Changing preamp tubes and the C5


New Member
Feb 26, 2011
Reaction score
Sacramento, CA
In my opinion, upgrading your pre-amp tubes, is one of the easiest, cheapest and funnest ways anyone can experiment with and improve the TONE of their AMP. First off you don't need a tech just a hand and upgrading your tubes will improve the tone of every amp! If you appreciate the TONE of classic tube amplifiers then I recommend you invest some time in learning to “tune” them-up by your ear. My goal is to give you enough info here to set you well on your way. In my ongoing quest to cover the C5 from one end to the other... I couldn't over look it's tubes... It's one of my basic 5 right... Tubes, Transformers, Speakers, Circuitry and components... as always don't do anything you don't understand. I am not responsible if you blow your amp or hurt yourself in anyway...

So what do you need and what do you need to know...

Well you need a bunch of tubes, some patience and a good ear. Now the more tubes the better here. It is true that every tube will sound different; even if the same brand bought on the same day... even if they came direct from the manufacture right off the assembly line, directly after each other, they will still sound different! And, yes NOS is almost always “better” to most peoples ears. The more time you invest in understanding the subtle differences in between tubes the better you will get at this and as a simple matter of fact, the more tubes you have the wider your selection is. In time you will come up with your own descriptions or system of distinguishing between tubes... such as Telefunkens sound thick and clear and a little dark to my ears, you may “hear” them different, it only matters that in the long run that if I am working with an amp I “hear” as to bright that I may want to try a Telefunken and then to my ears this may or may not work... it's all relative. So you got to just roll up your sleeves and get in there. It will do you know good for me to tell you how I hear each tube but trust me there is a method. Over time I have categorized my tubes to my descriptions... Mullards, great definition and mid-range, Mullard square-getters the same with creamier top end... and so on. You must understand the differences that overall GAIN will make (see chart below) as well as the overall tonal characteristics of the Brands you are working with and even the differences between tubes within brand and all the way down to the very Tubes in your box. The better you get at this the better your results will be. You see if I go home and decide, damn I want more gain or maybe more compression, less high end, more low end, more head room etc... I can move some tubes around and get closer if not what I wanted (often bypassing MODS customers may have otherwise “wanted”). What we want is for you to learn to do the same... and lots of tubes and experience are the key here...

The only way to ultimately find that perfect set of tubes is to test them one at a time and mix and match combinations in place and by ear... now this can take some time... but it is well worth the investment. With time and experience you will get better at this. There is, of course, a good deal of thought and theory that will help you along the way... I mean it is better to understand how every tube in your amp is operating, what it's function is and how they individually and in combination, effect tone... that way you know what you are doing and there is a method to the madness... we will cover all this...

First let's talk about the tubes themselves...

Well, as you probably have already noted, most every guitar uses the 12A?7? series of tubes and the most common is the 12AX7 also commonly know the ECC83. This is what's stock in the Class5. All 12A?7- tubes have much in common and in most all guitar amp applications, are interchangeable. The few exceptions to this usually involve reverb drivers and things of this sort but, even in these cases are you rarely able to cause any harm; instead it just won't sound good; Doubly so when talking about 12AX7's, as we are with the C5, so experiment away as long as your in the 12A?7 family.

So how do you get these tubes in and out??? Well gently... Most preamp tubes will not be so hot that you can't handle them with some care but you can get burnt so be careful... it's like handling a light-bulb with some caution you should be fine. Just gently rock the tube back and forth while carefully pulling upwards and it should come out with relative ease. Sometime these things can get stuck and you have to be relatively forceful but with care you should get it. On older amps this can be a place/source of noise and popping if the Pin/tube retainers have become loosened, so on all amps check to make sure that good tension is maintained and make sure the new tubes your putting in have nice straight pins. (You can get a pin straightener for this) Now in the C5 there is no easy way to reach the tubes, so you must take it out of the Cab to get to them. Once out you will have to leave it out to test a whole bunch of tubes side by side. So you will need a longer speaker cable to hook to whatever speaker even if the internal and you need to be careful as you will be working with a live AMP out of CAB and it can be dangerous so make sure you know what your doing... my advice don't hold the guitar when switching tubes... keep in mind that the tubes are on top and on the outside of the chassis and therefor there is no reason to put your hands on the inside of the amp even when it is outside of the cabinet... be careful. Some people will tell you to turn your amp on standby or power off to switch preamp tubes... all I do is turn the master volume all the way down, that way I can rapidly compare many tubes as the power tubes stay on and warm. I've been doing this way for years without any problems, but to each there own... (Never attempt this with power tubes, here you must turn the Amp OFF).

