Tubes explained in AB763 amps

Tube functions

Let’s explain what the different tubes do in the common AB763 blackface & silverface amps with two channels, reverb and tremolo. Some of these amps are the Deluxe Reverb, Vibrolux Reverb, Pro Reverb, Super Reverb, Twin Reverb, Vibroverb, Bandmaster Reverb. We start from right to left, amp seen from behind:

V1 – 12ax7

Preamp tube for normal channel. First gain stage. This tube amplifies the guitar signal immediately after it enters the jack input of the amp’s normal channel. This tube is very important for the tone and how sensitive the volume knob is. A weak, old and worn out V1 tube will give you a dull tone with potential lacking lows or highs and you might have to set the volume knob relatively high to achieve a certain volume. The stronger tube you have the more you will push the power amp section. If you play the amp at maximum volume, you’ll achieve more distortion and screaming tones with a strong tube that has a high voltage gain factor. The clean headroom is not affected by the V1 tube. The headroom is determined only by the phase inverter, power tubes and rectifier tube. It is important that the V1 tube is a high quality tube with little noise. If you have hum or microphonic issues, this will be blended into the amp tone and become a problem.

V2 – 12ax7

Preamp tube for vibrato channel. It operates identically to the V1 tube, just for the vibrato channel. If you are like us and use only the vibrato channel, you should pull the V1 tube (see for more info). The stronger tube you have the more you will push the next preamp gain stage of the vibrato channel. Hence, you will achieve preamp distortion earlier.

V3 – 12at7

Reverb driver tube. This tube is only in use in the vibrato channel, but even so, it means nothing for the dry guitar tone of the vibrato channel or the normal channel. It runs a strong current into the reverb tank. Both channels will work fine without this tube (just without reverb if you’re using the vibrato channel). It is not important to have a high quality, fresh and totally noise-free tube in this position. What is reverb after all if not a lot of noise? If you for some peculiar reason own a vintage Fender reverb-amp and play strange music without reverb (!), we recommend you to pull this tube.

V4 – 12ax7

Reverb recovery and 2’nd gain stage for the vibrato channel. This tube is only in use in the vibrato channel and for two different purposes. As the 2’nd gain stage in the preamp section this tube is just as important for the tone as the V2 preamp tube. If you have hum or noise issues, the tone will suffer directly. Old tubes may result in lacking lows or highs. We recommend to use high quality tubes here. If you want more preamp gain out of this tube, you should pull V1 to achieve a hotter signal out of the V2 preamp tube. If you want less preamp gain out of this tube, insert the V1 tube and use 12au7 or 12at7 in the V2 slot in stead of the standard 12ax7. You will then hit the V4 tube much softer and the signal comes cleaner out of the preamp stages. Never using the vibrato channel? Pull this tube.

V5 – 12ax7

Tremolo tube. This tube is only in use in the vibrato channel. It is needed to make the tremolo work and has no effect on the tone of the amp. You may very well use cheap and old tubes in this position. Never using the tremolo or just the normal channel? Pull this tube.

V6 – 12at7

Phase inverter tube. This tube is used by both channels and a part of the power amp section of the amp. It plays an important role for the clean headroom. If you seek maximum clean headroom make sure you have a new and fresh tube here. If you want to lower the clean headroom you may use 12ax7 in this position.

V7 + V8 – 6V6 or 6L6

These tubes mean a lot for tone and headroom. You need to adjust bias of these tubes when changing them or the rectifier tube. Some people change power tubes every year, others keep them for 40 years. There is no single, correct answer how often you should replace them. The more clean headroom you need, and the more often you play, the more often you need to replace them.

V9 – GZ34 or 5U4GB

Rectifier tube. These tubes affect the clean headroom and attack of the amp. 5U4GB results in a lower plate voltage in the amp, meaning less clean headroom and more sag. GZ34 offers a higher plate voltage. Hence more power available in all tubes, power and preamp.


Tube wear

Tubes gradually get worn out when an amp is played. The more playing hours the weaker the tubes get. Tubes are also weared when being shakened during transport, especially while they are warm. Higher bias currents also wear out the power tubes faster than with a lower/correct bias current. Since the tubes slowly and gradually get worn out you may not notice before you compare with another well performing amp that is louder, fuller and has more of everything. For some people the tone of old, weak and flat sounding tubes can be just what they want. They might have an amp that is loud enough in the first place. Weak tubes might tame the amp and color the tone the way they like it. Therefore it is not a common rule that you should have new tubes in all types of amps. Read more in the next chapter about tube strategies for small and big amps.

