Tube Talk
There are so many different tube types that are used for various tube amp applications that it may be a little overwhelming to some. It may leave some people to wonder "What's the difference?" or "Does it really matter?" On this page, we focus on the more common tube types and try to answer some of those questions. The goal is to clarify things for when you're making a decision on which tube amp - and tube set - is right for you.
Power Tubes
Power tubes - also called output tubes - are the larger tubes that perform the final stage of amplification before the output transformer. Along with the output transformer, they have a significant impact on the final tone of the amplifier. There are two main layouts for power tubes - octal (8 pin) and noval (9 pin), with octal tubes being the most common.
Below is a list of common octal and noval power tubes.
Octal Power Tubes
6V6
Max power: 12-14 Watts
A lower powered tube, typically used in clean or bluesy amps, but it handles overdrive well. It's a well-balanced power tube that has a clear and sparkly top-end, a controlled mid-range and thick low-end.
Available in the Delta Jr. and the 20-watt Delta Deluxe.
EL34
Max power: 25 Watts
A venerable and very popular power tube, used in many British-style amplifiers, lending itself to that "British" tone. It accentuates the mid-range, has a tight low-end and handles overdrive extremely well.
Available in the '67 Special 50W, '67 Special 100W, '81 Special, Loyalist, Mercenary 50W/100W, Snakecharmer and SE-12
6CA7
Max power: 25 Watts
The American version of the EL34. It can be a direct replacement to the EL34, but does have some slight tonal differences. The top-end is chimey, the mid-range is slightly less pronounced and the low-end is fatter.
Available in the '78 Special
KT66
Max power: 25 Watts
An alternate to the EL34, however it can only be a replacement to the EL34 if the amp is designed for it. It's a cleaner tube with a less pronounced mid-range.
Available in the '67 Special 50W and '67 Special 100W
KT77
Max power: 25 Watts
Another alternate to the EL34 which can be used as a direct replacement. Like the 6CA7, the KT77 has a smoother mid-range and fatter low-end.
6L6GC
Max power: 30 Watts
A common tube used in amps that need a big clean sound with a lot of headroom. The 6L6 tone is often associated with the "American" tone. It's got a sparkly top-end, scooped mid-range and a fat low-end.
Available in the Clarksdale, Patriot, Mercenary 60W/120W and SE-12
KT88
Max power: 42 Watts
A big bottle tube with tons of headroom. It has a pronounced mid-range and tight low-end.
Available in the SE-12
6550
Max power: 42 Watts
Another big bottle tube with tons of headroom. It has a more mid-scooped, chimey tone compared to the KT88.
Available in the Snakecharmer and SE-12
Noval Power Tubes
EL84 / 6BQ5
Max power: 12-14 Watts
An extremely underrated power tube with a very sweet overall tone. It handles both clean and overdrive extremely well. It delivers a warm clean tone and when overdriven, its tone is tight with lots of mid-range harmonics and a tight low-end.
Available in the Halfpint, RC15, RC30 and SE-6
Can I...?
The number 1 question asked about power tubes is "Are they interchangeable?" The answer is somewhere between "It depends" and "No." An amp circuit would have to be designed in a very specific way to allow certain octal power tubes to be used interchangeably. A number of things would have to be considered in designing an amp to accommodate different octal power tubes, including, but not limited to:
- The plate voltage would have to be in a range to support the different limits between the tube types
- The tube sockets would have to be wired to accommodate the different tube types
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The bias circuit would need to accommodate the different power tube bias requirements:
- In fixed-biased amps, the bias supply would need to provide a suitable range of negative voltage to properly bias the different tubes.
- In cathode-biased amps, the cathode resistor would have to be an appropriate value to accommodate the different power tube types.
Of course, in the case of octal and noval tubes, their form factors are not compatible, so they are not interchangeable for that reason. (Conversion sockets are available, but SonicTone cannot and would not recommend using a socket converter, as the parameters and limits between octal and noval tubes can vary tremendously and it could pose a great risk to the amplifier and tubes.)
As with anything dealing with tube amps, do not try to use tubes that an amp wasn't designed for and always consult with an experienced amp technician before attempting to use alternate power tubes. And remember, once power tubes are replaced, they need to be properly biased.
[ ^Top ]Preamp Tubes
Preamp tubes are essential in providing the first amplification stages of an amplifier, among other things. In those first stages, they get the relatively weak guitar signal amplified and ready to be handed off to the power amp section. Like power tubes, there are many different types of preamp tubes, but the most popular are the 12A*7 series, such as the 12AX7 - which is the most common preamp tube.
Preamp tubes like the 12AX7 are actually two tube stages in one, called a dual-triode. They are all noval (9-pin) tubes.
The main difference among the 12A*7 series of preamp tubes is the gain factor. The 12AX7 provides the most which makes it ideal for a preamp stage. The others can be used in place of a 12AX7, but with their lower gain factors, the gain and perceived volume of the amp will seem less.
- 12AX7: Gain Factor 100
- 12AT7: Gain Factor 60
- 12AY7: Gain Factor 40
- 12AU7: Gain Factor 20
(A preamp tube like the 12AU7 with such a low gain factor is typically used for something like a reverb driver and not a general gain stage.)
All SonicTone amplifiers use only 12AX7 preamp tubes.
[ ^Top ]Rectifier Tubes
The rectifier tube in an amplifier does not process the audio directly, but it plays a large part in rectifying the required voltage and providing that voltage to the amp. That said, the physical and internal factors of a rectifier tube contribute to things like the overall touch and responsiveness, because of the manner in which it provides the voltage to the amp. Some common rectifier tubes are highlighted below.
5Y3
The 5Y3 is considered more of a "vintage" rectifier tube with qualities that really lend itself to older vintage amp tones that had a lot of sag and early compression. It has a large voltage drop and when playing an amp that uses a 5Y3, the player can really feel the touch and response as the guitar signal compresses and breaks up quickly. The 5Y3 is an octal (8-pin) tube.
Available in the Delta Jr.
EZ81 / 6CA4
The EZ81 is great rectifier tube for lower powered amps. It has less of a voltage drop than a 5Y3 and a "faster" response time when replenishing the available voltage. It provides a great vintage style touch and response when playing an amp that uses one. The EZ81 is a noval (9-pin) tube.
Available in the Halfpint, RC15, SE-6 and SE-12
GZ34 / 5AR4
The GZ34 is a common and widely-used rectifier tube. It has less of a voltage drop than a 5Y3 and a "faster" response time when replenishing the available voltage. With that, amps that use a GZ34 give a little more "immediate" response and touch, but still not as much as an amp with a solid-state rectifier. The GZ34 is an octal (8-pin) tube.
Available in the Delta Deluxe, Clarksdale 60W, '67 Special 50W and RC30
Can I...?
A common question asked about rectifier tubes is "Are they interchangable?" The answer is "No." Each rectifier has a certain pin layout with internal properties that make it not possible to switch between different rectifier tubes.
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