Q: Do I need to rebias my amp when changing tubes?
A: Sometimes. When changing preamp tubes, biasing is not necessary. Preamp tubes are typically the smaller, 9 pin tubes in your amp. Typical preamp tube designations are 12AX7/A, 12AT7, 7025, 12AY7, 6072, 12AU7, 5751, etc.
When changing power amp tubes, whether to rebias or not depends on the type of amp you have. There are 3 general categories of amplifiers:
1) Self biasing which is also known as cathode biased. These amps do not require rebiasing in most cases. In rare cases, the cathode resistor will need to be changed to accommodate certain tubes.
Examples of self biasing amps include Vox AC15, Vox AC30, Matchless DC-30 and other Matchless EL84 amps, many Fender Tweed amps (from the '50s) such as the Deluxe and Tremolux, and ALL single ended amps (amps with a single power tube) such as the Champ or Vibro Champ.
2) Fixed bias with bias adjustment. These amps always require rebiasing. Examples are most Fender Silver Face, Black face and Reissue amps such as Deluxe Reverb, Pro (Reverb), Super Reverb, Twin Reverb and Vibrolux Reverb. Most Marshall amps also fall into this category.
3) Fixed bias without bias adjustment. These amps always require rebiasing but are more difficult to rebias than amps with built in bias adjustments. This process involves replacing an existing resistor in the amp with a new fixed resistor, or an adjustable resistor known as a potentiometer. In most cases, these amps should be biased by a qualified amp tech. Examples are Fender Princeton/Reverb, and most recent production Fender amps such as the Pro Junior.
Q: Why must I rebias my amp when installing new power tubes?
A: Setting the bias is like adjusting the idle of your car. If not set properly, the performance (tone) of your amp will suffer. If the bias is set too high for your new power tubes, you can destroy them in a matter of minutes. You can also damage your amp. If the bias is set too low, the amp will sound weak, thin and anemic.
Q: Ok Ok! I'm now convinced that I have to bias my amp. So, how do I do this?
Chances are, if you're asking this question, you should have a qualified amp tech do the work for you. Working inside tube amps can be very dangerous. Even after the amp is unplugged from your wall socket, DANGEROUS voltages remain inside your amp for HOURS, and possibly DAYS! These voltages are LETHAL! I'm not kidding.
For the more adventurous, there are devices on the market, known as bias probes, which allow you to bias your amp without opening the amp. This makes it far less dangerous to do the job. Using these devices without opening your amp, however, requires that your amp has a bias adjustment which is accessible from outside your amp. Practically all Fender blackface and silverface amps have such adjustments which are accessible from the underside of the chassis, near the left side of the amp behind the power tubes, while facing the back of the amp. These amps include Deluxe, Deluxe Reverb, Twin Reverb, Pro Reverb, and Vibrolux Reverb.
II highly recommend that you get at least 2 books on amp servicing (I have about 6 in my library) if your interest level is high enough that you're reading this FAQ. There are several available at JK Lutherie http://www.jklutherie.com and other online book stores. None of these books is perfect, but they all have useful information. This is why I suggest buying at least 2. They're well worth the investment.
Q: What function does each tube in my amp perform?
The easiest way to answer this question is to reference some common amplifiers. Since Fender is the most prolific of all amp manufacturers, I'll make reference to their silver/black face amps.
Q: What's that blue glow inside my power tubes?
A: The easy answer is, don't worry about it. To read more, click here BLUE GLOW
Q: My new tubes have a problem.
A: Which one?
It's highly unlikely that every new tube you put in your amp has a problem. To figure out which one is the culprit, replace all the new tubes with the original tubes. Play the amp. Has the "problem" gone away? If not, there's likely a problem inside the amp or with one of your original tubes. If the problem is gone, then read on.
Replace your original tubes one by one (if your amp uses a pair of power tubes, replace both at the same time) and play the amp with each new tube. If the problem surfaces, replace the newly inserted tube with the original. Play the amp. If the problem is now gone, you've located the (or one of the ) problem tube(s). Repeat this procedure for all new tubes.
Q: My tubes are microphonic, what do I do now?
A: In reality, every tube is microphonic to some degree. The question is, is the tube usable in your application?
Tapping on hot tubes while in operation can indicate how microphonic a tube is, but it's not a good idea to do this. You can damage a hot tube by tapping on it, so tap GENTLY if you insist on doing this.
You will likely hear thumping through your speakers when you tap on tubes. Some thumping is definitely normal in average tubes. Ringing can sometimes be heard. This is not necessarily a bad thing either. Note that the first gain stage (or high gain stages) in most amps will be more susceptible to microphonics than later stages. If a tube acts badly in the first or high gain stage, try it in a later stage. Phase inverters, for instance, rarely have problems with tubes that have higher than average microphonics.
VERY microphonic tube can be unusable. They will cause your amp to feedback with no signal input, much like a microphone feeds back when held in front of a speaker. Other unusable tubes will create notes of their own while you play your guitar. These tubes are obviously too microphonic and must be replaced.
Q: So then, can microphonics be a good thing??
A: Yes. Many players and audiophiles feel that microphonics add to the tone of the tube. I agree with this. Comparing totally "dead" tubes, ones that make no discernable sound when tapped, to more "lively" tubes, most people prefer the tone of the more lively tube.
Q: Is there anything I can do about noisy or intermittent tubes?
A: Try cleaning the tube's pins and your tube socket. Get a spray cleaner like De-Oxit or other electronics spray cleaner. If all else fails, the cleaner sold by Radio Shack will work fine. Spray some cleaner on your tube pins, insert the tube into your amp socket, remove, insert, etc....about 6 times. This will clean oxidation on your tube pins and in your socket. This often eliminates tube "noise" and intermittent operation..
Q: I've recently acquired a little Fender Pro Junior (15W 1x10" combo). It's currently running two 12ax7's. I was wondering what lower gain pre-amp tubes I can use to allow me to get the power tubes "cooking" at lower volumes?
A: I don’t know why people continue to perpetuate this fallacy.
Using lower gain preamp tubes makes the power (and preamp) section run cleaner, pulling it even further from distorting. The only way to get the power section cooking is to use the highest gain tubes possible and play LOUD. Alternately, you can use an attenuator. That’s it!