Tube strategies

In the bigger and most powerful tube amps we don’t replace tubes often to get max effect out of the amp. Yes, we might replace tubes to reduce risk before important gigs where we don’t want the amps to fail. But we don’t replace tubes every 6’th months to achieve maximum Power in the big amps. A Super Reverb is more than loud enough and older tubes with a gentle treble and bass response contributes to the whole vintage “thing”. Brand new tubes and speakers can sound too much “HiFi”.

For low wattage amps, like a silverface Princeton Reverb used on practise and gigs, we want maximum clean headroom and volume available. Therefore we make sure that the tubes are fresh and high performing in such amps. We experiement with different old and new tubes and select the tubes that results in the most volume and punch.

Some players replace tube by tube when they fail. Others replace all tubes regularly, i.e. every 12’th month or every third year. It all depends on your need for reducing risk and utilizing 100% of the voltage gain capacity of the tubes. The best way to reduce risk is to have two amps with you on stage, or keep a spare set of tubes in your gig bag with your guitar strings. You never know when a tube will blow or get noisy. In our experience, the risk for failure is the same with old and new tubes. The old RCA tubes get weak after 50 years, but man, do they seem to last forever. Playing our amps amost every day, we try to keep a pair of 6V6 or 6L6 power tubes for at least 4-5 years. They will still sound good and we only replace them in amps where we need maximum power available in the amp and if we can hear the amp is weak by comparing it to another. We never throw away old tubes that are still working, but keep them in a box. You’ll never know when you or your friends are desperately in need for a tube. The phase inverter tube is playing an important for the clean headroom of your amp, and it should be replaced as often as the power tubes and for the exact same reason; clean headroom. The recitifier tube we keep longer than the power tubes as long as it is noise free and can provide a high enough bias current for the power tubes. Some players are very selective in choosing specific brands for the different tube functions. We think it is more important to have fresh tubes that work well (not microphonic or noisy) and pick a good brand in general. TAD and JJ tubes are some of our favorites. Price is good as well as superb tone and robustness. In a bigger perspective we consider the brand of the tube to be less important for your tone than i.e. the guitar strings, pickups and speakers. We recommend to spend your time and money here in stead of on expensive tubes.

Tube failure

Tubes can fail in several ways and with different symptoms. The easiest tube failure to handle is blow-out where there is no signal or noise being produced at all. Tubes can also become microphonic meaning that they whistle and resonate with certain frequencies, sometimes sounding like broken glass. These tubes can be harder to locate since the error is not reproducable. Tubes can also start to create “pop” and “crackle” noise when you are playing or when the amp is just idling. This can also sound like a terrible and scratchy noise when you play certain notes/frequencies. Some tubes are noisy only when they’re cold and within the first ten minutes after the amp has been fired up, while other tubes start to act noisy after some time. All this can happen to preamp, power and rectifier tubes. Corrupt tubes are far easier to notice than a worn and less efficient tube. The erronous tone is usually reproducable making it possible to identify such bad tubes. See below for an elimination technique to locate bad tubes in dual channel AB763 Fender amps.

Locating and replacing bad tubes

Sometimes the tubes get noisy, microphonic or rattling. Some players replace all tubes at once to save time, but to save money and the environment we share with you a simple strategy to find exactly which tube that needs to be replaced. Assume a dual channel AB763-type of amp, Deluxe Reverb, Pro Reverb, Super Reverb, Twin Reverb, Vibrolux Reverb.
  1. Verify the normal channel. You only need V1 and V6 present and of course the power and rectifier tubes. Pull out the rest of the preamp tubes. If the problem dissapeared when you pulled out V2-V5 and the normal channel works fine, you’ve narrowed down candidates for the corrupt tube; V2-V5. You can move on to the next step verifying the vibrato channel. However, if the problem was still present in the normal channel, you need to replace V1, V6, power tubes and rectifier, one by one, until you hear the problem again.
  2. Verify the vibrato channel. You need V2, V4 and V6 present. Pull V1, V3 and V5. If the problem now dissapeared you may assume one of those were corrupt. Insert V1, V3 and V5, one by one, until you discover the bad tube.

We hope you get the technique of elimination.