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Survival Tips for Tube Amps

by R. Aspen Pittman(Reprinted from The Tube Amp Book)

Tube amps are simple, and so they are easy to keep running smoothly. However, if you neglect to follow a few simple rules, you can buy yourself some expensive trouble. What follows are some suggestions you can try that will put your amp in top condition and keep it there.

TIP #1 Speaker Impedance

The proper matching of the impedance between your tube amp and speaker is extremely important. Improper matching will cause severe tube wear and is a common cause of early tube failure. Some amplifiers are more sensitive to this than others . Among the most sensitive are Marshall amps. Pay attention that the Marshall's impedance selector is on 16 ohms when your running a common 16 ohm Marshall cabinet, and reduce it to 8 ohms when adding a second identical cabinet. Always check your cabinets by measuring with a volt meter on the ohms scale (these meters read low, i.e.: an 8 ohm cabinet might read 6 ohms while a 4 ohm cabinet could read 3 ohms). Another way to determine the impedance of your cabinet is to read the individual speaker impedance and note how they are wired. If there are two 8 ohm speakers wired in parallel (+ to both +'s and - to both -'s) then the cabinet will be a 4 ohm load. If the two speakers are wired in series (+ to spk #1 +, #1 - to #2 +, #2 - to -) then the cabinet will have a 16 ohm load. In other words, parallel wiring halves the impedance of the speakers while series wiring will double it.

Find out the specified output impedance of your amp by asking a service station or perhaps your local dealer. The common amps are: Marshall, variable 4, 8, and 16 ohms; Fender Deluxe and Princeton, 8 ohms; Fender Twins and Dual Showman, 4 ohms; Fender Super/Reverbs and 4-10 amps, 2 ohms.

Beware the dangers of using a power attenuator with your Marshall as most power attenuators do not match impedance closely enough for these amps. Using a power attenuator might let your Marshall distort at lower levels, but at the expense of much more rapid output tube wear -- premature failure of the output tubes is common in Marshalls used with power attenuators. Fender amps are not as sensitive to power attenuators as Marshalls, because of differences in design in the output section. However, since the tubes are putting out full power into the attenuator, they will wear out quicker than if they were just coasting at a moderate output level. If you like the sound you get with the attenuator, be prepared to spend a little more on power tubes.

TIP #2 Power Tube Replacement

The regular replacement of power tubes is normal in amps with regular use. Just when to change them can vary with the type of use the amp gets and how often it's used. Most players should change their tubes once a year if they play moderately loud and fairly often. As the output tubes wear out, both the bass and treble responses of the amp will begin to suffer. This power loss from worn out tubes isn't always noticeable because it occurs gradually over time, and because power level differences aren't easily noticed. It takes twice the power for the ear to hear just 3 dB more, and that's just barely audible! Worn tubes will usually have poor, mushy bass response. Regular power tube replacement will guarantee consistent and reliable performance. It's cheaper in the long run.

TIP #3 Drive Tube Replacement

The driver tube operates in conjunction with the power tubes to form the power-amplifier section of the amp. The best power tubes will sound bad with a weak driver tube, as this is the tube that controls the output tubes -- if it can not control the output tubes, the amp can't sound its best. This will show up particularly at higher power playing, or when playing the amp distorted. REPLACE THE DRIVE TUBE WHENEVER REPLACING THE OUTPUT TUBES! In most amps, the driver is the smaller tube (12AT7, 12AX7, 7025, 12AU7 or similar), which is adjacent to the output tubes.

TIP #4 Re-Tensioning Tube Sockets

TIP #5 Capacitors and Resistors

.The most common problem we see in tube amps (other than tubes) is worn out capacitors and bad resistors. What follows are some common symptoms of bad resistors and capacitors, why they can go bad, and how to locate and fix the problem.

NOTE: Tube amplifiers contain high voltages which may be lethal, even if the amp has been off for some time. We do not recommend that you open your amp, or try to perform any repair operations unless you are properly trained in electronic servicing. Again, there are large voltages present in your amplifier that can kill, even with your amp unplugged from the wall. Having said all that, you may now read on.

