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Thread: Changing the tubes of a Marshall JCM900 (EL34)

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    Ultimate Tone Member Francois's Avatar
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    Default Changing the tubes of a Marshall JCM900 (EL34)

    Hi ! I have some tube questions :
    1) The tubes of my 1990's Marshall JCM900 need to be changed. I never did it by myself before but I would really like to try. I could be dangerous (I know a soundtech who got once shocked). Is it OK if the amp have not been used for one or two days ?


    2) Currently I have JJ EL34 and the three pre-amp tubes, all made in Slovakia and changed by a shop in 2016.
    The sound is crackling horribly if I boost the clean channel "A" with a some powerfull vintage booster (Sola Sound Colorsound PowerBoost but even with the pedal gain at 9 o'clock if you really hit with the pick). Is that the power tubes of the pre-amp ? I change first the 4 EL34 and see ? Or the other way round ?

    3) I am an dummy for amp circuitry : does both channels uses the 7 tubes ? I suppose so but am I wrong ?

    4) And lastly, the most important question : Any recommendation ? For me the JJ EL34 sound was OK when new. Mullar are expensive and I share the studio with a band that will always crank the master twice a week ... (They emulate Messhuggah). And I'm myself also guilty as I add a MXR 10-band EQ in the loop. So maybe I should avoid deluxe/pricey tubes... But this is also my recording amp.
    So any nice sounding tubes robust enough to hold their course ?
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    Mojo's Minions Gtrjunior's Avatar
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    Default Changing the tubes of a Marshall JCM900 (EL34)

    Tubes are easy to change but you absolutely need to bias them once installed (power tubes).
    That means you’ll need to take measurements of your plate voltage with either a multi-meter or buy a bias probe.
    Then there is an easy math formula to compute which will tell you what your bias setting should be.
    I’d you have never done this before I highly recommend that you take it to a qualified tech.... there are very dangerous voltages present in an amplifier even if it is not turned on....you absolutely can get hurt or killed by these voltages. A Marshall will carry about 500V coming off the plates....get bit by that and you’re gonna have a bad day.

    Preamp tubes can simply be plugged in and that’s all you need to do.



    Preamp tubes usually last a very long time....decades sometimes.
    Power tubes last much shorter. With heavy use maybe as little as a year or 2....or they may last longer. There is no set rule.

    The amplifier uses some tubes for the clean channel stages and some for gain stages.

    If you were happy with the JJ tubes, no reason to switch. They are well made, sound good and are reasonably priced.
    Last edited by Gtrjunior; 05-30-2020 at 05:01 AM.

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    Darkness on the edge of Tone TwilightOdyssey's Avatar
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    Default Re: Changing the tubes of a Marshall JCM900 (EL34)

    Quote Originally Posted by Francois View Post
    Hi ! I have some tube questions :
    1) The tubes of my 1990's Marshall JCM900 need to be changed. I never did it by myself before but I would really like to try. I could be dangerous (I know a soundtech who got once shocked). Is it OK if the amp have not been used for one or two days ?
    Replacing the valves on a JCM900 doesn’t require going inside the amp. It’s just like replacing lightbulbs, just be aware of the gap (preamp valves) and nub (power amp valves) facings. As long as they are cool, it’s fine, you don’t need to wait for days.


    2) Currently I have JJ EL34 and the three pre-amp tubes, all made in Slovakia and changed by a shop in 2016.
    The sound is crackling horribly if I boost the clean channel "A" with a some powerfull vintage booster (Sola Sound Colorsound PowerBoost but even with the pedal gain at 9 o'clock if you really hit with the pick). Is that the power tubes of the pre-amp ? I change first the 4 EL34 and see ? Or the other way round ?
    In all likelihood it’s the preamp valves that are worn, but if it’s been a couple of years, replace them all.

    3) I am an dummy for amp circuitry : does both channels uses the 7 tubes ? I suppose so but am I wrong ?
    That’s correct. Each channel uses one half of the preamp valves. The JCM 900 also has a solid state clipping circuit added to the lead channel.

