Longtime guitarist Chris Muth is best known as a partner (along with Bob Muller) of Dangerous Music, where he designs high-end recording and mastering equipment. But he also rebuilds guitar amps for rock stars and lesser-known guitarists. Here, he answers a few questions on keeping your amps tuned up.
What do you usually replace on an amp when it comes in for basic maintenance?
Mostly with the amps I see, the owner does routine things like change the tubes occasionally. When they come to me, there''s either something wrong with them, or it''s time for work outside the scope of basic maintenance. My specialty is restoring classic amps to better-than-new condition so they can be used both in the studio and on the road.
This involves replacing the electrolytic capacitors and any other parts that are age worn. The speakers are evaluated for tone and punch, and then cleaned or replaced with quality units according to the client''s wishes. Tubes are selected to match the era, and the bias is set so the tubes run strong but don''t get burned out early. Then the box gets repaired and possibly reinforced and installed in a touring case.
How important are the different makes of tubes?
There are some new tubes available today that work very well. The Russian types have been okay for me. Usually, for the work I do, I find new old stock (NOS) American tubes or, even more often, “pulls” from old equipment. I can call Richard Matthews at Leeds Electronics in Brooklyn and ask him to find me a box of pulled 6V6GTs or 6L6s and get them for a good price. Then I''ll put them up on my curve tracer and select them into pairs and quads as needed. My opinion is that used but strong American tubes from the ''50s through the ''70s sound better and last longer than anything made today that I''ve come across.
What''s the most common problem that you have to fix?
Dry electrolytic caps, followed closely by leaking paper or wax capacitors, are the usual suspects in amps that hum, have lost their punch, or eat tubes for lunch. After that, there is the occasional bad resistor and dirty pot that gets in the way of having fun.
What single component makes the biggest difference in sound?
The player''s skill—oh, I said that out loud! No, really, setting the power tube bias makes or breaks an amp. Some amps are easily adjusted, others need resistors changed, and still others need tubes selected for them that draw the proper amount of idling current. I keep track of this on return customers so I can select some tubes that will drop right in. Sometimes a client wants me to select a few sets of tubes so they can go for a few years between visits.
What might someone do at home to keep their amp maintained nicely?
Play it! Amps like to be on. It keeps the electrolytic caps formed so they don''t leak.