I received an email from one of my readers recently regarding their Vox AC15 Tube Amp. Apparently their amp volume was going up and down inconsistently. I thought I would share what the solution was, and a little info on how to look after a Tube Amp.
The official solution I provided to fix the volume problem in the AC15 is as follows:
My first thought would be to replace the valves. I’ve just taken my amp to the “Amp Doctor” here on the coast, and it only cost me a few hundred. Amp valves need to be replaced regularly (at least once every 2 years).
Also, I would suggest turning your amp off completely between practice and the gig just to extend the life of the tubes.
Why Do You need To Change The Tubes / Valves?
There are 2 main reasons why tubes need to be replaced. The first reason is that tubes will eventually burn out, which will cause all sorts of problems with the amp (see the list below). Tubes have no definate life span and can burn out at any time. However depending on how much you drive the amp will affect how long the tubes last.
The second reason to replace the tubes in your amp is because it improves the ‘tone’ of your amp. It’s the same as changing the strings on your guitar. How much better does it sound when you have fresh strings. This is the same with your tubes. The wear down process is very gradual, so you most likely won’t notice the difference until you put new tubes in. I would suggest getting your tubes replaced by a professional technician, as there is an art to doing it right.
Common Sign That Tubes / Valves Need Replacing
These are the most common signs that tubes need replacement.
1) Excessive noise (hiss, hum) including squealing or microphonic tubes
2) Loss of high end. Little or no treble.
3) A muddy bottom end. Sounds like there is too much bass and note clarity is lost.
4) Erratic changes in the overall volume. Can go up and down but generally it goes much lower.
5) A blown fuse.
6) The actual amp doesn’t work!
Tips to take care of your Tube Amp
1) Take a little extra care in transporting them. Tube amps need to be handled with love (ie. don’t drop it on the floor, or bang it into things).
2) Make sure that there is proper ventilation for the tubes. Tube amps run hot–especially if they are wired in class A and they need proper ventilation. You shouldn’t set the amp right against a wall so that there is no air to get to the vents.
3) Use the standby switch when powering up and powering down. The standby switch allows the amp to sort of “warm up” and allows the tubes to settle into a sort of equilibrium state before it gets slammed with high voltage. If you generally let the amp warm up or down in standby for about 30 to 60 seconds when powering up or down, you’ll find that the tubes last longer.
4) Don’t move a hot tube (ie. don’t move a tube amplifier until it has a chance to cool down). If you do, you shorten the life of the tube.
Your Opinion
What do you guys think?
How often do you replace your tubes, and if so, how do you know it’s the right time to do it?
Related posts:
18 Responses
Cheers for that, think I’m going to play it safe and get it done by a tech, it shouldn’t cost much if I’m supplying the valves right?
I have read several times where people are replacing brand new tubes for other brands. WHY? Didn’t you buy the amp because you liked the sound? Just wondering…
This is good practical advise, too often the signs of tube failure are evident yet they are ignored because it’s deemed too expensive to replace them. The effects can be terminal not just for your amp but also to your playing career.
Good call Tom!
blackOrange, for my AC15 the valves only cost about $100. The technician who replaced and biased them set me back another $100. He also checked my amp over completely for any faults, etc.
“I have read several times where people are replacing brand new tubes for other brands. WHY? Didn’t you buy the amp because you liked the sound? Just wondering…”
Tubes produce clipping and higher-order harmonics, and different tubes do this to different degrees.
I’m no expert but I would think how the amp is used will determine
how often the tubes need changing. Thanks for the tips on amp care.
It is my understanding that preamp tubes can last for years and power tubes wear out much faster.
Also IME, how much tubes are used are how much is asked of them also greatly affect life.
One thing you might note about Vox amps and potentially others. Commercial builders have a bad habit of soldering resistors above the main board and leaving them sticking up in the air. look inside your Vox 50 valvetronic and you will see. These things can vibrate loose and the connections break. Sounds like your tube is going, but it’s really just a joint that has vibrated loose. These things should be glued to the board.
It really depends on the quality of the valve.
My Futurama (by Selmer,) 15w Bass Major is 52 years old with the
original valves and works fine. I used it at a rehearsal last
week and it was more powerful and had better tone than
modern SS amps at twice the rating.
