Same question for me. Just got my first tube amp, a fender hot rod blues junior. Lob the sound so far. Is a tube amp the same as a valve amp? That’s how little I know about amps. Also, I was wondering too, how will I know when my tubes need to be replaced an is that something I can do myself or do I need to take it to someone?

@scotty117 wrote:

Same question for me. Just got my first tube amp, a fender hot rod blues junior. Lob the sound so far. Is a tube amp the same as a valve amp? That’s how little I know about amps. Also, I was wondering too, how will I know when my tubes need to be replaced an is that something I can do myself or do I need to take it to someone?

If you know someone that has a tube tester, tubes can be tested pretty quickly and gives a good indication of their overall health. If you don’t have that luxury, there are a few signs to look for that I’ve pasted below.

I had a Kustom amp that sounded great but on occasions the output would go almost dead. I pulled the output tubes and even with a tester, they looked OK – until I kept the heater engaged longer and then they readily failed.

As far as replacing the tubes yourself – check your owners manual and see if your amp is self biasing. If it is, you can safely do it yourself. If it needs to be biased with the new tubes installed, take it to a shop. You would likely be dealing with voltages upwards of +600 volts DC and that is lethal if you get your hand in the wrong place.

Keep in mind too that your output tubes will go bad much quicker than the preamp tubes. Don’t let someone talk you into changing all of them just b/c of age. Preamp tubes can help shape your sound but just to replace them b/c the output tubes are bad isn’t warranted

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!

http://www.thedailyguitar.com/6-common-signs-that-amp-tubes-valves-need-replacing

Both of the amps in question here (Fender Blues Jr & Marshall JMD1) will need to be biased when changing output tubes. They are both known as a “fixed bias” amps, meaning you have to manually “fix” the bias in place. It’s not that hard to do, but it does require taking measurements inside a live amp. It’s not the voltages you need to worry about, it’s the current….and there is enough current flowing to kill you. If you know what you are doing, it’s perfectly safe….but if you don’t know what you are doing, take it to a qualified service engineer.

Although it doesn’t apply in these cases, the term “self-biasing” is a misnomer. Amps of this type are cathode biased, meaning there is a resistor (or set of resistors) that allow for a certain range of bias current depending on what the tube requires…that doesn’t mean that bias can’t be adjusted. If a tube falls out of the bias range set by the resistors, then you need a soldering iron to adjust it. It’s usually not that important on low wattage output stages (like 1W, 5W etc.), but when you start moving up in power it’s better to stay away from cathode biased amps.

The word tube and valve are interchangeable, although valve more accurately describes it’s function.

As far as whether you need to change the output tubes, the answer is “it depends”. For a used amp, I would highly recommend it. When you buy a used amp, you really don’t know how long the tubes have been in there, how much (or how hard) they have been used or if they were properly biased when changed. Output tubes usually die in one of two ways: a slow degradation or in a blaze of glory. The first four symptoms that Bluezhawk describes are examples of slow degradation. The last two are examples of the latter….boom they just go. The problem with that is many times they take out other components (usually grid resistors) as well, making repair even more expensive. So, in my opinion, after buying a used amp, I would take it in for a “checkup”. Let a tech test the tubes, check the bias and give the amp a “once over” to make sure everything is OK.

For a new amp, you can change some of the tonal characteristics by changing output tubes. One of the things I have found with most new production amps is that they tend to put inexpensive, re-branded tubes in. In other words, you might buy a brand new Marshall amp and the tubes are labeled Marshall, but they are actually just Shuguang tubes (Chinese made). The amp might sound good, but if you put in a pair of say Winged C tubes, it will come to life.

If you are gigging often (2 or 3 times a week) and pushing the amp at high volumes each time, it’s a good idea to have the tubes changed at least once a year. For the rest of us, every two to three years is a good rule of thumb. Preamp tubes should be changed every other output tube change….with one exception: If it’s a push-pull amp (most Marshall’s and Fender’s are) and it has a tube for the phase inverter (PI), it should be changed with the output tubes. It is literally the hardest working tube in a push-pull configuration. It works even harder than the output tubes. 😉

Below is an article from The Tube Store that will provide a little more info.
Article found here: http://www.thetubestore.com/Resources/Matching-and-other-tube-info/Bias-Types

As The Phantum pointed out, output tubes can be chosen as far early or late break up to help shape the sound. Preamp tubes can also be chosen to have less drive so that the power amp will break up and go into OD at a slower rate. Some will replace a 12AX7 tube with a 12AT7 or a 12AU7 tube to accomplish this.

