#1
Hi guys,

Was wondering if anyone knows a good place to start to learn the basics of tube amps. I feel like I've been getting ripped off by repair techs who tell me things are broken that aren't, like a vulnerable person getting their car fixed who knows nothing about cars.

I have a book on it, but it's so complex I can't even begin to comprehend it.

So, anyone have some tips on knowing the basics of tube amps? (And not getting ripped off )

Thanks
Last edited by Mlagz at Mar 16, 2013,
#3
The nice thing about tube amps is that you can fix most problems yourself. Unless the amp is 20 or 30 years old, most of the problems you'll encounter will involve replacing a tube. I have 4 tube amps sitting in this room and all of my issues have been resolved by replacing preamp tubes, or output tubes.

Beyond that, you need to know whether your amp has adjustable "fixed" bias or non-adjustable "fixed" bias. This only affects the power tubes. I normally refer to these as adjustable and fixed bias, but the purists often complain.

For the first type, you'll likely need a digital voltmeter and perhaps a special test socket. You use the test socket and meter to measure the bias voltage, which really sets the current draw.

On the second one, you simply replace the power tubes and turn the amp on and play. Mesa Boogie and Peavey are among the manufacturers building amps with non-adjustable "fixed" bias. With these types of amps, you simply buy power tubes having the same specs as those originally supplied and you're all set. Mesa, for example, assigns a color code to their power tubes. With Peavey, you can simply tell the tube vendor which amp you have and they should be able to provide you with the correct match.
#5
Unfortunately my amp is from 1967! What kind of amps do you have?

Hmm, what about the speaker cone, transformer, all the circuitry? How do you test that, and how long did it take you until you could diagnose and fix your amps?

Ok, need to go study this now...
#7
You're not even really talking about the basics of tube amps, really, you're talking more about general electronics.

Tube amps can be dangerous because they are able to retain lethal voltages long after being turned off. If there are issues that are more complicated than replacing a reverb tank or replacing some tubes I would not suggest poking around without some working experience with electronics in general.

Work on some small circuits like pedals first. Mod some cheap pedals, build a pedal from GGG. Its a fun, inexpensive way to cut your chops troubleshooting, soldering, etc, on a circuit that isn't dangerous and that you won't be heartbroken if you destroy. Tube amps aren't much more complicated, but there are much more valuable components inside one and they are much more dangerous.
I don't give a shit if you listen to me or not
#8
My amps are all newer Mesa, Rivera and Peavey models.

Ok... So you have an older amp. One of the concerns regarding an older amp are the electrolytic capacitors. These capacitors, or caps, are used for filtering in the DC power supply and other areas of the amp. Electrolytic caps use an electrolyte material, which gradually breaks down and can dry out. Once this happens, the older caps will become noisy - you will hear a loud hum on your signal, even when nothing is plugged into the amp. Caps in this condition are prone to overheating and can frequently fail, with explosive results. One way to identify a bad cap, is to look for bulges, or leakage.

The general recommendation for replacing caps is about every 20 years - that's about the lifespan of a cap under normal operating conditions. When we replace old caps in an amp, it's often referred to as recapping or a cap job. Unfortunately, this is a job that's best left to a competent and reliable technician. Another component that might be inspected and checked during a recap is the old carbon resistors. Multiple power cycles on an amp will cause these to heat up and cool down. This can cause their resistance value to change out-of-tolerance, which can effect tube circuits, or any circuit for that matter. This is another job best left to your trusted technician.

The speaker cone... Simply inspect it, the dust cap and surround material for any damage. Older speakers are prone to drying out, cracking and insect damage. The good news is, some of these problems can be repaired.

The transformer... They either work, or they don't. When they're bad, you'll know it. There are special testers for measuring the impedance and windings of transformers, but you generally need to know its specified value. Even then, the windings on these older transformers weren't always exact, so you could end up with variations. You also need to know how to use the special test equipment used for measuring transformers. Generally, it's not worth it to purchase a tester that only gets used rarely.

The circuitry... We pretty much already covered that during our discussion of caps and resistors. Another area of concern on old amps would be the potentiometers, or variable resistors. These can become noisy over time. The noise is caused by the resistive substrate breaking loose from its bond. As the pot is rotated, the wiper arm encounters this loose material and it results in a static sound from the speaker. The good news? There are companies that sell specialized cleaning products designed to clean old, noisy pots and restore their lube. The bad news? The cleaner is only a short-term solution. Once a pot starts getting noisy, it's usually on the downhill slide and it's only a matter of time, before the pot needs to be replaced.

Since you have a vintage amp, you need to be careful how repairs are made to it. Once you start modifying and repairing it, the value can be affected.

How long did it take me to learn to diagnose and fix my own amps? Well, that's the beauty of tube amps. For the most part, they're really designed to be fixed by the common Joe. Anyone can order a complete set of tubes and replace them in just a few minutes, restoring an amp back to full life. With a little knowledge though, you can learn to intelligently troubleshoot an amp and replace the minimal amount of tubes, to restore it back to health. A little logic and good thinking can go a long way here. Once you realize that the preamp section of an amp only needs a few tubes to accomplish the job - once you know what each tube's function is, that makes it easier to troubleshoot. Oh, by the way... I'm an electronics technician, but you really don't need to have training to work on tube amps.

One channel dead? Replace the preamp tube for that channel. Probably V1 for channel 1.

Both channels dead? Best to suspect the phase inverter, since it's the last tube before the power tubes. Still dead? Could be power tubes. Can also suspect the rectifier tube, if the amp has one.

Reverb not working? Replace the tube responsible for spring reverb.

Do all the tubes get warm after several minutes? Does one or more tubes have red, glowing plates? On some tubes you won't be able to see the glowing heater. Don't always suspect one of those tubes as being bad.

Most of this stuff is just common sense and stuff that you learn over time. Remember, you can always ask questions in here. There are some smart tube folks around here - Cathbard is one of them. If the book you have is too deep, pick up another one. I've seen some good tube amp books at my local book store. Also, the internet is a good place to learn and gleen knowledge.

Lastly... realize that tube amps are dangerous to work on. Tubes are voltage devices, while semiconductors are current devices. That means that tubes use some high voltages to make things work. 300 and 400 volts is not uncommon in tube circuits. I have one tube amplifier that uses around 2000 volts on the tubes. Get mixed up with that and you're in for a hurting. That's another reason why I recommend using a qualified, reliable tech for certain jobs. It's not worth losing your life to save $100 on a repair.