    4) And lastly, the most important question : Any recommendation ? For me the JJ EL34 sound was OK when new. Mullar are expensive and I share the studio with a band that will always crank the master twice a week ... (They emulate Messhuggah). And I'm myself also guilty as I add a MXR 10-band EQ in the loop. So maybe I should avoid deluxe/pricey tubes... But this is also my recording amp.
    This is a MASSIVE subject in its own right. Tube swapping is great fun, but can get expensive!

    I would say, if you like JJ, stick with them. Comparatively, Mullards will sound much darker/smoother. Alternatively, you can get a single Mullard ECC83 and place it in V1, that will give you the best idea of how they sound. If you don’t like it, place it further down where it will have less impact on the overall tone.

    However — many dealers offer full replacement sets at a discount, and this is the route I would recommend for your first ever swap.

    Good luck, have fun!
    Last edited by TwilightOdyssey; 05-30-2020 at 05:06 AM.
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    Default Re: Changing the tubes of a Marshall JCM900 (EL34)

    Quote Originally Posted by Francois View Post
    Hi ! I have some tube questions :
    1) The tubes of my 1990's Marshall JCM900 need to be changed. I never did it by myself before but I would really like to try. I could be dangerous (I know a soundtech who got once shocked). Is it OK if the amp have not been used for one or two days ?


    2) Currently I have JJ EL34 and the three pre-amp tubes, all made in Slovakia and changed by a shop in 2016.
    The sound is crackling horribly if I boost the clean channel "A" with a some powerfull vintage booster (Sola Sound Colorsound PowerBoost but even with the pedal gain at 9 o'clock if you really hit with the pick). Is that the power tubes of the pre-amp ? I change first the 4 EL34 and see ? Or the other way round ?

    3) I am an dummy for amp circuitry : does both channels uses the 7 tubes ? I suppose so but am I wrong ?

    4) And lastly, the most important question : Any recommendation ? For me the JJ EL34 sound was OK when new. Mullar are expensive and I share the studio with a band that will always crank the master twice a week ... (They emulate Messhuggah). And I'm myself also guilty as I add a MXR 10-band EQ in the loop. So maybe I should avoid deluxe/pricey tubes... But this is also my recording amp.
    So any nice sounding tubes robust enough to hold their course ?
    1) Swapping tubes only is a safe activity -just only touch the tubes and the tubes covers/springs etc. Those 2 blue circular components are the deadly things -but on a JCM900 they are easy to avoid -especially considering the bottom side is where they dissipate energy -just avoid them. Also wear a glove or wipe the new tubes down before turning on -you don't want the oils from you hand on the glass once they warm up. If you expose the electronics under the chassis to bias or change the electronics -then electrocution is a risk if you don't know what you are doing. If you lest an amp sit, yes over time the capacitors will bleed off the energy stored -but nobody is going to make that calculation of when they are no longer deadly for you -the safer approach is to bleed them off with a giant resisitor before going in the chassis (there are plenty of videos online of safe ways to do this)

    However, when you swap tubes -unless the values of the new tubes are very very similar to the old ones -a rebias of the circuit is performed. This requires getting inside the chassis and turning a pot while measuring plate current or voltage and converting on a JCM900 -which is best done by a professional unless you want to learn how and buy a voltmeter and bias probe. But swapping the tubes and listening is fine too, as a hot bias is preferred by some as is a slightly lower bias -so you can just swap tubes and see how your amp performs -if it sounds bad -like it's starving or way to hot -then take it to a professional. This is why companies like Ruby and Groove Tube rate their tubes with a number system-so you can just swap a set in with the same value without a re-bias.

    Here's a video if you are interested in learning how to rebias your amp -it's simple to learn https://www.youtube.com/watch?v=Sw2JQqwYjvE

    2) That cracklin is likely one of the 3 small tubes. V1 is the preamp tube V2 is the Reverb V3 is the Phase Inverter tube. V1 is the furthest away from the EL34s. Swap and experiment with different tubes in those positions. I'd do this first to see if you solve the issue before messing with the EL34s, that way you know your EL34s are good to save as backups. also save the 12Ax7s that aren't causing an issue.

