More Technical Info
Contents of this Page:
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.
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.
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
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.
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.
V (Volts) = I (Amps) x R (Ohms)
Max Current Draw = P Max Plate (
Use 70% of this for your Bias Current Target.
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).
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
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
Fixed Bias 6V6 Push Pull 20 to 25 Watts
Fixed Bias 6L6 Push Pull 35 to 60 Watts
Fixed Bias EL34 Push Pull 50
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