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TO RE- OR NOT TO RE-?

 

Your amp might need one or more new tubes if its volume, tone, or punch has gotten weaker.

You might also need a new tube if you occasionally hear feedback from your amp for no reason.  In this case, access the tubes and tap on them one at a time.  You might quickly find the problem tube.

If scratchy noises accompany the wiggling of a tube, its pins or the tube socket may need cleaning or tightening.

 

WHICH TUBES SHOULD I REPLACE?

 

Replacing all the tubes can be costly and even invite new problems.  Most often, only a few tubes might need replacing.

POWER & RECTIFIER TUBES are the bigger ones.  They run hot, do all the heavy lifting, and wear out faster.  An overall muddiness might be due to weak power tubes or a weak rectifier tube.

  • If there are two power tubes, you could install a new, matched pair.  If there are four, you could install a matched quad.  However, there is nothing wrong with replacing just two of the four, or even just one.

  • Power tubes include the 6L6, EL34, 6V6, 5881, and 6BQ5 types.

  • A rectifier tube is used in some amps and it can also become weak and degrade the power of your amp over time.

  • Rectifier tubes include the 5AR4/GZ34, the 5U4, and the 5Y3.

PREAMP TUBES are the smaller ones.  They run cooler and last much longer.  The preamp tubes that are most likely to cause problems are:

  • The INPUT or GAIN STAGE tube:

    The input tube is usually the farthest one from the bigger tubes and is the most likely to cause problems.  Tap on the tube and listen for any strange noises.

    The input tube is usually a 12AX7 type (a.k.a. 7025 or ECC83).

  • The PHASE INVERTER (P. I.) or a DRIVER tube:

    The phase-inverter tube is usually the tube closest to the bigger tubes.  An amp with spring reverb usually has a reverb-driver tube near the reverb cables.

    Phase-inverter and driver tubes are usually a 12AT7 type (ECC81) but they could also be a 12AX7 (ECC83).

PREAMP tubes can become microphonic, causing howl or squeal.  A microphonic preamp tube can often be singled out by tapping on each tube, one by one.

You should also wiggle all the preamp tubes.  If there's a crackling noise when you wiggle a tube, the tube pins and socket probably need cleaning.  You can use a spray contact cleaner to clean them.

 

WHAT ABOUT BIASING?

 

Hidden inside some amplifiers is a control for adjusting the power tubes' bias or balance.  Not all amps have this control.

If your amp's bias is adjustable, it's a good idea to have the bias setting checked when replacing power tubes.  But it's not dangerous to swap tubes without rebiasing.

Newly purchased used amps are good candidates for inspection at every level.  And if you install a different, but compatible tube type, the bias will likely need readjustment.

 

PULLING THE TUBES

 

Some power tubes have an over-the-top tube retainer, held on with springs.  Lift the retainer off the top of the tube (right) before pulling the tube out of its socket.

Some power tubes have a leaf-spring clamp with teeth that grab the base of the tube (right).  Push down on the wings of the clamp while removing or inserting a tube.  You can tighten the wings by pulling up on them before inserting a tube.

Always grab large tubes by their plastic base, not by the glass, and wiggle the tube as you pull it out of its socket.

Preamp tubes are often covered by metal shields (right) that have a twist lock.  Push down on the shield and turn it counterclockwise to remove it.  Then rock the tube out of its socket.

Over-The-Top Tube Retainer Tube Clamp Twist-Lock Tube Shield




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