More article found here
http://www.thetubestore.com/Resources

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Different Types of Amp Bias
Just like the idle of your car may need adjusting when a major change is done to the engine, new power tubes need their idle (the “bias”) adjusted to make sure the amp sounds as good as possible. Some amps do this automatically, but many do not – particularly in the world of guitar amps. Many tube amp owners are unaware of this simple regular service for their amp that will keep it sounding its absolute best. The bias methods of amplifiers fall into three basic categories:

Non-Adjustable Fixed Bias amplifiers (no bias adjustment potentiometer)
Adjustable Fixed Bias amplifiers (adjustment potentiometer to set the “fixed” bias point)
Cathode Bias amplifiers (also commonly referrred to as self-biasing)

Each of these types is unique.

Adjustable “Fixed” Bias
For adjustable fixed bias amplifiers, the amp should be biased by a tech with experience with this procedure. It’s not difficult and it doesn’t take long but if you don’t know what you’re doing you may hurt yourself (amps can give lethal electric shocks even when unplugged, and NO, we’re not just saying that to scare you). Also, an unbiased amp may sound horrible or cause undue wear and tear on the tubes or power supply. Until you learn how to do it correctly by yourself, it’s best to pay a technician a few dollars to do it for you.

There can be some exceptions. If you already have tubes with our Perfect Pair rating, or rated tubes from another seller such as Groove Tubes, you can replace them with a new set that has the same rating and safely avoid rebiasing. When ordering you should mention the rating you require in the “Comments” box on the checkout page.

Non-adjustable “Fixed” Bias
Non-adjustable fixed bias amplifiers without an adjustment pot cannot be bias adjusted. These amps (for example, all Mesa Boogie models) use a fixed resistor to bias the tubes, not a bias pot, and are designed to run with tubes that have a specific current draw range and should only use tubes that fall within that certain range. Fortunately, these amps are very forgiving and will work well with a pretty wide range of tubes.

For the best results, when you order be sure to specify the make and model of your amp in the Comments box on the checkout page (or simply order a Shop by Amp tube package) and we will make sure you get the best tubes for your amp.

One more thing: some fixed/non-adjustable bias amps have a “bias switch” to select for a specific tube type. Often it is a switch that can select bias for EL34 tubes, or 6L6 tubes depending which is installed. These are still fixed/non-adjustable bias amps as there is no internal bias trimmer pot to fine tune.

Cathode Bias (Self Adjusting)
Cathode bias amplifiers shouldn’t require any adjustments and will work with a wide range of tube plate currents, as the circuit is “self adjusting”. Generally speaking these amps are considered plug’n play when it comes to replacing the tubes.

Bias exceptions
As with anything, there are always weird exceptions. In the rare case where your amp is an “exception” we suggest you consult your manual or a tech to learn if any special service is required to change the tubes.

These are excerpts from a post I wrote on the another board regarding the differences between Class A and Class AB amplifiers as well as Single Ended vs Push- pull. In that post I was actually comparing two specific amp models, but I pulled out the generic bits to post here:
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[strong]Class A vs Class B[/strong]
The main technical difference between the two is:
* In Class A amps – current is always flowing through power tubes : or they are “always on”
* In Class AB amps – the tubes will “idle” when no audio is being processed : or current flow reduces

Because of that, each type has it’s own sonic qualities:
– A Class A amp tends to have less headroom. They also will go through tubes faster. In addition they will put much more demand on the output transformer, requiring it to be heavier an tougher. Because of those “drawbacks” however, they are much more touch responsive, have better compression and will sing with high end harmonics. They also will sound louder (a 30w Class A will sound louder than a 30W Class AB).

– A Class AB is not as responsive or sonically rich, but they tend to have more headroom, tighter bass response and longer tube life.

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[strong]Single Ended vs Push Pull[/strong]
– Single ended is just that, they take a single input and create a single output. It is the simplest of all the amplifier designs. While only one output tube is needed, multiple tubes can be run in parallel to create more power. A single ended amp will always be a Class A amp.