A common result of cheap tubes failing is that they will take out a screen grid resistor with them (usually located across the inside of the tube socket, or near by). These take the heat when the tube shorts and can fall out of specification easily. This will cause improper function of any power tube you place in the faulty socket -- if the resistor is open, the tube may as well not be in the socket! In any case, the amp will not be reliable until the screen grid resistor(s) have been replaced. Fender amps usually have a 1 watt 470 ohm screen grid resistor, while Marshalls generally use a 5 watt 1000 ohm resistor for this purpose. The screen grid resistors can be checked using an ohm-meter to measure their resistance. The measurement should be within 10% of it's marked value.

Another common source of poor sound quality would be worn-out filter capacitors in the output or supply stage of the amp. This is especially common in amps over ten years old. These are fairly large components and are often mistaken for "metal tubes" at first glance. The filter caps "filter out" the 60 cycle hum from the power source and through the years they dry out and filter less and less. As the 60 cycle hum is now present in your audio output, it will create an odd harmonic that will seem to follow your notes up and down the scale. It's almost like having somebody singing off-key all the time. In addition, since the amp is now producing sub-harmonic notes, the power is sapped and the overall response of the amp will become weak and sound mushy.

Inspection of filter caps can usually determine if they are bad. These large metal cylinders are easy to spot. Fender amps have them on the under side of the chassis, between the transformers, covered by a 4" X 6" metal pan. It is therefore not usually necessary to remove the amp chassis from the wood cabinet. Remove the pan and "drain" the capacitor by touching a screwdriver from the hot side of the caps to ground. Now inspect the top site (or positive) of the part, looking for a broken or swollen seal. This can look like a little bubble about to pop, or it could have already burst and have powder coming out. Capacitors have this relief seal to expose when they go faulty. Be sure to replace them with the same value (or greater value) and make sure they are placed with the proper polarity.

Marshalls have their filter capacitors placed upright on the chassis held at the base with a clamp. The chassis must be removed from the wood cabinet to inspect the filter caps. Observe the same procedure for inspection of the capacitors. It should be mentioned that if you replace your filter caps, you should connect your amp to a variac and power the amp up very slowly to allow the caps to charge and form properly

NOTE: Because the tube sockets are connected to the very highest voltages in the amplifier, we suggest that the following work be done only be those having the proper knowledge of electrical safety.

When tubes are changed again and again over time, the sockets female parts begin to stretch and not make good tight contact with the tube pins. This can lead to arcing and intermittent connections between the tube and the amp. This condition can be aggravated by the vibration from your speakers and so may occur on certain notes on your guitar or keyboard. You can correct this by replacing the socket (last resort) or by re-tensioning the socket with a large safety pin, jeweler's screwdriver, or small ice pick. Use a tool with an insulated handle if at all possible.

First: disconnect the amp from the AC outlet and allow the amp to drain off any voltage by leaving your speakers hooked up to the amp with the standby "ON". This takes just a few minutes and could save an awful experience later. Now remove the tubes and notice the contacts located inside each pin hole of the socket. These contacts spread the pin hole -- do not push the contacts in so far that the tube will not re-insert. After you've re-tensioned all the contacts, replace the tubes and notice how much tighter the tubes are held.

You may also find corrosion on the contacts. Try spraying a little contact cleaner or WD-40 on a tube and inserting it into the socket a few times. This will improve the connection to the tube and prevent future corrosion.

