Pre-amp valve selection! The valves/tubes you choose to use effects your tone in a big way (almost as much as a new pickup!) luckily it doesn't cost as much (unless you have more than 4 preamp valves). It is also 'lucky' that you HAVE to change them. Otherwise you could end up with a tone you hate for the rest of your life! Most people who aren't up on valves think "an 12AX7 is a 12AX7" This is totally wrong! There are many types of ALL valves (EL34s, 6L6s I mean ALL valves). They are normally have a designator, which follows them (e.g. 12AX7-C) This represents different types of tube, It also Represents their origin.
C- Represents China made Valves
R2-Different Russian factory
S-Formally the Telsa factory now owned by JJ. In the Czech republic
GE-USA only Groove tubes use this key (I think)
These are the stock 'codes' for Groove tubes. Although some Dealers advertise all valves this way. I am not going to go into the argument of makes because I am sure I will offend somebody who swears by a certain company.
Also in my personal opinion we shouldn't get too wrapped up in the NOS game (unless you have a vintage amp and lets face it if you can afford a vintage amp you would be able to afford those ridiculous tube prices!) . I know many say that they love Mullards and I agree but it is really risky to buy them! Firstly they cost a bomb more than a set of new valves! Also as they are 'old' it means they weren't used when they were new, This leads me to believe that they were 'duds' in their prime in some cases. So always ask a supplier to test valves before you part with your cash EVEN NEW VALVES! Also you don't know whether NOS tubes have been used or not!! so you could end up with some really old valves that are dead!! Also the original box is a must (all though I believe lots of people make fakes and charge top $$$).
Also despite common belief there is only a small number of tube manufacturers in the world. So a China valve from one company will sound similar to another (although their will be some differences due to different active components used and glass thickness)
Right onto selecting valves. Like choosing pickups you need to know a few things:
1) What type of amp you are using? This is similar to what pickups go with what body woods. But as with pickups some people like weird combos! A 'good' way to determine good combos is to know what tube the amp was designed around (stock tube) because most companies are looking to save money the use the cheapest valves they can get their hands on. So to make up for it they optimise the amp around them to sound better. So a valve from the same origin is considered to be 'right'. Also vintage amps were designed around NOS tubes so those should sound 'right'.
2) What style of music do you play? If you need a lot of headroom a valve with a lower gain rating would be desired (for output tubes a slower break up)
3) This is most important and is the same as choosing a pickup. What is your sound lacking or have too much of?
4) An obvious one. What type/brand of tubes does your amp have in the first place? If you have a REALLY bright set of tubes in there and think it is too bright go down to a medium brightness unless a huge loss is required. Also if it is sounding weird even after the tube change TAKE IT TO A TECH. It will most probably be drifting values of some kind.
Right now that is said lets move on. So the main type of valve (and the most used audio valve of all time!) is the ECC83/12AX7/7025 despite a minorities belief these are the same. The 7025 never used to be though, it started out as a higher grade ECC83 but now most people regard it as the same valve.
Lets start off with the China made these normally have a fair bit of gain. And have a fat warm tone. There are rumours of low quality valves coming from China but I hear the new ones are quite good (hopefully). It also meets the original ECC83 spec very well (almost perfect) so they work well in vintage amps! Although in modern amps (usually designed around Russian valves) it may have too much gain and hum a bit. Although I know a few people who love these in modern amps. Although there is a bad sign over at www.watfordvalves.com
which policy states:
For the performing Musician and professional audio industry we have found that these items to be lacking in both terms of reliability and performance. Watford Valves therefore cannot recommend fitting such items and as company policy do not stock or offer items of Chinese Manufacture.” They also slate their tone and regards it as ‘wasp in a jam jar’ and fizzy. So they are good for reliability but not for tone!!
Secondly lets look at the good Old Russian valves. (AKA SovtekWA) These are probably the most used in modern amps. Reliability is pretty good with these although not many people like the tone. It is generally lower gain and darker than most other valves on the market. (So maybe good for bright Fenders?) It is also very mid orientated. The balance between the 2 sides of the valves is normally very poor so I wouldn’t use them in a phase reverser or driver stage!