    3) Yeas both channels use all 7 -they share the preamp, reverb and phase tubes and all 4 power EL34 tubes.

    4) Original Mullards don't actually exist anymore (unless you pay a mint for an old set) -if you are thinking you want to play the tubes Hendrix played -Sovtek just bought the name and charges more and build them in the Reflector factory with Svetlana, Tung Sols, and EHX branded tubes

    JJs are great tubes in my experience -they are from the TESLA tube factory in Slovakia -if it rains, they all go pick mushrooms in the hills -so no tubes get made on rain days.

    I traditionally like Svetlana brand in my Marshalls -which Sovtek also bought and still produces but I don't believe they are marketed under the Svetlana name any more -they are the Saratov Russia mades tubes offered under several names from Sovtek -but they are essentially American RCA style tubes -you want the ones with a C logo on them -but S are fine too.

    Outfits like Groove Tubes and Ruby are not tubes manufacturers -they buy Russian, Slovakian and Chinese tubes and stress test and rate them for quality and consistency and to get that number rating I was speaking about. It's a great way to know you're getting quality and to avoid biasing when you swap tubes.

    If you amp is getting hammered by other people in your practice space, I don't know -I wouldn't spend for big money on tubes -I'd probably buy some chinese rated Groove tubes and keep a Russian version on the side with the same number rating to swap in for recording -I don't like the idea of another man touching my woman. But that's extreme for most people.

    I like big Power tube distortion with Speaker distortion -so for me the El34s were the important part, but for many people playing around with different 12Ax7s is where they fool with their tone -12ax7s are inexpensive, I'd buy several different brands and experiment with your tone -you can swap them in seconds -it's a lot of fun.

    Also remember your JCM 900 100 Watt can be changed into a ferocious 50 Watt amp in seconds. just pull out either the outer 2 EL34s or inner 2 -just know that when you do -the EL34s pairs start to get different age on them. You might prefer 50Watt for recording or playing without a band.
    Last edited by NegativeEase; 05-30-2020 at 06:06 AM. Reason: spelling
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    Default Re: Changing the tubes of a Marshall JCM900 (EL34)

    Quote Originally Posted by TwilightOdyssey View Post
    Replacing the valves on a JCM900 doesn’t require going inside the amp. It’s just like replacing lightbulbs, just be aware of the gap (preamp valves) and nub (power amp valves) facings. As long as they are cool, it’s fine, you don’t need to wait for days.



    In all likelihood it’s the preamp valves that are worn, but if it’s been a couple of years, replace them all.


    That’s correct. Each channel uses one half of the preamp valves. The JCM 900 also has a solid state clipping circuit added to the lead channel.


    This is a MASSIVE subject in its own right. Tube swapping is great fun, but can get expensive!

    I would say, if you like JJ, stick with them. Comparatively, Mullards will sound much darker/smoother. Alternatively, you can get a single Mullard ECC83 and place it in V1, that will give you the best idea of how they sound. If you don’t like it, place it further down where it will have less impact on the overall tone.

    However — many dealers offer full replacement sets at a discount, and this is the route I would recommend for your first ever swap.

    Good luck, have fun!
    We should have coordinated lol.

    If I had known you were typing out a well thought response,...... I could have saved some time...
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    Darkness on the edge of Tone TwilightOdyssey's Avatar
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    Default Re: Changing the tubes of a Marshall JCM900 (EL34)

    Quote Originally Posted by NegativeEase View Post
    We should have coordinated lol.

    If I had known you were typing out a well thought response,...... I could have saved some time...
    HA! I thought the same thing! When I read your response, I was like, well, my post is now rendered invalid LOL
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    Default Re: Changing the tubes of a Marshall JCM900 (EL34)

    Quote Originally Posted by TwilightOdyssey View Post
    HA! I thought the same thing! When I read your response, I was like, well, my post is now rendered invalid LOL
    We were remarkably consistent in our posts.

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    Default Re: Changing the tubes of a Marshall JCM900 (EL34)

    Quote Originally Posted by NegativeEase View Post
    We were remarkably consistent in our posts.

