More Technical Info



Contents of this Page:

Tube Designations

Power Tubes

Phase Inversion

Power Tube Biasing

·       Cathode Biasing

·       Fixed Biasing


·       Power Transformers

·       Output Transformers


Tube Designations

Common Guitar Amp Rectifier Tubes – 5Y3, 5U4, GZ34

Common Guitar Amp Preamp Dual Triodes – 12AX7, 12AT7, 12AY7, 12AU7

Common Guitar Amp Power Tubes – 6V6, 6L6, EL84, EL34

The first number signifies the heater voltage, so the “6”’s signify 6.3 Volt. The “12”’s are a bit confusing since these tubes you can hook up the heaters for the two internal tubes in parallel or series. So if they were hooked in series they would use ~12 V, but they are commonly hooked in parallel so they nominally use the 6.3 volt current.

The Preamp tubes are listed in order of highest gain to lowest gain. “Gain” is the amount of amplification, however, this is also commonly referred to by guitarists as a measure of distortion, since often more amplification in a stage results in signal distortion. So a 12AX7 will sound slightly louder and more distorted than a 12AT7 on the same setting. Too much distortion is also not a good thing, which is why the 12AT7 and 12AY7 are often used to “soften up” the tone of an amp.

Power Tubes

Simple amplifiers are “Single Ended” which means they have a single power tube that is configured similarly to any preamp tube. The Fender 5F1 model Champ or AX84 P1 are single ended designs. Single ended design can produce enough volume for home use, but not to play in a band or onstage. You can also configure single ended designs in parallel for “double single ended” or “triple single ended” designs for more volume.

“Push Pull” power tubes are configured in a pair. Signal input and high DC voltage is applied as with other tubes, however, each tube provides its own output signal to each end the transformer leads. The output signals “alternate” very quickly. This configuration provides more power than two single ended tubes in parallel since each tube is only “on” part time and can handle more current flow and thus produce more power. This is the common design for tube amplifiers intended to use in a band setting (anywhere from about 20 to 100 Watts). Since the output signals must alternate, the input signals must go into each tube out of phase, or must alternate positive and negative voltage. To do this we must use an additional tube as a Phase Inverter. A signal’s phase (“+” or “-”) is inverted when it passes though a tube, so can always get a “copy” of the inverted signal. We send the mutually opposed signals to the two power tubes to get the alternating effect. Generally the two power tubes not only need to be the same type to sound good, but need to be “matched” to at least some extent. Tubes can draw different amounts of current, even the same type and manufacture. They are considered matched when thy draw about the same amount of current (within ~5%). Most like the tone and sound of push pull designs much better than single ended, which can tend to be a bit harsh. There are different push pull designs such as Class A, Class B, Class AB etc. You can search the Internet or look in books and find detailed explanations of each. Frankly, you can build amps and enjoy them without really understanding all the Class subtleties, especially if you are using proven designs and schematics as a guide.

Phase Inversion

There are a several ways to do Phase Inversion. A “cathodyne” phase inverter uses one triode to produce two mutually opposed signals by tapping the cathode side of the tube for the inverted signal. Look at Fender Tweed Amps for this Style (5E3, 5E8, etc). A “long tail pair” uses two triodes connected at the cathode to produce the two opposed signals. Look for Fender “Black Face” design for this type (Deluxe Reverb, Super Reverb, etc). One advantage of the long tail pair is that is also produces some appreciable gain where the cathodyne type does not. “Self Split” phase inverting is a neat design where the power voltage is put across two tubes in series and the second power tube acts to produce the inverted signal, not really acting as a power tube, but as a phase inverter. No additional preceding phase inversion step is needed. So we get the advantage of the tone of push pull, but at a power/volume level that is not much higher than a single tube. You can also use mismatched tubes for this.

Power Tube Biasing

Biasing is another word for “controlling the amount of current that flows through a tube.” Too much current will cause premature tube wear and possibly even failure. If too much current is flowing the plates will glow red and the coating can be stripped off the cathode. Too little current and the sound of the amp will be thin and weak. There are two main types of biasing, cathode biasing and fixed biasing.


  • Cathode biasing uses a resistor on the cathode of the tube which is in line with the current flow. This “physically” limits the current flow. Cathode biased amps will sound different, not as clean, break up (distort) easier. In order to change the bias you need to physically swap out the resistor. Look at most preamp tubes and Fender Tweed Amp Power tubes for examples of cathode bias. A drawback is that the resistors need to be of pretty high wattage rating. It is really impractical to use cathode bias for large wattage amps (greater than about 25W).

