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Because there is a wide variety of playing styles, sounds, guitar types, and personal preferences, the answer, of course, is that there is no one answer. To say one amp is "better" than another, apart from personal preferences, is meaningless.
Players of more traditional jazz and big band jazz, often go for a very clean sound, especially when playing acoustic archtop guitars. Other players, especially of the more modern styles, often make heavy use of a lot of electronic gadgets and effects. Also, in the last decades, a certain amount of distortion has made its way into the realm of basically traditional jazz sounds (not styles!), though I have a few Django Reinhard recordings from the early 50's, where he cranks his amp up highly, cute!
Players who prefer clean sounds often go for solid state amps, while others, even some who abhor distortion, feel tube amps make their sound more 'alive'. Some players rely on a certain brand and model of amp, while others feel comfortable using what comes along.
Mike Stern, who uses a lot of chorus, is said to use a solid state amp. I have seen John Scofield, who plays through two amps for a stereo sound, use Boogies and Fender Deluxe tube amps on different occasions, but Scofield does not travel with his amp and apparently uses what the tech will put on stage. Scott Henderson used to play through custom made tube gear.
In conclusion, it needs to be pointed out that most of the tone preduced comes out of the fingers of the player. Almost equally important as the type of amplifier are the guitar's pickups, and the amp's speaker - the same amp can sound completely different with various setups of speakers. To find out which amp works best for yourself, you need to know what kind of sound you want to have, and then simply try different amp types and models, consistently (not only half an hour at your dealer's), and then decide which setup works best for you. There is no such thing as a "jazz amp."
While I am pretty sure that Robben Ford or Pat Metheny may just have a great sound playing through a transistor amp, personally I have yet to find a solid state
amp I feel comfortable with. When I play through a transistor amp I don't like my sound, it feels cold or dead, even when it's a well known jazz favorite such as a
Polytone or a JC120. The worst ones, I really dread, are Peaveys.
On the other hand, in nine times out of ten, if there's already an amp on stage I'm allowed to play
through (e.g. at jam sessions or festivals) it comes out if it's a tube amp I just plug in and feel comfortable, my guitar sounds great, alive and warm, and at most
I need only a minor adjustment of the amp's controls.
However, there's no denying that tube amplifiers do have disadvantages, at least in theory, compared to solid state design, and require more careful treatment in several ways. Therefore, I still keep looking.
Several companies offer very highly priced matched output tubes, usually in pairs or quads. While it is technically true that the amp's power stage is designed to operate symmetrically, and therefore in theory the bias (idle) current through each tube should be the same, I never experienced any problems using unmatched tubes, not even loss of sound. The symmetry of operation largely depends on a lot of the amp's circuitry and parts itself, not only the output tubes.
I once wanted to know it, and deliberately put extremely unmatched tubes into my amp (which has neither an adjustment pot nor a bias balance pot such as many silverface Fenders have). I expected bad sound and a strong hum. But the sound was as always, and the hum was barely audible, I was really amazed. Can't say for sure if that will be no problem for any amplifier, however.
Quality tubes.
Alas, there are only about half a dozen manufacturers of tubes world wide still making tubes, namely in Russia and China. There used to be another manufacturer in Yugoslavia, which apparently had taken over the old German Telefunken tube making machines, and they made very good preamp tubes, only second in quality to the original Telefunken tubes, but sadly, since the civil war in Bosnia I haven't seen new tubes from there again (latest rumors have it these tubes are available again).
It is true that most original tubes made in the 60's and 70's in the U.S.A. (GE, RCA, Sylvania) and Europe (Philips, Telefunken, Valvo, Mullard) were superior to the tubes made today. However, I've found that the Chinese preamp tubes - though they do not last as long - are quite good, and have been factory installed by guitar amp makers (eg Fender, Marshall, Boogie). I cannot really say the same about their power tubes, what I've seen a few years ago was not exactly superior, maybe they have improved since. The Russian-made power tubes I've seen (and used) usually were quite good sound wise, though some of them can cause problems I'll discuss in the next paragraph; however this can be an issue with any power tubes, even new old stock tubes (original tubes manufactured in the 'good old days' which were never used).
Preamp tube caveats.
Some people seem to be quite confident to plug in different tube types than the ones the amp was designed for. I strongly disagree! For example, one sometimes hears a recommendation to plug in a 12AT7 type tube instead of a 12AX7 into the preamp for a more clean sound, because the former has less gain.