 
 

Survival Tips for Tube Amps(2)

1. Always make sure the speaker is plugged in properly before turning on the amplifier. Failure to do so may cause expensive catastrophic results.
2. Turn on the main power switch and wait at least 30 seconds before turning on the standby switch to extend your tubes` lives. If your amp uses 6550 type output tubes, wait about 60 seconds before switching on the stand by switch.
3. Use a grounded A.C. outlet and don`t defeat the ground pin on the three pronged A.C. plug on the end of your power cord. Better hardware stores sell a device for testing the integrity of the A.C. receptacles to see if the building`s ground wiring is connected properly. This is a wise investment in your safety and future. If the testing device indicates a faulty or missing ground path, you may be in danger of severe electrical shock.
4. Have your power output tubes changed (6V6, 6L6, 6BQ5, 6550, 6CA7, EL34, EL84 etc.) about once a year, or more often if you play every day for roughly six or eight hours, or if you notice a dullness in your sound. Always purchase matched pairs, or quads, of output tubes; they will sound and work much better than unmatched output tubes. Even if you only need a pair of matched output tubes, it is very wise to buy a matched quadruplet set so you will have a pair that is ready to use without a big change in tone . If you see glowing red plates in your output tubes, STOP! You either have failed tubes or circuitry trouble, and failure to shut the amp off usually results in major blown parts ($$$). The preamplifier tubes, the 12AX7`s and 12AT7 smaller type tubes might last for several changes of output tubes, but this is not a rule. If you hear jingles, rattles, pops, squeals or if the gain or attack decreases, it may be time to have these tubes changed out.
5. Transport your amp on a padded surface. Amps transported on the bare metal floor of a van or unpadded trunk of a car may have the elements in the tubes shaken loose and cause microphonic rattles or worse, short out when next powered up at a gig. Treat the amp gently and it will last longer. Consider the purchase of an professional shipping case if you plan to transport your amp frequently. If you are going to place your amp on an airplane, a professional shipping type case is a must.
6. Carry a spare fuse or two with you and tape them to the inside of the amp cabinet. Always unplug the amplifier from the power source before changing any fuses. Follow the amplifer manufacturers recommendations about fuse changing. Never, ever use a fuse of a higher rating than called for, or you may wind up with a ($$$) blown power or output transformer.
7, Do not plug your amp into an A.C. outlet where heavy appliances or industrial equipment are also plugged in, such as refrigerators, freezers, heaters. Their off and on transients may cause severe voltage spikes on the power line which could take out weak components. Your amp will not like the brownout condition a 15 amp heater will cause if plugged in together.
8. If your amp has an impedance selector, such as Marshall, HiWatt, some Ampegs, etc., place the amp in stand by before changing the impedance. Also, be sure to select the correct impedance for the type and number of speakers being used.
9. To determine the impedance of the speakers used, follow these general guidelines; two 16 ohm speakers in parallel equal 8 ohms; Two 8 ohm speakers in parallel equal 4 ohms and two 4 ohm speakers in parallel equal 2 ohms. And 2 ohms is about as low an impedance as any amp can withstand. Many solid state amps will fry with a 2 ohm load. If there is an impedance stated near the amplifier' speaker output jack, do not use a speaker combination lower than this stated value.
10. Use a thick wired zip cord for speaker hookup. Don`t use thin coaxial guitar cables as speaker wire if possible. This is specially true for bass, where damping factor, tone and watts could be easily lost. However, if your are experiencing radio or T. V. interference, a shielded guitar cable might help out with this problem, as the culprit interfering radio frequency energy could enter your amp through a speaker cable, as well as through the input cables. Additionally, shielded coaxial cables used in the speaker path might cause some amplifiers to break into uncontrollable and dangerous oscillations. Caution is advised here. If the amp doesn't sound right or if it behaves oddly after installation of a shielded coaxial speaker cable, go back to using the zip cord type.
11. If you hear the amp cutting in and out, reduce the amp volume then wiggle the speaker cord. If this influences the cutting in and out, STOP! An intermittently open or shorted speaker connection or cord might blow up your amplifier. This one is very important.
12. Keep all cable ends clean. Dirty input jacks cause intermittent crackles and hums sometimes attributed to more serious problems.
13. Don`t unplug your guitar from the guitar end of the input cable while the amp is powered up. The loud hum you will hear could be the death knell of some component or speaker if the sun is not shining well on you that day. Some otherwise well informed people think this is an old wives tale, but if you have several hundred watts available, the results could be catastrophic. If you believe in trial by fire, then this one`s for you.
14. After powering up your tube amp, look at the output tubes. If the tubes' plates are glowing red hot, STOP! You could be in big trouble if you continue to operate with tubes running away, as red hot plates are called. This symptom takes moments to show up and just a few more moments to destroy the output transformer or other parts. The problem could be as simple as worn out tubes, or you could have other trouble, such as bias supply failure. A new set of output tubes plugged into a seriously malfunctioning amplifier can be ruined in a very, very short time. When in doubt, have your amp tested by a competent technician.