    Teamwork!
    That's just because facts are facts.
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    Default Re: Changing the tubes of a Marshall JCM900 (EL34)

    Quote Originally Posted by TwilightOdyssey View Post
    That's just because facts are facts.
    BTW -I got my first IR box recently too.

    I play guitar way more with it hooked up in my control room to my guitar and mixer ready to go
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    Default Re: Changing the tubes of a Marshall JCM900 (EL34)

    Quote Originally Posted by NegativeEase View Post
    BTW -I got my first IR box recently too.

    I play guitar way more with it hooked up in my control room to my guitar and mixer ready to go
    Nice! I have found it works best/most realistic with my amps set to clean or edge of breakup, stack a drive or two in front, watching for noise, CRANKING the master volume, and then padding down the input to my interface. A preamp with a really wide dynamic range, like an OctoPre, works best, imo.
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    Default Re: Changing the tubes of a Marshall JCM900 (EL34)

    Quote Originally Posted by NegativeEase View Post
    Also remember your JCM 900 100 Watt can be changed into a ferocious 50 Watt amp in seconds. just pull out either the outer 2 EL34s or inner 2 -just know that when you do -the EL34s pairs start to get different age on them. You might prefer 50Watt for recording or playing without a band.
    On my JCM900, you don’t have to pull tubes. There’s a switch on the back to make it 50 watt.
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    Ultimate Tone Member Francois's Avatar
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    Default Re: Changing the tubes of a Marshall JCM900 (EL34)

    Wow ! Thanks for all your answers with so many insightful advices. I learnt a lot today thank to you all.
    And special thanks to NegativeEase for the whole article.

    And I was surprise to learn that my amp has a solid state section. But then I understand more why people prefer the JCM800.

    I like the idea of a different sets of tubes for practising and recording.

    I will first change the V1 and so on.

    But If I get two matched pairs they can work OK ?
    I do not want to go into the multi-meter / bias probe business (I am ElectroHyperSentive (originally due to cellphone base stations: And now I can even feel the electromagnetic field of the pickups under my fingers).
    So can I try ready matched pairs of EL34 and then take it to the shop only in case it sounds bad ?
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    Default Re: Changing the tubes of a Marshall JCM900 (EL34)

    Since most amps (not all) only have one bias point it’s always a good idea to get a matched set.

    As far as the bias question, you’ll get different answers from different people but I ALWAYS bias a new set of tubes. It’s just good business.
    ALWAYS...

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    Default Re: Changing the tubes of a Marshall JCM900 (EL34)

    Quote Originally Posted by Gtrjunior View Post
    Since most amps (not all) only have one bias point it’s always a good idea to get a matched set.

    As far as the bias question, you’ll get different answers from different people but I ALWAYS bias a new set of tubes. It’s just good business.
    ALWAYS...
    Yes, with most Marshalls it's always a good idea to re-bias. Every time.

    The 900s don't have a bias measuring point stock. Back in the pre DSL era, you took them to a tech and had it done on a scope. If your multi-meter, which is almost all of them, only measures millivolts and not milliamps you can create bias measuring points by soldering in 1 ohm one or 2 watt precision resistors between pin 8 (pin 1 on EL34 amps) and ground. You can do it for each power tube or just 1 or 2. Once you have 1 ohm resistors installed you measure the voltage across the resistor. The 1 ohm resistors mean that millivolts = milliamps. If you can measure milliamps directly you don't need 1 ohm resistors. You measure between cathode and ground. You then turn the mini pot on the board with a mini screw driver until you get the desired numbers on the multi meter.

    The desired numbers is a range, not one particular ma reading. For example, if the plate voltage (pin 3) reads 460 volts you then divide 25 (the plate dissipation of an EL34) by 460. You get 0.054 in this example, or 54 ma. This is 100% plate dissipation. You want it to read between 60% and 70% of 54. You could just split the difference and set it at 65%. Or could try and see how you like it at 70%, or try it at 60%.

    Then after you decide where to set it, you play the amp and see how it sounds. Try it 2 or 3 ma above and below the ma you set it for. Leave it where you like the sound the best.

    If you are at all uncomfortable or unsure about any of these procedures, by all means take it to a tech.