  • Fixed biasing is a bit of a misnomer since you can have an adjustable fixed bias. "Fixed" does not mean it is “permanent.” Also adding ot the confusion, cathode biasing looks “fixed” since the resistor sets the bias and you would need to change it out to adjust. What fixed biasing means is that the voltage into to the tube (the signal voltage) is “fixed” or “intentionally set” which effectively changes its relationship to the voltage of the cathode and will influence the flow of elections, in essence controlling them and therefore controlling the bias. A more negative voltage of the signal will reduce the current flow through the tube. For this type of biasing, there is typically
    another voltage tap from the transformer, maybe 70 V or so, that is use with a diode to produce a the negative bias voltage. This arraignment can be used in conjunction with a potentiometer to allow adjustment of this "fixed" voltage. Look at Fender Blackface Designs for examples of Adjustable Fixed bias.

As far as guitar tube amp biasing goes, how much current to allow is a hotly contested topic. I believe in simplicity. I like the current draw method which is allow about 70% of the max current rating at idle (for fixed bias). You can tweak it a little to your own ear from there. For cathode bias I run my amps pretty much up to 100% of the max current rating. I am a firm believer that tubes are quite robust devices and a little too much current won’t hurt them. So if, say you are at 75% of the max rating they should be fine, as long as the plates are not glowing red. You can find max plate ratings on Tube Data Sheets (see Links). You will need to convert the Power to Amps based your measured plate Voltage.  

P (Watts) = [ I (Amps) ]2 x R (Ohms)


V (Volts) = I (Amps) x R (Ohms)

So therefore:

P (Watts) = I (Amps) x V (Volts)


Max Current Draw = P Max Plate (Watts) / V Plate (Volts)

Use 70% of this for your Bias Current Target.

Here is a Spreadsheet to make a graph (Already Set up for 6V6 and 6L6)



Remember to subtract the voltage across the cathode resistor for cathode biased amps!



There are two primary types of transformers Power and Output (or Audio).


  • Power Transformers - Ok, so we need a power source to send through the rectifier for the high voltage, we also need a heater current source. If we have a rectifier that operates its heaters on 5V, then we need that in addition to 6.3 Volt. So a guitar amp transformer will generally have two or more coils on the amp side to give the high voltage service and the low voltage heater current service(s). The high voltage service may be something like 300 Volts at 150 milliamps. The heater service may be something like 6.3 Volts at 3 Amps. So a transformer must not only provide the right voltages, but be rated to handle the current required for all the services. The voltage it
    produces is a function of how many turns of wire in each coil, the rating is a function of how thick the wire is (to handle the current flow). The preamp tubes might only draw 1 or 2 milliamps, but the power tubes will probably draw 30 to 40 milliamps or more each at idle (with no music through the amp). For home brewers, a rule of thumb is that the high voltage current rating should be twice the total idle tube current draw (as a minimum). To learn more about how much current the power tubes should be drawing, see the Biasing Section.

The heater current rating does not need such a 2 times safety factor, just add up all the currents of all the heaters plus the power on light (if any) and be sure it is less than the rating of the transformer. You can get heater current draw from a Tube Data Sheet (see Links Page). Some heater current draws are:

12AX7 (or any 12A series)                   300 mA

6V6                                                      450 mA

6L6                                                      900 mA

EL34                                                    1500 mA

Typical “On” Light                                        150 mA


  • Output Transformers – the output transformer takes the final amplified signal (still riding on the high voltage of the power tubes) and converts it to a lower voltage, higher current signal. This signal has enough current to go though the coil in your speaker and induce a strong magnetic force that moves the cone of the speaker and produces the sound. Output transformers are rated also, but typically not by Voltage and current, but rather by Wattage. There is no real direct way to measure the power (Watts) an amp is putting out. Here is a list our the typical guitar amp power tubes and about how much power an amp built with them will produce. Output transformers
    and speakers should be at least these ratings.

Single Ended EL84                                        6 Watts

Single Ended 6V6                                6 Watts

Single Ended 6L6                                7 to 10 Watts

Single Ended 7Au7 or 12Au7               1 ½ Watts

Double Single Ended 7Au7 or 12AU7  3 Watts

Cathode Biased 6V6 Push Pull            15 to 20 Watts

Cathode Biased 6L6 Push Pull            25 Watts

Fixed Bias 6V6 Push Pull                    20 to 25 Watts

Fixed Bias 6L6 Push Pull                     35 to 60 Watts

Fixed Bias EL34 Push Pull                  50 Watts



One aspect of transformers that I have always found intriguing is the issue of how the type of transformer can affect the tone. Some swear by “boutique” transformers such as Mercury Magnetics. Others don’t seem to attribute any particular style of transformer with good tone. Vintage amp transformers weren’t necessarily chosen for tone, they were probably the cheapest thing they could get to produce a satisfactory product! I personally have had great results with “Average Joe” transformers (such as Hammond, Weber, and generic remakes of fender transformers). I am probably never going to go out and spend several hundred dollars extra for real top end transformers for any build. With that said, my only data point of different transformers in the same amp was with my 7AU7. I had three different Hammond output transformers in there, and I must say, I did not notice any difference in tone at all. In fact only one of them was for a single ended amp, the other two were push pull where I simply did no hook up the center lead! Granted, with the low output, none of them were probably loaded up at all, but I am still not sold on “boutique” transformers for guitar amps . . .