Preamp tubes for Marshall JCM 600 Head

Preamp tubes for Marshall JCM 600 HeadAbout preamp tubes for a Marshall JCM 600 head

I recently bought a Marshall JCM 600 head and would like to keep spare preamp tubes on hand. The preamp uses ECC83, but I’m told I could also use either 12AX7 or 12AX7A tubes. Will substituting these tubes damage the amp or change its tone? And would it require having the head rebiased?


You’re in luck, Max. First, preamp tubes do not need biased, only output tubes (i.e., EL 34, 6L6, 5881, 6550, etc.) require biasing upon installation. What’s more, there is no difference between 12AX7s, 12AX7As or ECC83s. They are essentially the same tube with different model numbers. In England and around Europe they are commonly referred to as ECC83s, but in the U.S. they are referred to as 12AX7s and 12AX7A. Rumor has it that in the late Fifties and early Sixties, General Electric offered the 12AX7 in its product line and made a change in the cathode material used in the manufacturer of this tube. To differentiate the new tube, GE stuck an “A” on the end of the model number. Whether that’s fact or fiction, the bottom line is either tube can be substituted for the ECC83. While doing so will in no way damage your amp, it will in all likelihood change your amp’s tone. Any musician with good ears will tell you that tubes have their own unique tonal character—even tubes of the same model number and from the same manufacturer. For this reason, many musicians like to buy more tubes than they need, then pick those that sound best to their ears.

Lastly, while we’re on the subject, you might find it interesting that the ECC83 can be replaced not only with the 12AX7 and 12AX7A but also with the 7025, M8137, B339, 12DT7, B759, 6L13, 12AD7, 12DF7, 6057, 6681, 12AT7, 7729 and ECC809S. Whew!

Read more about tubes at “A Field Guide to Tubes”


  1. so i possibly might be trading a lot of things for a jcm 2000 super lead 100 watt with a lead cabinet. and a relatively new gibson explorer. i was wondering about tubes and such. i know nothing about them, and have just spent a good 4 hours trying to learn some basics. i only know what the people trying to sell them have said, and what some forums recommend. so of course my logic will be flawed. nevertheless, i just opened up word pad and blurted out my master plan for tone. ill attach it unadulterated. i guess i’m loooking for any insight or advice. in short, set me straight.
    please respond to
    6550 tungstol in place of 6l6
    el34 as is stock…idea being el34 to give lots of compression and breakup and 6550 for crunch…this is the kind of logic
    that comes from reading, not actually hearing or knowing. i just figure if im lucky enough to get a jcm 2000 dsl,
    ill need new tubes and the amp i’m after is a classic rock throwback…so if i wanna be like everybody else and play heavy music
    then i should try and get more gain out of it. so…with el34 being compressed and breakupy and 6550 being more crunchy that stock 6l6,
    the formula leads me to believe that i have found the recipie for crunchy and thick tone. mix this with a boost to compensate
    for the jcms relatively mediocre gain level, a power attenuator to burn the tubes rightly, and a prospective (if i can
    convince that other dude) gibson explorer with the hot (498 etc.) pickups, and i should be exploring the upper limits of what you
    can do with a jcm 2000…at least as far as what i know, minus modding the wiring and adding things to the amp itself.
    the idea is that im utilizing all of the knowledge i have simultaneously. dunno if itll be what im hoping for tho.

  2. Hi Devin,

    Also bare in mind that some musicians can hear a slight difference even in the same brand tubes.

    If you’re looking for the Marshall sound that performs a good classic rock to a tight hard rock crunch, then I would recommend staying with the EL34’s. Remember, tone is subjective, what one guitarist may like, another may not.

    With that said, powertubes will change the dynamics to some degree. For example KT77’s are known as the “in between” 6L6 and EL34s. And each brands vary as well. through personal observation I find that EH have more breakup than Groove Tubes. Each amp reacts differently with different tubes.

    To get more or less gain, you’ll look at the preamp tubes. For example, 12AX7 are hotter than 12DW7. Through trial and error you’ll find which slot section that you like better for said preamp tube. For example, I like a 12DW7 or 12AT7 in V1 slot for the Peavey XXX.

    I wouldn’t concentrate as much on tubes, instead look into a EQ to help shape your sound. Take a peak:

    By the way, the JCM2000 super lead with the Marshall cab is a great choice. The sound will be thick and full with the Gibson Explorer. Great combination!

    If you would like to email me, my mail address is

    Now go have a look at the EQ article, Devin.


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