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    Default Re: Changing the tubes of a Marshall JCM900 (EL34)

    Quote Originally Posted by Lake Placid Blues View Post
    Yes, with most Marshalls it's always a good idea to re-bias. Every time.

    The 900s don't have a bias measuring point stock. Back in the pre DSL era, you took them to a tech and had it done on a scope. If your multi-meter, which is almost all of them, only measures millivolts and not milliamps you can create bias measuring points by soldering in 1 ohm one or 2 watt precision resistors between pin 8 (pin 1 on EL34 amps) and ground. You can do it for each power tube or just 1 or 2. Once you have 1 ohm resistors installed you measure the voltage across the resistor. The 1 ohm resistors mean that millivolts = milliamps. If you can measure milliamps directly you don't need 1 ohm resistors. You measure between cathode and ground. You then turn the mini pot on the board with a mini screw driver until you get the desired numbers on the multi meter.

    The desired numbers is a range, not one particular ma reading. For example, if the plate voltage (pin 3) reads 460 volts you then divide 25 (the plate dissipation of an EL34) by 460. You get 0.054 in this example, or 54 ma. This is 100% plate dissipation. You want it to read between 60% and 70% of 54. You could just split the difference and set it at 65%. Or could try and see how you like it at 70%, or try it at 60%.

    Then after you decide where to set it, you play the amp and see how it sounds. Try it 2 or 3 ma above and below the ma you set it for. Leave it where you like the sound the best.

    If you are at all uncomfortable or unsure about any of these procedures, by all means take it to a tech.
    My JCM 800 has the resistor mod you described. I also have the Eurotubes bias probe which reads plate voltage and mA. But you still need to do the math that you explained.
    I typically bias to 70% which works well in that amp.

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    Default Re: Changing the tubes of a Marshall JCM900 (EL34)

    Quote Originally Posted by Gtrjunior View Post
    As far as the bias question, you’ll get different answers from different people but I ALWAYS bias a new set of tubes. It’s just good business.
    ALWAYS...
    To be clear, I always bias mine with new tubes too. Just letting him know it's not an absolute requirement if you use a rated tube system like Groove Tubes.
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    Default Re: Changing the tubes of a Marshall JCM900 (EL34)

    Quote Originally Posted by NegativeEase View Post
    To be clear, I always bias mine with new tubes too. Just letting him know it's not an absolute requirement if you use a rated tube system like Groove Tubes.
    Understood.
    I just like the peace of mind in knowing I have the correct voltage flowing.
    I buy tubes from tube depot and they use a rating system to match tubes. I always ask for tubes rated the same or close to what I am taking out of the amp. 9/10 a minor adjustment is all that’s needed.

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    Default Re: Changing the tubes of a Marshall JCM900 (EL34)

    Tubes are very easy to change yourself.

    Be sure to get a balanced set of power tubes. I love Mullard EL34 tubes. For the preamp tubes, the V1 tube is the most important. Spend more money on that one tube. If you want most gain go for 12AX7/ECC83. If you want less the 12AT7 is an option. Biasing tubes for your amp should be done by a pro.

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    Ultimate Tone Member Francois's Avatar
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    Default Re: Changing the tubes of a Marshall JCM900 (EL34)

    I ordered one Tung-Sol 12AX7/ECC83 and one TAD JJ ECC83S/12AX7 to experiment V1 swaps. Thanks everyone !

    I have a last dummy question : What is the main risk for not biased new power tubes (two balanced sets of EL34) : It will sound bad ? It will wear faster ? Or both ?

    Someone mentioned a adding clipping on the Lead channel of the JCM900 perform by a solid state circuit. Do get this only if you crank the gain past a certain value (like 12 for example) ? Or always ? (I mean with guitar vol. not rolled down)
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    Default Re: Changing the tubes of a Marshall JCM900 (EL34)

    Quote Originally Posted by Francois View Post
    I have a last dummy question : What is the main risk for not biased new power tubes (two balanced sets of EL34) : It will sound bad ? It will wear faster ? Or both ?
    Both. In the worst case scenario the new tubes could red plate.

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