So what do these numbers 12A?? mean anyway... well the 12 means it's a 12 volt tube... the A means it part of the 12-A family all of which are 9 PIN... The 7 at the end means that the Tube has 7 internal elements. The 12AX7 is a dual triode valve which means that this tube is not a single 12V valve but instead two tiny 6v valves each with 3 elements or dual triode; so you have 2 Cathodes, 2 Anodes/Plates, 2 Grids and then the Heater/filament (7)... so what about the ? In 12A?7... in our case this is an X. Many think that the 'second place/numeral' designates “gain” in a sequential manner, as in a 12AX7 has more gain then a 12AT7, but one look at the chart below and you see that simply isn't the case... it's arbitrary...

[FONT=Times New Roman, serif]12AX7 - ECC83, ECC803, E83CC, 6681 = 100 GAIN[/FONT]
[FONT=Times New Roman, serif]7025 = 95 GAIN[/FONT]
[FONT=Times New Roman, serif]5751 = 75 GAIN[/FONT]
[FONT=Times New Roman, serif]12AT7 - ECC81, 6201, 6679 = 60 GAIN[/FONT]
[FONT=Times New Roman, serif]12AZ7 = 60 GAIN[/FONT]
[FONT=Times New Roman, serif]12AY7 - 6072 = 45 GAIN[/FONT]
[FONT=Times New Roman, serif]12AV7 - 5965 = 40 GAIN[/FONT]

Now some people will give slightly different values and the “official” value may even vary from that but for the most part these values are right on the money and the values I work with, but since things vary from tube to tube these are just a ball park figure; however they can be useful in determining which tube is right for your given application. So, in a nut shell, this second letter ? Is vital in identifying the tube but won't help you much more then that; however knowing the gain structure of the tube in question is vital so learn the chart or keep it handy.

Years ago I realized that, how pre-amp tubes sounded in combination with each other, what slot in the AMP they went into, the amp in question and what the customer wants, all play a role into determining what's the "best" tube for the given Application. It's easy enough to just get Matching Mullards across the board and call it a day... as this will sound better then the stock tubes did for certain but it still may not be the “best” your Amp can sound nor the “tone” your after... No to do this it also takes knowledge of the amp your working with and a good understanding of what your trying to achieve tonally and the time to sit back and start testing them one at a time in place, like I do... and you may be surprised what you can achieve tonally especially when you consider that all the 12A?7 tubes I listed above are interchangeable!!! Want more gain??? Less gain??? Better defined lows??? More Mids??? Creamer top end? Softer response? More current draw?? etc... all questions I have answered before I sit down with tubes and start messing around. This to me is a real TUBE tune up and can only been done in shop...

OK lets cover some specifics to help guide you in your ways...
The classic pre-amp tube set up is a three 12AX7's deal where the first tube represents your Gain Stages, the second tube you Tone stack and the third tube your inverter. This is a great place to start in understanding how preamp tubes work in combination. If you have one of these types of amps get in there and start messing around and take good notes as most amps will more or less follow this pattern and those that differ some, like our Class5, still follow many of the same overall principles. You of course must have a schematic of the amp to know exactly what each half of each tube is doing and we will do this with the C5 in a bit, for now just understand that each pre-amp Tube Slot, whether you have 2, 3 or 5, is assigned to a specific application in the amp... simple enough right... that way, you will know if reverb is getting weak, which tube is your reverb driver (usually a 12AT7 in black face Fenders) and which tube to try replacing. This is so with all the AMPS applications. With the schematic and a little knowledge you can identify what each half of every 12A?7 you are burning is doing! Hey you want more gain...less gain well what SLOT is the gain channel... got to many mids well where is your tone SLOT etc... knowledge is powerful.

