are listed some subjective views to this matter.

Replacing components and modifying circuit is easy and fast if amp has been designed properly.
With this technology you can design very unique and customized amps.
P-to-p-amp can contain also physically bigger components which are long standing and reliable.
P-to-p-amp can be designed such a way that component+wiring layout prevents capacitance problems. These problems normally occur if component placement and wiring layout has been done in a bad way. Capacitance problems can alter guitar sound for example killing some high frequencies.

A real p-to-p amp has to be built by hands because it is almost impossible to use automation lines to assemble this kind of amp. That is why it is more expensive to build compared to an amp which has been built in a automated assembly line. P-to-p-amp can be physically bigger and heavier compared to pcb amp due to construction and component size.

This kind of amp can be designed such a way that it is easy to assemble in an automation line with minimal costs.
Due to design methods, this kind of amplifier can be built physically to a smaller space than a p-to-p-amplifier.

It is pretty difficult and time consuming to fix or modify a pcb amp. A pcb based amp with bad design and cheap components can break easily. today it is very normal to integrate all components, tube sockets, jacks and potentiometers to an integrated pcb board to save costs and make automated assembly possible. When this kind of amp is up and running, heat can cause damages especially in combo models, where tubes have been installed upside down to a thin pcb board.

VVR (Power scaling etc.), Variac, PPIMV

VVR = Variable Voltage Regulator/Reduction is a method to control amp output power using modern mosfet technology.
This control can be used different ways. Here are some examples of different approaches:
– VVR controls power tubes plate and grid (bias) voltages when it is a matter of fixed bias amp. In a cathode bias amp only plate voltages are regulated. In this case the best master volume is PPIMV (Post Phase Inverter Master Volume) to prevent power tubes from too much overdriving. If power tube plate voltages are set really low (30-50 V) and there is a high guitar signal level (30-50 V) coming from PI without any master volume, the result is really ugly sounding distortion.

– VVR controls power tubes + phase inverter voltages. In this case it is good practise to have a master volume before phase inverter, to prevent power tubes from too much overdriving as mentioned above.

-VVR controls all tubes plates voltages. In this case there is no need for a dedicated master volume, because VVR acts like a master volume to reduce noise level.
VVR is a good way to drop noise level even to bedfroom level. To our mind it works sonically best in cathode biased amps, but works technically well also in fixed bias amps when needed. VVR/Power scale does not overload power tubes compared to attenuators an dummy loads. These methods shorten power tube life cycle radically.

VVR works best in one channel amps where there is a need for power amp distortion. If you have a multi channel amp where the biggest part of distortion is generated in preamp, VVR is not the best way to go.


Variac=adjustable transformer, which can be used to control tube amp voltages and output power + power amp distortion. Traditional variac is a separate unit, which will be connected between amp and wall voltage. When variac is adjusted, amp voltages will change at the same time. When using traditional variac, all voltages will drop including heater voltages. This situation can cause faster tube wearing. Some variacs can also be used to increase voltages. This can be dangerous with tube amps if you exceed amp nominal tube voltages. Amp can be damaged and cause to it`s user a dangerous situation.

Variac can be built such a way, that amp heater voltages remain normal, despite all other voltages will be adjusted. This kind of solution demands a special circuit inside amp.

Dynamics control

Dynamics is a method to control preamp tubes plate voltages with VVR technology. If preamp tubes plate voltages are high, there is more headroom and more dynamics depending on configuration. If plate voltages are low, there is more compression and preamp distortion depending on amp technical architecture.


PPIMV= Post Phase Inverter Master Volume is a master volume, which is located between phase inverter tube and power tubes. In old school amps phase inverter distortion is a big part of power amp distortion. PPIMV is a pretty cheap and trusted method to generate power amp distortion even in bed room levels.

An active and buffered effect loop

Many guitar players want to use effects, which can be connected to amp effect loop. These effects normally work with 1 dBv operating level compared to traditional guitar effects (-30 dBv). Normally effect loops work best with higher operating level. A big challenge is following:

In a high gain guitar amp preamp generates guitar signal level, which can be tens of volts when it is driven hard. When this signal reaches effect loop, it has to be reduced/compressed to appr. 1-3 volt level, so that effect loop can handle it properly. After effect loop process the same signal has to be raised back to 10-30 volt level before power amp stage, so that it behaves the same way than without effect loop and can overdrive amp phase inverter and power amp. All this should be done without altering original sound and adding any extra noise to signal chain.

This kind of loop should also somehow handle guitar effects, which operate even lower operating levels (-30 dBv).

In Bluetone Amps we have developed a buffered seriers effect loop with adjustable send level. It works with one 12Ax7 tube and can be also used as a clean boost if necessary. By now we have installed these loops to many Bluetone amps with success. When this loop is disconnected from signal chain, it is completely transparent soundwise.

After these experiences we have come to conclusion that an effect loop can be a useful tool in your guitar amp if it has been designed properly.

Preamp distortion and power amp distortion

Many guitar players try to find a single guitar tubeamp, which is capable to create traditional dynamic clean sounds, lightly distorted sounds for rhythm playing and screaming heavily distorted sounds for lead playing. Traditional one channel “old school” guitar amps normally demand high volume levels before you get any power amp distortion. Anyhow these amps are great tools for a guitar player who can control amp distortion level with his guitar volume potentiometer. In this kind of amps the biggest part of distortion is created in power amp. This kind of amps are for example old Marshalls and Bluetone Express.

To have a practical solution for all distortion styles, guitar amp manufacturesrs developed multi channel amps, which were also capable to create distortion in preamp sections. Normally the biggest part of distortion is created in preamp section in this kind of amps. A couple of examples of these amps are Soldano SLO 100, Marshall JCM800/2204, Bluetone Dusty Road and Bluetone Crossroad.

Preamp distortion and power amp distortion differ from each other soundwise, because they have been created with different topology and technology. It is always a challenge to build an amp which is capable to do all distortion things mentioned above. That is why we developed Bluetone Dusty Road to meet these demands. It copmbines Fender world clean sounds and old school Marshall world distortion sounds. After developing this amp, we went even further and created Bluetone Crossroad with three independent preamp channels and a sound palette from “clean to modern mean”.

A combo version or a head and speaker cabinet ?

Many guitar players want to play with a small and lightweight guitar combo, because it is compact and easy to transport. A well designed combo is good choice in many cases. Anyhow there are a few cons, which are good to know. One problem are today`s tubes, which can cause all kind of mechanical noise when playing amp. This can be annoying, especially when you play alone in bed room levels. The other problem is the lack of low frequencies. A combo back wall is almost completely open, mainly for tube heat reasons. This causes lack of bass frequencies, which are essential in some music genres. A small combo can sound “small” despite it sounds good. The smaller the cabin and speaker, the smaller the sound.

A head+ cabinet combination is more versatile. You can use different amp and speaker combinations depending your needs. With a sealed or partially sealed back wall, a speaker can produce low frequencies more easily than a combo model. However head+cabinet combination can be heavy and not so easy to transport.