– Push-pull, on the other hand, requires at least two tubes (or a tube with twin triodes). A single input signal is split into two signals (by a Phase Inverter or PI): one is in phase and the other is out of phase (inverse). Each tube (or triode) amplifies one signal and then the output transformer takes the difference between the two signals and sends it to the speaker.

In guitar amps, generally speaking, Class A’s are typically Single Ended and Class AB’s are typically Push-Pull. As stated earlier, a Single Ended amp is always Class A….but not all Class A amps are single ended. Push-Pull amps can be Class A or AB (there are more amp classes but they do not apply to guitar amps).

This is a post I wrote on another board regarding generic steps for biasing a Class AB, Push-Pull amp. It is by no means definitive (far from it). Rather, it’s just an introduction to the different methodologies as well as some of the tools out there that can be used to take the measurements.

***********************************************
Disclaimer: Setting bias requires working inside a live amp containing high voltages and lethal currents. If do not understand the information stated below and/or you are uncomfortable doing this, you should continue to have the amp serviced by a professional. I will not be held responsible if you kill or maim yourself. I will also not be held responsible of you fry your amp or otherwise turn it into a glorified cinder block.

First of all, a lot of people are going to tell you “Just set bias to ‘xyz'”. These people do not understand what they are doing. Proper bias setting is going to depend on the specific tubes in the specific amp. Every amp is a little different. Every tube is a little bit different. The only proper way is to measure, calculate, adjust and repeat until it is dialed in.

More specifically, proper bias depends on the voltage measured at the plate (pentode) of each tube in a push-pull pair. Bias is the current running through the cathode as the tube is at idle. Since current is flowing from the plate to the cathode, as you adjust bias, the plate voltage will change. This is why it takes time to dial in the proper bias and why there is no “one size fits all”.

It’s not necessary, but if you do not understand how a tube works and/or how it amplifies signal and/or what single ended vs. push-pull means and/or what an amps Class means (A vs. AB, etc), I would suggest you read up on those so that you better understand what you are doing.

[strong]

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MEASURE & ADJUST

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[/strong]
There are two basic methods for reading and setting bias current for a push-pull tube configuration. The first is the cathode resistor method. The second is the OT shunt method. A lot of people will say that the OT shunt method is more accurate….but that’s only because it requires very accurate (and thus expensive) Digital Multi Meters (DMM’s) to do it. The same meters using the cathode resistor method would be just as accurate.

Because not everyone has a pair of DMM’s that cost $200 each and the fact that it’s a little easier, the cathode resistor method is detailed below:

The basic procedure is:
1) Remove the chassis from amp
2) Remove old power tubes
3) Insert new power tubes
4) Connect load (speaker) and power on
5) Measure plate voltage (each power tube)
6) Calculate bias based on desired dissipation rate
7) Measure bias for each tube across a 1 ohm resistor
8) Set bias for each tube using the bias and balance pots
9) Repeat steps 5-8 until dialed in
10) Install chassis back in amp

The calculation itself is fairly straight forward.
(Plate dissipation /Plate vdc) x 0.7 = Bias per valve in amps

– Plate dissipation for an EL34 tube is 25W.
– Plate vdc is measured
– 0.7 is assuming you want to run the tube at 70% of the tubes maximum dissipation rate

So let’s say you measured 496V at the plate. The math would be (25W/496V) x 0.7 = 0.0353A. Take that and multiply it by 1000 to convert it to mA…so 35.3 mA per tube.

The actual measurement is taken by inserting a 1 ohm resistor between the cathode and ground (hence the name cathode resistor method), thus allowing it to be measured in mV (Ohms law). You would then measure bias at the cathode of each tube. The BIAS adjust pot will bring the both values up or down. The BAL (balance) pot will “pan” the value between the tubes.

In the above case, you would fiddle with both pots until you get 35.3mV on each tube. Then you would measure plate voltage again…if you adjusted the pots even a little, it will have changed. So you’ll have to do the math and adjust again. Rinse and repeat until it’s dialed in.

[strong]

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TOOLS

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[/strong]
You could measure the voltages and currents directly at the pins of the tubes. All you need are DMM’s then…but there are much safer and easier options. The easiest way to do all these measurements is with some type of bias probe.

