Preamp Tube Academy

An illustrated guide to preamp tubes, featuring examples from simple to complex amplifiers.

You'll see it's all quite simple when you learn how tubes work together in your amp.

Tube Safety

The best part about preamp tubes is that you do not need a technician to change them. However, there are certain precautions that should be followed to ensure you are doing so safely. This information applies to all tubes, including common preamp tubes such as 12AX7, 7025, ECC83, 12AT7, 12AY7, 12AU7, and their substitute replacement types.

 

5 Rules of Proper Tube Handling

  1. Tubes do get hot enough to scald your skin. When removing tubes you should allow time for them to cool off just as you would a light bulb.

  2. Treat the tubes extra gently when they are warm.

  3. Allow tubes to heat up for 13 seconds before you turn the high voltage "standby" switch to the "play" position. The tubes will last longer. It does not matter when shutting off.

  4. Do not install tubes with the standby switch in the play position.

  5. Do not tinker inside the amp; you do not need to disassemble the chassis to change tubes (NO exceptions) - do not go inside the amp unless you are a qualified technician). Lethal shock is possible if you avoid these simple warnings.

Gain Tubes

When you look at the layouts, picture yourself standing behind your amp. From right to left the tubes are designated V1, V2, V3...and so on.

The most important tube is the one your guitar sees first (V1) . It sets the tone and character that all the subsequent tubes have to work with. Usually it's the tube all the way to right, more on that when we see the models with 4 inputs. Small amps may not have a subsequent preamp tube.

Example #1 -  Fender Champ Amp:

Fender champ amp tube layout

The guitar plugs in the top right and goes to the only preamp tube, shown in RED.

RED is signifying the importance of this tubes performance, select a tube we Application Grade™ as "GAIN". You can see that the input jacks wires up to the GAIN tube. Notice that there is one output tube; a single 6V6. The 5Y3 is a rectifier tube.

REPLACE ONE TUBE AND DRAMATICALLY CHANGE YOUR AMPS SOUND.


Function Tubes

The next tube down the line also has an audible impact on the tone, but not as pronounced: The Vibro Champ gives you another gain stage so you can further refine your sound. Although it is a gain stage, the amount of signal voltage gain is not high. Therefore you can use a tube that is not the equivalent of a race horse, and you don't need to spend that much on the tube.

Example #2: Fender Vibro-Champ Amp

Fender Vibro Champ Amp Layout

Simply select a "FUNCTION" Applications Graded™ tube. The "FUNCTION" tube is marked in YELLOW. This will keep your costs down, and you won't have to worry about the tube being microphonic.


Driver Tubes

This amp uses more than one output tube. So matched output tubes make sense, even more sense when using a "DRIVER" tube. The DRIVER tube is a rarely qualified tube in that the two triodes in the glass envelope are working at the same potential. This in turn assures that the matched output tubes have the same signal voltage to work with.

Example #3: Fender Vibrolux

Fender Vibrolux Preamp Tube Layout

The DRIVER tube is colored in GREEN. Just to make clear: an amp with more than one output tube will benefit from a "DRIVER" tube.

Common examples of output tube types; 6L6, 6V6, 6BQ5, EL34, EL84, 5881, 6550, 7591, KT66, KT88.


Amplifier Examples

Get ready to graduate! Let's move on to some more complex amplifier models and learn how the tubes affect their tone.

Fender Bassman Amp AB165 preamp tubes

Fender Bassman

This schematic shows the Fender Bassman which has your typical four tube layout. The "Bass Instrument" channel hogs up two tubes, V1 and V2.

If you are using the bass channel you will benefit from a GAIN tube in V1. Experience tells us that while a GAIN tube can be used in V2 it is not really needed and a "FUNCTION" tube will work well as a 'further refinement' tube.

*Note that the 7025 tube specified is merely a 12AX7 with additional testing for low hum and low microphonics.

If you use the Bassman as a guitar amp and you use the "Normal" channel then it is V3 that is of the most interest to you. The "bass" channel tubes are not used when you plug into the "normal" channel. Most people really don't care what tubes sound like in a channel they never use, so as long as they work, save some cash here.

Notice the 12AT7 driver tube, it can be substituted with a 12AX7 "DRIVER" for greater tonal choices. The 12AT7 has a very strong tendancy to be microphonic, and tonally is very brittle sounding, harsh. You can listen to it in the "GAIN" position if you are interested. It wont hurt your amp, but bring the volume up slowly as it almost certainly will squeal. Putting in a 12AX7 will add more gain - at higher volumes you will be able to hear the gain, at lower volumes you will be able the tonal improvement.