15. NEVER PLAY ON A WET OR DAMP CONCRETE FLOOR OR WET WOODEN STAGE! If a shock potential exists, you will be bitten badly, especially if you wear leather bottomed shoes. Wet or damp leather is a fairly good conductor of electricity, and hand to foot shocks can be quite fatal. Don't play in the rain. Beyond the obvious shock potential, your amp and particularly your speakers may be ruined. If using an extension A.C. cord outdoors, be absolutely sure you are plugged into a grounded outlet and that all the ground pins on A.C. plugs are intact or else.....
16. Never touch the grounds (input jacks) of two amplifiers at the same time. One may be properly grounded and the other amplifier may have any number of dangerous volts present waiting to shock you. Hand to hand shocks are the most dangerous type. Avoid them like the plague.
17. Use moderate sound volume levels. Sustained high levels of amplified sound (above 80 D.B. S.P.L.) can cause permanent and irreversible hearing loss. You may want to purchase a sound pressure level meter (and learn how to use it properly by carefully reading its instruction manual), to protect your very precious hearing capabilities.
18. Give the amp plenty of ventilation. A fan blowing on the output section of the amp will keep things cooler and generally increase the service life of the electronic components in the amp greatly. An easy way of accomplishing the cooling process is to purchase a small table fan at a discount store (around $19) and place the fan behind the amp blowing into it. The cooler your amp runs, the longer it will run. Your capacitors will especially love you if you keep them cool. Never place the amp with its back against a wall. This will severely limit the natural ventilation the manufacturer has hopefully built in.
19. Do not move the amplifier immediately after shutting it off. Let the amp cool down for a few minutes before moving or transporting it. It is also a very good idea to shut off the standby switch first before turning off the mains A.C. power switch. This extends tube life.
20. Do not take your amplifier apart. Do Not remove the chassis (the metal box containing all the electronic components) from the cabinet. There are no user serviceable parts inside your amplifier. There are capacitors in amps which store 500 deadly volts long after you shut it off. Contact with these lethal voltages will lead to only one outcome. You could be playing harp with Jimi and Janis immediately upon contact with many parts in your amp. Some amps have bleeder resistors in them to automatically discharge the caps within a few minutes of shutting off the amp, but many amps do not have this feature (especially older amps like tweeds and most black tolex Deluxes). BEWARE! DON`T EVER GO INSIDE YOUR AMPLIFIER! Leave all amplifier work up to your trusted technician. He knows how not to get killed by the 500 or so volts in your amplifier. (Ampeg SVT's top out above 650 volts at about three amps capability). Your author has also been badly bitten by high voltage several times. I am lucky to still be here.
21. A little knowledge can be a dangerous thing. What may start out as a simple problem can turn into an expensive repair when you have a well meaning friend "who knows something about amplifiers" try to fix your amp for you inexpensively. I can attest to this situation personally, as I have repaired several amps which started out with a simple problem, and which turned into a catastrophe when some well meaning novice let his meter probe slip off the desired test point and caused a smoky short blowing several expensive tubes and other parts. Bring your troubled amp to a qualified technician only. If you are new to an area and do not know who`s good and who`s not, either ask fellow musicians or call a music store and ask for a referral.
22. Use common sense at all times. Follow your first mind. I have helped many musicians who said something like, " I thought that hookup might blow my amp up (speakers, tubes, transformer etc.). I wish I had done what I thought was right instead of blowing it up"
23. Never, ever, plug the speaker output of one amp into the guitar or line level input of another amp. There is a way to do this without harm, but you need a special interface. This can be a very expensive mistake if not avoided. If your amp has a line output jack, this is a safe signal level to plug into another amplifier's line input. The line output of some amps is fairly large (a volt or more) and may distort the guitar level input of your amplifier. Try using the second input jack on your input channel which is typically 6 db less sensitive, and therefore, less likely to distort the preamplifier stages when a large input signal is presented.
24. Never try to parallel the output of one amp with the output of another amp. Never try to connect two amps to the same speaker. (These two "nevers" are exactly the same). This is the best way of generating the most smoke and most expensive repairs I know of.
25. Amplifiers which have been sitting unused for many months may have their power supply electrolytics deformed to a lower voltage than required for proper operation. Bringing the amplifier slowly up to 120 volts with a variac is a good idea. (Your amp tech should have a variac.) Amplifiers which have been unused for years usually have dried out electrolytics. Powering up one of these amps frequently has one outcome: smoke. Electrolytic capacitors have a useful life of about ten years. (Although many last much longer, some have much shorter lives.) Any old tube amp with its original electrolytic capacitors should have those power supply caps changed out by your tech promptly if you intend to plug the amp in and use it. If you are a collector of amplifiers and are never going to plug the amp in and use it, then you may forego this vital maintenance. While collectors want to see all original components in an old amplifier, players should have the proper maintenance performed on an old amp before any appreciable use is given to it. This is because old electrolytics may seem to be performing sufficiently, but may be leaking current and imitating a resistor. The current may not be enough to blow the fuse, but could be enough to overheat and burn up the old, often very hard to find power transformer.