Back in the day Chinese valves were considered a joke and
a standard Mullard or Osram were what we fitted if we didn`t want
a call back. Now, of course, there is not the choice – what we once
took for granted is now a valuable rarity.
Will
It is my understanding that preamp tubes can last for years and power tubes wear out much faster.
Also IME, how much tubes are used are how much is asked of them also greatly affect life.
That is true but last year I bought 4 new EL84s for my Vox AC30 (before I sold
it,) and their emission measured just 50% as against the original Mullards
that were still up at 80%.
If you have older valves it is a good idea to take the amp to a technician who
knows what he is talking about (this excludes most music shops,) and get
him to actually test the valves.
Will
Tubes are like strings? They wear out gradually and sound better if ‘fresh’?
Does this sound right to you guys?
When they go microphonic, or lose power, it’s time for new (NOS) ones. I have had the same RCA 6L6′s in my Fender Twin since 1970, and they still sound super!…I actually think you should replace your caps more often than the tubes..
Hi,
I am a guitarist and longtime tube tech. One thing to keep in mind If you are using a vintage amp, say from the 1960′s, you are working with a device that was designed during a time when the mains voltage in the United States was lower than it is today. Back then, 110-117 volts, today it is more on the order of 120-125 volts in most places. Mine here in Eastern Mass. is usually 123 volts. Running an amp such as this at higher voltages than it was designed for can cause the tubes to age faster. I recommend to users of vintage tube equipment that they invest in a stepdown transformer that will enable the amp to operate at the voltage it was intended for. A good one to look for is called an RCA Isotap. They show up now and then on ebay. You can adjust the input voltage and select an output that matches the AC voltage that your old amp was designed for.
A good tube tester is always helpful if you do a lot of work with tube amps. US Navy TV7′s are still the preferred workhorse. However, these tube testers do not always give accurate readings for common preamp tunes such as 12AX7′s. Make sure yours has been calibrated.
You can buy matched sets of tubes for your amp from reputable vendors and install them yourself, saving yourself a good deal of money. If bias needs to be reset, then you go to a reputable tech.
Best,
HP
“I recommend to users of vintage tube equipment that they invest in a stepdown transformer that will enable the amp to operate at the voltage it was intended for.”
Or a step up transformer, if you happen to be using old AC30s, like I
am. The one I have gives the option of stepping up or down. Works great.
Your Comments
I am of he mindset that almost any new tube produced today is garbage, with the exception of the original Svetlana, now Winged C i think and to a lesser extent JJ EL84s just because they are capable of withstanding the brutality of an 18 watt Marshall.
Than being said. I am a big proponent of old production glass. I have never had a n original, Mullard, Heerlen Holland Delta 12ax7 or vintage tube fail. WIth about 35 years of tube experience I am confident in this statement.
Hi there,
My blackstar series one 100 watt head I think has a faulty valve, and am in the process of getting it sorted. Just wondering would this effect the tone of the amp? I’m working on getting a good real heavy, metal tone with bite to it. Though in the tone there seems to be something missing, I think it may be due to the valve though, any advice?
Hey Simon, this will definitely affect the tone of the amp. It’s sort of like changing your strings.. As you are listening to the strings go ‘dull’ over a period of time, it’s not until you change the strings that you realize how terrible your old strings sounded.
My advice would be to take your amp to an amp ‘doctor’ and get them to replace the tubes. I did it recently to my AC15 and it only cost me $180. But the sound difference is amazing.. seriously, you won’t regret it!
Thanx. GREAT information. My Peavey XXX has ALL of the 6 symptoms fore mentioned. Taking her to the “doctor” this week for a FULL “tube-ectomy” with a brand-new set of TungSols. Can’t wait to get her home & fire her up.
I have a 1980 Marshall JMP 50 watt head. My problem is there isn’t any bottom end and hardly any overdrive when guitar pickups are in the lead position but when my guitars are in the rhythm position it sounds great. I have tried this with both of my les pauls and same result. Are my tubes going out? This is very puzzling to me. Plus it doesn’t look like the preamp tubes or power tubes or getting hot like they should. Haven’t had this amp long. It has a FJA mod. Any suggestions?