You can read more about microphonic tubes on our blog that contains lots of very helpful free information.
ABOUT TUBES

What about an older Fender Twin?

The preamp tube layout is the same for the classic Marshall Super Lead amplifier (model 1959, others). If you have 3 preamp tubes, odds are this is the layout. Extremely common.

*Note the 4 input jacks located together. This would indicate both channels are sharing V1.

I'll bet you have this all figured out. The RED indicates the GAIN graded tube is needed. The YELLOW indicated a FUNCTION tube will work just fine. The GREEN symbolizes that a DRIVER type tube is needed.

ANOTHER LOOK at the twin; Great Improvement, Nearly Perfected now, and MY AMP NEVER SOUNDED THIS GOOD!
Fender Twin Amp Preamp Tubes 5F8
Fender Super Reverb Preamp Tubes AB568

Fender Super Reverb and similar

The preamp tube layout is the same for the Fender Deluxe Reverb, and the Fender Twin Reverb.

This time we have a "Normal" channel sucking up 1 tube - V1. This channel has no reverb and no vibrato.

V2 is the "GAIN" tube for the "vibrato" channel.

V3 is driving the reverb springs.

V4 is doing two things:

1. Refines the vibrato channel sound (the second gain stage for that channel)

2. Amplifies the reverb springs sound. Any amp that lets you use 2 separate tubes in the signal path offers you a creative opportunity to improve your sound.

V5 is not even dealing with an audio signal! It just makes that very slow vibrato effect, and the guitar signal in no way passes through this tube. Its an excellent place to rob a tube from if you are in a pinch with a microphonic tube because a microphonic tube will work just fine here!

V6 is the "DRIVER".

Fender Deluxe Amp


The Fender Deluxe...
The Normal channel is associated with V1.
The Vibrato channel is associated with V2.
V3 is strictly a FUNCTION tube.
V4 requires selection of a "DRIVER" tube.

*Note the 4 input jacks located together. This would indicate both channels are sharing V1.

Fender Deluxe Amplifier Preamp Tubes AB763

Tube Tips

  1. If you don't know if you are replacing a tube on the channel you are plugging into then remove the tube. No harm will come to your amp. If you still get sound then the tube is not in the circuit. Skip replacing such a tube, instead consider it a spare tube holder. Do replace the tube back to its socket.

    If you don't want to pull that tube then turn up one channel, turn down the other and plug your guitar in. When you tap gently on a tube with your fingernail or an eraser tip then you will be able to hear what tubes are in that channel by the presence of your tapping on the output of the amp. The tube that taps most loudly is the gain tube.

  2. You can identify microphonic tubes by tapping them as described above. Consider that since all tubes have a mechanical construction that they all possess microphonic characteristics to some degree. Microphonic tubes are problematic only if they are causing changes in the amp sound while playing. So to tell if the tone you hear when you tap is really a problem you must first be hearing something is wrong with the amp, and then you can nail down which tube it is by turning the amp up a bit and if your tapping results in a sound that sustains itself then BINGO. Substitute the tube without changing your amp settings (except power it off) and then retest.

Example On refining your amps sound - for some Marshall players, some MetaL forms. You know your amp well. You play a rapid fire style that is percussive and you find the tone nice and clear but too razory in the treble zone. For the GAIN stage you will most of all need an accurate tube to articulate that speed and percussive quality without getting squishy. Then as a secondary concern you want to tame that treble, so your FUNCTION tube should sound warm.

What if you want balanced tone - dark driven beefy cream? Okay. "Warm with mild COMPRESSION and early distortion" GAIN tube, perfected with "Clean and COMPRESSED" to get the complete attitude. Rock on. Almost but not quite? Switch the 2 tubes...give another listen (you will need each tube to be Applications Graded™ for "GAIN" performance in order to swap). This is FUN!

For a great clean sound start with what you think your amp is lacking, and or what its needs more of.

Ex: I need to play louder without the amp breaking up where there is bass. I like to be loud and clean, try the "clean and accurate" tube in the gain position, and make sure you have a good working DRIVER tube (a potential source for clipping specially at louder settings). That selection of a gain tube will help to keep the bass tight and never add distortion to a clean channel. The driver needs to be a driver. Its can be anything that is not dark or easily distorted. Of course its tonal qualities further refine the amps response but not in a critical manner.


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