 
 

Replace tube for your amps

You know a tube had gone bad when:
1) Your amp develops and uncontrollable high pitch squealing feedback. (A tube has gone micro phonic. a micro phonic tube can happen any time, but old age, or a sudden jar while still warm will be the most common causes)
2) Output power loss.
3) Loss of gain in a high gain amp. (can also be caused by a bad cable occasionally. check that first)
4) Tube color changes from warm orange to a blue, pr purple cast. Even if it still works fine, it's on it's last legs. Change it.
5) Amp used at the same settings is suddenly noisy with hiss, or hum.
6) Oscillating tonal changes.
7) Amp suddenly won't stay powered up, and you keep blowing the fuse.
8) When turned off, the tube is not clear but has a slight whitish cast.
These are the most common things I've experienced over the years using a tube amp.
Things to do when re-tubing:
1) If possible buy matched sets. Tubedepot.com offers to test the tubes for compatibility when you buy new ones.
2) When changing the power tubes also get the amp re-biased
3) If you have a shuangguan, use shuanguan tubes, and you don't need a re-bias.
4) Never touch the tube you are replacing bare handed. If you do, clean it with alcohol, let dry, and re-install it.
5) Never use anything metal around tube sockets to help pry it out. Not only can you shock the hell out yourself even if it's not plugged in, you can damage the tube socket. The reason you can still get shocked with the amp unplugged is based on the use of a capacitor in most tube amps. It's basically a high voltage battery, and accidentally arcing across the right set of pins will result in a good jolt.
6) Check tube compatibility. There are lots of replacement tubes that can be used like a Marshall uses an ECC83 preamp tube, but you can use a 12AX7 in place.
Tube life depends largely on use. I've seen pre-amp tubes last years, and seen them go in 8 months. High volumes, frequently moving gear, and overly high voltage at the wall are all factors in how long a tube set lasts.
It's never the whole set that goes, usually just one, but that's the indication it's time. With the exception of accidentally breaking a tube while moving the amp.
Tube amp tones are heavily colored by the type of tubes in them. You can find tubes that brighten a dark amp, darken a bright amp, lower or raise gain, change power output, so on and on.
Here's a couple of links that are helpful.

 

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