The R2 Russian valves seem to be really bright. But can be prone to microphonics. The quality control is meant to be an issue with the Groove tubes though. It has a higher gain than the other sovteks WB.
Sovtek LPS are pretty good too. They are pretty similar to the EH brand tubes but a few people regard them as better because of a flatter freq response. Although I have never had both side by side to compare.
There are also Electro Harmonix Sovteks these are really great valves. Low microphonics and good gain. Also it has a spiralled filament so AC wiring noise is kept to a minimum. It is definitely based around the higher frequencies! Real trebly!!
Next comes the Telsa factory (Czech Republic) . The newer JJs are regarded as some of the best tubes of today. They are also very similar to the Electro Harmonix tubes in sound. It has a huge output much bigger than any other new valve. They are fairly high treble response and is favoured for hard rock and metal. (Especially in high gain amplifiers) These have a very thick glass (on JJs) Also really low microphonics on the new JJ/Telsa’s. And a long life. It is clear to see why they are so popular!
Next we have the Yugoslavian tubes. They are immensely bright and biting and may become brittle in Marshall’s! But they complement Fenders very well (and early Marshall JTMs which are basically fenders!)
American Brand tubes are always quality! Groove tubes have their GE series, which are great from what I hear but are expensive. Sylvanias are also regarded as great tubes. Very rich and warm sounding. It’s a standard rock valve with average gain but cleans up nicely.
Another brand is Svetlana.They are replicas of the Mullards which is a good sign! But are not regarded as top quality by ‘audiophiles’ I have never had any experience with these so I can’t comment.
I am unsure of their origin but the Harma valves have been a favourite of mine. In the USA I think you call them European STRs? Anyway I love these valves! They have a sound reminiscent of the sort after Mullards but are cheaper and easier to get hold of. They are also Brian May’s favourite of the new valves. He now uses them in all his AC30s! I am probably going to use these in my amp when it is built.
Also Mesa Boogie now have their own brand tubes. They are said to be the only valves that work in their amps. But they aren’t being strictly true
Any tubes can work in Mesas fixed Bias amps. It is not that hard to do with either a slight knowledge for amps or a good tech
Firstly you need to locate the Bias resistor. And take it out. You then need to replace a trimmer in there. Bias like normal. Get the trust VOM/Multi meter and measure the resistance. Replace the trimmer with a resistor of the same value and you are done! This will only work with valves with good quality control though. Other wise it would all go wrong! There are also numerous mods to install a bias trimmer to mesas you can find them on any good schematic site.
Another thing to add to choosing pre-amp tubes is they are pretty much universal! You can (almost) use any 12xx7 in place of a 12AX7! Why would you do this? For a different sound. It doesn’t need any modification to the amp to do this (unlike output tubes.) I am inclined to get a set of 12AY7s for my Marshall DSL. It reduces the preamp gain by over ˝! It also makes modern Marshall’s to (almost) like the old Plexis! I am just looking for a model that will make it a little darker sounding as well. Then adding fast ‘break up’ EL84s will make things really scream
The 12AT7: This is usually used in the driver stage section of modern amps, but in Fenders early days was also used in the preamp as well. It has less gain than an ECC83. Another name for the AT is an ECC81. In modern high gain amps it can be used to reduce the gain (and most probably noise).
The 12AY7: It has a tiny amount of gain, which leaves the big distortion up to the power stages
It is less than ˝ of the gain of the AX. Leading to a lot less noise and a ‘cleaner’ feel. But it has over double the output! This will really pound your output/power tubes to the compressed saturated distortion! So if you think your amp is too high gain try these!
The 12AU7: It has a tiny gain compared to an AX. Only just over a 1/10!! But the output is almost 10 times as much!! It has huge clean headroom though. Favoured by fingerpickers and adds a softer touch to the tone. Another name for this is the ECC82.
I have hunted around the internet for all these ‘reviews’ but mostly it is from Watford valves. Also regarding Some of the output and gain info is from the amazingly good book by: Aspen Pitman and is called “The tube amp book”.