The Tube Amp Repair Myth

     Seems like suddenly everyone who works in the music industry can repair tube amps. From companies 

that specialize in guitar sales, used gear sales, or luthier work, everyone says that they can service tube 

type guitar and bass amplifiers. It may be true that tube amps are not rocket science. Quite the contrary,  

actually. Most designs in use today are at least 50 years old. The first Marshall amplifiers were nothing 

but reverse engineered Fender Bassman amps made using English tubes. The designs and patents of a 

basic tube amp have long since become public domain. Every “boutique” amp that I have serviced at 

Deltronics follows the same basic design, with a few minor additions or subtractions depending on the 

amp builders target customer. 

This being said, a customer may ask, “Then what difference does it make who services my amp?” A good 

question, and one I intend to answer. While anyone with a brain can change tubes, and anyone with a 

brain who reads a basic “how to” book on amp service can learn to replace capacitors and obviously 

burned resistors, there is a lot more to this. To assure top performance of any piece of electronic 

equipment, transistor or tube, the technician needs to know theory of operation. It is also necessary to 

have top notch test gear in which to verify proper operation. Most “tube amp techs” that I have come in  

contact with do not own an oscilloscope, yet in order to correctly bias any push pull type amp, this is an 

essential tool. The word “bias” has become the new “buzz word” in the tube world, but I would bet a 

dime to a dollar that less than 10 percent of the people who service guitar and bass amps can actually 

explain what it is or why it is necessary. Believe me, I, and the people at Deltronics do know.

Finding problems that are unusual, or not consistent, present a huge obstacle to techs who are not well 

versed in theory. Weird noises, oscillation, crackling, buzzing, and other abnormalities are hard to track 

down. If a tech truly understands what the purpose of each capacitor, resistor, diode, tube, and 

transformer is, the chance of actually finding the problem is greatly simplified. Any tech can start 

changing parts (and keep charging you) until the problem goes away. That is not the correct way to 

approach repair. Believe it or not, I have found problems that turned out not to be a bad part at all. 

There are times that a ground loop or high resistance solder connections can cause a multitude of 

problems. A “parts changer” tech will never find the problem but you will pay anyway.

Your amp is a big part of your sound. It also represents a large financial outlay. Most guitar players, upon 

having a problem with their amp, go out and buy a complete set of tubes and re-tube the amp. Nine 

times out of ten, they have thrown their money away. Unless you know for sure that a tube has failed, I 

would suggest leaving it alone. Bring the amp to a known good service center. An honest tech will tell 

you what is wrong with the amp and what is OK. At Deltronics, we evaluate the entire amp before 

quoting a price of repair. Should the customer want modifications done, we can provide that service 

also, and make sure that the amp meets all manufactures specs. Every amp that we service is checked 

for correct output power, bias, signal to noise ratio, reverb and tremolo operation and a final sound 

check using a guitar and speaker cabinet.  Don’t be fooled. We do not sell product at Deltronics. We do 

not do fret work or luthier services. We service electronics and that is all we do.

Mike DelValle

Posted on February 8, 2016 .