Tube amp question
Scoot82
Posts: 78
I recently got a Fender Hot Rod Deluxe off of ebay and i was playing it today for a while and it was fine for like the first 30min, but then it started getting messed up. It would play at normal level for a while and then all of the sudden it would get really quiet for a few seconds and then kick back in at full volume? ANy help????thanks
Well, it's one louder, isn't it?
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"The Care And Feeding of Tubed and Other Amplifiers and Other Facts"
c1990-c2002 by Sal Trentino
of Sal Trentino Electronics
(415) 453-6217
trentino@earthlink.net You're visitor number
Caution is always advised when dealing with electric or electronic materials or valuable musical instruments. Any and all work should be performed by a qualified tube amplifier technician. This document reflects the opinions of the author. The author assumes no responsibility for the use of any information contained in this document.
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.
tubes wear out. i'd ask the guy if/when it was ever retubed just to know but it's not something to argue the sale over. now, if you get it checked out and there's something heinously wrong with it, then maybe. but tubes are a consumable.
the 6L6 section is where you are needing to go.
I'd either get the SED or the EH matched pair. These are the closest to the old RCA's used in countless vintage fenders.
Do not let your heart be troubled. New tubes will make your amp sound a lot better.
The only two things I ever bought off ebay had problems. One, a G&L guitar was all messed up. I made the guy a deal that he would refund a bunch of money, and since I could do the work myself, I'd fix it. He agreed because he didn't want his good rating to be tarnished on Ebay, and it was blatant covering up of the problems.
I would let the seller know right away and at least he'll know you had a problem. Like exhausted said, tubes are consumables, but if the seller knew, he should have told you. He may not have known, because a tube or something could have been jarred loose in shipment, too. Pretty common.
Don't be mankind. ~Captain Beefheart
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