OK this is when you will need to go get your schematic of the C5 in order to follow along... if you don't have one, then go find it on line and download it... that's what I did. There is a very good and reliable copy readily available... OK let's move on...

Many Amps will follow this classic 3 tube preamp set up and then add a fourth tube as an extra gain stage or a fifth for the reverb driver etc... get the picture? But, in many simple Amps like the Class5 you will find only one or two preamp tubes and this is simply because the amps circuitry is less complicated and therefor demands less tubes. I've even seen amps that don't use both sides of each of it's 12A style tube... it's just a design thing. As we have already discussed, the 12AX7 is really two 6AX7's or twin triode. Each triode is made up of three elements the (K)Cathode, (G) Grid and (A) Anode. Our Class5 has two 12AX7's and therefor our schematic reflects 4 Triodes. Look closely at the Schematic and you will easily identify all four, they even have the G-A-K assigned to make it easy for you... Note that the guitar input is on the far left of our schematic and that our signal then flows from left to right in our schematic, thru the varies gain stages of each preamp tube and all 4 triodes. Note how the guitar input directly hits the Grid of the first half (V1A) of the first preamp tube and that each (G) in session receives the signal from the previous stage all the way down the line. Also note that each (K) runs to ground all the way across the board and all (A) attach to the power supply side. There is a complex relationship at play that can also be manipulated by changing certain values on the board but we are talking about tubes and will get into this down the road when we finally get around to looking at the board but for now I will state out right that I do not recommend changing any of the values of any of these RI... at least not yet as we are nowhere near there yet... first we must address many other areas and right now we are talking tubes. Anyhow on our C5 schematic the triodes of the first tube are identified as V1A and V1B or Valve 1, triode A & B and of course then the second tubes triodes are V2A and V2B. If you look closely you will see that V1 is the first gain stage and that V1B feeds into the Tone Stack and V2A (note that the Bass,Treble, Mid knobs are all here) which then hits the Final stage in V2B. The difficulty in working with an amp of this nature is that the bi-function of individual tubes serve different purposes. In other words it ain't as simple has tube 2 is your tone stack but there are some generalities we can still conclude that will help us with are C5... in a nut shell... Valve 1 is mostly Gain but will effect Tone some and Valve 2 is mostly Tone but will effect Gain somewhat.

So now you should know how every tube will effect tone and how each tube works in your amp, now unless you know someone like me, your going to have to role up your sleeves and get to work. Just experiment with any tube above and let your ears be the judge. Hey you got to much gain in the first stage try a 5751 a-la SRV or maybe a 12AY7 like the old Tweed Bassman or maybe a 7025 is just what your after.. or maybe just like me, you will discover that you prefer two 12AX7's in the C5. No real surprise here as this is almost always the case in Amps that divide their tubes triode applications. Because you turn down the gain and you also turned down the tone maybe not what you wanted but let your ears be your guide. For me it was a matter of what type and brand of 12AX7 and then what specific tubes of that type and brand from my stock...

OK before we close out preamp tubes I want to discuss one more thing... tubes shields and their kin.

Tube shields are often found in tube Amps. The hope is that these devices will reduce microphonics, noise and interference, which they may; However, they like every other component in your amp, affect tone... and usually not in a good way. If your Amp makes a ton of noise without these Shields on, then you may need them on temporarily... but it's been my experience that a well tuned amp don't need these and most amps will instantly sound better the moment you remove these things... there chocking your tubes man! Why??? Well because the shield messes with the flow of electrons between the cathode and the plates... in a nut shell the electrons are being pulled in two directions... just think about it... you take a conductive metal tube Shield which connects to the chassis or ground... which is negative in relationship to the plate... this essentially forms a small value of capacitance... which is not good. Your amp may be quieter but your chocking your amps tone... But don't take my word, go and try your amp with them on and off... maybe some on... some off??? let your ears guide you... I never use these things on any of my amps and most people prefer them off...