First of all, such a substitution only works because incidentally these have a similar pinout, and some people seem to be ignorant of the fact this is pure coincidence! There are thousands of different pinouts of tubes. Generally the substitution of different tube types is a definite no-no (especially with power tubes). For example, the 12AX7 tube is designed for a maximum current of about two milliamperes, while the 12AT7 handles over 10 milliamperes. Depending on the specific circuitry, chances are therefore, you'll overload the plate resistors.
In general, you must make sure the replacement tube type is either a valid equivalent (such as a 7025 or ECC83 instead of the 12AX7), or the specific data are similar enough to prevent possible damage. For example, thou couldst replace a 12AX7 with a 5751 and a 12AY7 tube, both of which have slightly less gain but similar bias current. Still, if the circuit was designed for 12AX7 tubes, the question is: what's the point? You may end up with increased hum and noise. If possible, a better idea is to reduce the gain of the preamp stage by setting the preamp volume lower, or use the lo sensitivity input jack.
After saying that, as long as you make absolutely sure no damage can result from such a substitution, if you really prefer the sound of your amp with different tube types, go for it!
Power tube caveats.
Different amps can have different maximum output power, depending on their respective design. For example, some amplifiers with a pair of 6L6 power tubes have a nominal 40 watt max power output, others have 50 or 60 watt. Some of this is due to different design, some of it depends on the exact conditions the nominal power is measured.
The output power is limited however by the type of its power tubes, namely the power tubes' maximum heat dissipation. The industry specification of each tube type designates a maximum plate voltage, plate dissipation and so on. Here we have another problem with various guitar amps and replacement power tubes. I've found this when I tried to install Russian made 6L6 power tubes into several amps.
You see, industry standard specifies the maximum plate voltage to be 450 volts for 6L6 tubes, and their maximum output power 55 watts per pair. But several amps, especially ones made in the 70's, vastly and mercilessly overload their output tubes. I once measured a plate voltage of 560 volts in a silverface Pro Reverb which specified its maximum output to be 75 watts on its back panel. Whoa! Now, when these amps were designed, that was not really a problem, since tubes were not exactly expensive then, and most of these amps used the Sylvania 6L6 tube, easily the best guitar amp power tube, which had been designed on Fender's request for this purpose (rumor has it that a remake of this tube will be available sometime again).
When I installed Russian made new 6L6's into the mentioned amp, I noticed these were glowing red all over, and I had to hastily switch the power off again. These tubes obviously would not accept a more than 20% overload.
Now I am not saying these were bad tubes, on the contrary. Mechanically inside, and sound wise, they were a lot better than some Chinese tubes and even a pair of GE originals I had at the time. I found that they worked just perfect in my '69 Bandmaster head, and the sound was really sweet. When I measured the plate volage inside this amp, I found it to be 455 volts.
Another example of this same problem is my .22 Studio Boogie, one of the first made, which uses a pair of 6BQ5/EL84 type power tubes. In this case, the situation was more or less the other way round. This type of tube is specified for a maximum plate voltage of 300 volts and 17 watts output per pair. When I checked the plate voltage I did not believe it: there were well over 430 volts! This time, however, the Russian EL84 tubes I've always used work quite well, since they actually are remakes of a military grade tube with a heavier plate material than the original EL84 tubes had. Mechanically, however, they're not as stable, and tend to loosen inside when operated under heavy vibration (in the same cabinet as the speaker). That's another issue I'll discuss in the next section.
Summary: have your amp's operating conditions checked before installing new tubes. For amps more than twenty years old this is a must! If the amp is designed to overload its power tubes either make sure the new tubes work well under this condition, or consider installing a replacement power transformer which keeps the plate voltages within the original tubes' specifications. The small loss of power is barely noticeable, in most cases, and you may double your tubes' life. If you can afford to change tubes every now and again, of course, don't bother (but in my opinion it would be a shame to reduce the existing supply of new old stock tubes by torching them off with overload every so often).
A tube's life may end in several ways.
The most obvious is broken glass.