One like this is fairly cheap and used with a DMM. Ideally you would want two so you can measure both sides of a push pull pair simultaneously. They will cost you around $25 each plus the cost of 2 DMM’s. Assuming you pick up a couple of inexpensive ones, you are looking at around $100-$150 total.
[img]https://encrypted-tbn2.gstatic.com/images?q=tbn:ANd9GcTtU1gDpFaBB-VhATw9n6wKD3we4YqmmGdJr9fLssJ4vhbvWALtyA[/img]

Another option is something like the Compu-Bias. It’s purpose built just for measuring tube bias. It will however set you back over $200.
[img]http://www.missionamps.com/images/cb1_2oprobe_rs.JPG[/img]

There are other options out there as well. If you do some Google searches you’ll find them.

Hey phantum

Do you find that Class A amps prior to distorting are regarded as ‘better sounding’ by most? They definitely put off a lot of heat

I had a Jet City 20 watt combo for a short time, took it in on trade, and around 3 on the volume dial, it started breaking up. and by 6, it was brutal.
Compare that to some Fender amps that will stay clean until they are almost wide open…..

Oh yea. If it’s a true SE Class A, there’s tones that can be tough to get anywhere else. I find them to sound more open, dense and twangy sounding….it doesn’t matter if it’s clean or over-driving. The problem is keeping the tone clean. Like you said, the key is getting one on the edge of distortion, so that the tubes are compressing, but not clipping. It’s tricky, but if you get one set right in that clean sweet spot…there’s nothing like the additional harmonics and swirl you’ll get.

That’s not to say you can’t get really close to that with a Class AB/Push-Pull. A lot of people knock some of the newer tube amps, but some of the things they do in the preamps these days really help in replicating a Class A sound. I have a Blackstar HT-20 that people swear is a Single Ended Class A. I had one friend that, after playing on it for a few minutes, made me prove to him that it wasn’t a SE Class A. I had to pull the chassis and show him bias measurements to make him believe it was just a straight up EL34, Class AB, Push Pull amp.

Using Blackstar as an example though…play an HT-20 head in a store and you’ll be impressed. But then go try an Artisan 15H (which is a SE Class A) on the same exact cab: your ears will literally have an orgasm. 🙂

Aside from having almost no headroom, the other downsides to Class A’s is that they are heavy & cumbersome, eat output tubes like potato chips and typically don’t have a lot of power.

assuming you are requiring sound to fill a normal size room, and aren’t trying to compete with a someone bashing a huge drum kit, and a 200w bass amp,

get a class a amp no bigger than 5 watts output.
point to point wiring preferred..
if it’s got a master vol, and the odd gain switch, you be amazed at the range of and warmth of tone you’ll be able to draw.

best thing, is you can pot down the vol to a very neighbourly level and still achieve
a full range of tone…unless you’re looking for skin peeling feedback effects!

it’ll drive a quad box for more, or a single 8″ for least..

something like this one…
in US$ these things are a steal..

From what you have said, I would definitely take the amp in for a checkup. Since the amp works now, my guess is that a new set of tubes and re-bias will make it very happy…and it will reward you by sounding a whole lot better afterwards. 🙂

I think we did get a bit out of hand on giving advice, but the point of all our techno babble is that tube amps require some level of “care and feeding”. Here’s a couple of simple tips about tube amps to keep in mind:

[strong]Turning the amp on:[/strong]
1) Turn standby to on
2) Turn power on
3) Wait 30 seconds or so
4) Turn standby off and play

This procedure let’s the tubes warm up before attempting to process any signal. It will help preserve the life of the output tubes. Technically it prevents what’s called “cathode stripping”.

[strong]Turning the amp off:[/strong]
1) Turn (or leave) standby off (as if you are going to play)
2) Turn power off

This allows any residual electrical charge stored in capacitors to drain into the tubes. It’s debatable if this helps preserve anything, but it does render the amp safe to work on…so it’s a good practice to follow.

[strong]Only operate the amp at room temp.[/strong] In other words, don’t pull the amp out of your car when it’s freezing out, bring it inside an turn it on. Let it come to room temp first.

[strong]Never move a tube amp while it is hot[/strong]. The tubes are in their most delicate state when they are hot. Let them cool down for 15 or 20 minutes before moving the amp.