Hopefully by now some of you have been running some of these tests and doing these MODS have learned that everything effects tone... but for those who still don't believe or want further proof as to why even adding something like shields can change your tone... I mean what's the science behind this???... Go and try this... get two leads, with alligator clips on each side, and a Capacitor between the values of .02 and .1; attach one clip/lead to each end. Now go clip the other ends anywhere on the metal ground of your chassis. (If your ON and OFF switch is metal, like on the Class5, that will work perfect so just clip them here) Now Play while taking either clip end on and off... notice a change??? It is subtle but try a few times and you will probably pick up on it. The tone probably got a little brighter, tighter and a little thinner??? Now try this, take one alligator clip off entirely... so all you have is the Cap with a single lead connected to ground... now play again taking just this on and off... can you still notice a change.... this usually effects just the “tightness” while tone remains closer to the normal... if you can tell a difference you may be saying WTF??? how does a Cap connected on only one side and to only ground effect tone??? I mean isn't this thing essentially invisible to the circuit??? (scratch your head)... OK one more, now try this, leave just the single alligator clip attached to ground and turn the Capacitor around and back and forth... note that even the direction/orientation of our Cap, which is only attached to ground and has no current going thru it still effects tone... usually it will sound brighter in one direction and creamier the other way. I have been orientating my TONE CAPS like this for years, labeling one side Cream and the other Bright then utilizing this to my advantage when soldering in and selecting my Tone Caps. In essence the same principle we see at play with our WTF? CAP is essentially what's going on with our tube shield and a perfect example of how even the simplest changes effect tone. Some of you may not hear anything but I'm betting many will... I do... but then again some may say I'm mad... to a lesser degree even those rubber rings designed to reduce your tubes internal vibration and noise, also effect your tone... although much less then shields do and so some may prefer this approach. To my ear they sort of “dull” or flatten the tone a bit and can be useful if you find a tube your using to be just a little to bright and in my opinion, a much better way to to deal with noise then Shields... but I prefer my Tubes Naked and will usually try many tubes until I find the one I can run this way.

So in conclusion, what does a Tube Snob/Guru like myself burn in his C5... I settled on a early square-getter Blackburn Mullard EL84 for the power (no surprise here) and a matched pair of USA Sylvania 12ax7a's from the early 60's in the preamp section... these came in green and yellow labels, I settled on this certain yellow pair hit the spot for me...see pic. So why not Mullards here as well??? Simple...the Sylvania's sounded better to me. This came as no surprise to me as I have been using them for years in most my Marshall applications. I only mentioned these two brands because when dealing with Marshall amps and that classic British sound, these two almost always come out on top to my ear (with some exceptions but few)...I lean towards Mullard across the board when dealing with most of the vintage stuff but a mix of tubes works best IMO from most Marshall amps built after 1980 or so...maybe everything from JCM800 and above and Sylvania are GREAT tubes to burn in these amps. To my ear they are creamier and "tighter compressed" then Mullards though not as clean. They were also rumored to be the favorite tubes of EVH back in the day and if true I say for good reason! They were the perfect tube for this amps "Bright"...just the right amount of cream...

For me personally when choosing tubes, above all other variables, I am looking for compression. I want a certain tone and a certain amount of gain but usually several of the tubes in my stock will do this for me... but then there is that one sweet one... with just the amount of kiss/compression... and I go... ahhh that's the one...

Hopefully I given you some food for thought...Enjoy!


Some Pics

Our WTF? Tool

My tubes...


Well-Known Member
Oct 11, 2011
Reaction score
I'm overly verbose and annoying on forums, and that wall of text is even too much for an @$$hole like me.

It may be brilliant, but I have to wonder how many people read the entire post?


Well-Known Yinzer
Gold Supporting Member
Apr 7, 2020
Reaction score
the Burgh
I changed the tubes in my Class5.
I can sum it up in one word: meh.
Different tubes don't change the Class5 much.
Sold it.
I like the Class5, but I can get those tones from other amps I already have.
It was kinda redundant, so I turned it into cash.

Latest posts