The inevitable is slow burn out. The cathode surface material inside is designed to emit electrons when heated to a dark red glow, and this surface eventually wears out. This results in a reduction of available maximum current for this tube. Nominally, the tube is considered dead when this value has decreased, for example 10% below the specified value for a given tube type. Depending upon the exact operating conditions, however, it may still work well some time after that. In any case the more a tube is pushed to its limits, the sooner it will be dead (therefore, using a power soak is not really such a good idea).
Another way a tube's life can end is mechanical disintegration. This is prone to happen when a tube operates under vibration. Basically, therefore, a tube combo guitar amp is a misconstruction, placing its tube electronics and mechanically delicate tubes in the same housing as a high power, vibrating speaker. This is the most unnecessary way to end the useful life of tubes. Inside the tube, the various parts are held together by mica spacers. Especially in the case of power tubes with their larger plates and grids, sometimes also with preamp tubes, these spacers begin to lose their tight grip and the tube's inner parts get loose. This first makes for a microphonic tube, that is, the tube generates its own electrical signal when vibrated (like a microphone), which results in ugly distorted sounds, sometimes a continual buzzing hum, feedback and other problems (note that sometimes even new tubes are microphonic, either due to poor design, as in the Sylvania 12AX7's, or simply because it just is a bad one). Eventually, the tubes can shorten inside, in which case your amp's intestines may end up in smoke.
Remedies and mods
CAUTION! Before tinkering around with tube electronics make sure you know what you are doing! Tubes operate with potentially lethal voltages, often several hundreds of volts! These voltages may be present even if the amp has been off for some time!
I intentionally do not give specific instructions, if you don't know what I'm talking about, keep off! After this necessary warning, let's continue:
To prevent early burn out, obviously, I recommend using the standby switch most amps have. Always turn on the amp with the standby in the Off position. Let the tubes warm up thouroughly, about a minute at least, before turning it on. Otherwise inside the tubes the cathode material may get bombarded with remaining gas ions which poisons it and causes early burn out. I even recommend adding a standby switch if your amp doesn't have one. The obvious place is between the rectifier and the filter caps. Don't drill a hole into your amp. Try using an existing hole, for example by replacing an unecessary input or output jack at the back. You can also consider to use the ground switch at the back panel which isn't really necessary if you use a properly grounded power supply cord, and for safety reasons that is a must anyway. In the case of vintage gear, don't do anything which irreversibly changes the original condition. For example, if I were to install a standby switch into an old Princeton, I would simply replace the volume pot by another one with an integrated switch, which has the additional advantage that the volume is automatically at zero when the amp is turned on. Another way is to replace the original power switch with a three position switch (as Sundown amps used to have). Be creative!
Don't keep the amp on with standby off longer than about an hour. In this case again, the cathode surface material may deaden itself, it is not designed for operating at no current conditions.
The solution to the vibration problem, obviously, is using an amp with separate electronics and speaker cabinet. But what about existing combo amps? I know of only one maker who thought of that, namely some Boogie amps are equipped with a rubber suspension mount of the electronics, that is, the metal housing of the electronics is not mounted directly to the wooden cabinet but hanging from rubber shock absorbers. I can really recommend to consider adding such a shock mount into combo amps - unless you mess up some antique vintage gear. This is most easily done in the Studio Boogie series, you just replace the original cabinet with a Mark series cabinet of the same dimension. When I called the local Boogie service, the tech was not in favor of this modification, as he said, the specific rubber elements were designed for the much heavier electronics of the higher powered versions. But after I did the conversion nevertheless, I found my power tubes' life easily doubled if not more, and now they tend to burn out rather than shake up.
Another possible way to increase tube life is to add a small fan inside the cabinet or amp head, for example a 12V DC fan operated from a rectifier tapped from the heater supply where it will only get about 8 volts DC so it runs a little slow, making less noise. It is usually enough if it moves just enough air to prevent a heat block. You can also try running a 240 volts AC fan from the 110 volts AC supply, but make sure it really starts to run, sometimes this is not enough.
Another way to prolong tube life is to slightly increase the power tubes' bias voltage over the originally specified value, if possible (here, surprisingly, the Boogie repairman was in favor of this mod). In most cases this will not change the sound noticeably. However, the tubes may run a little cooler. But, on the other hand, since this reduces the bias current and hence the voltage drop in the power supply, this may increase the plate voltage somewhat, especially if there is a rectifier tube or if you have a class 'A' power stage, so this must be checked, too.
(To be continued and edited when I have the time...)
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