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Matched tubes for preamp and driver sections for SVT-CL?

Discussion in 'Amps and Cabs [BG]' started by bthiemann80, Jul 8, 2007.

  1. I know power tubes benefit from being matched, is it the same for the preamp section and driver tube section of the Ampeg SVT Classic? The preamp section calls for 2 12ax7's and 1 12au7 and the driver section calls for 2 12au7's and 1 12ax7, is it good to have the pairs of 12ax7's and 12au7's matched or does it even make any difference at all since they are being used w/ either a single 12ax7 or 12au7?
  2. Would be not to worry about "matching" preamp tubes too much. If anything, your phase inverter or splitter preamp tube - which I would guess would be the 12AU7 - might benefit a bit from being of the matched variety.

    If you have a selection of tubes, just roll them through your amp and decide what sounds better...then you've found your solution. I subject my wife to the torture of listening to my tube-rolling efforts for all my preamps and amps, and her "objectivity" (as she can't see if I actually change the tubes or not - or which tube I'm putting in for any given trial) really help as my own ears get tired after trying a number of tubes.

    This has proven very beneficial, and I'm very happy with my preamp/power tubes at the current time.

  3. Thanks for the advice Jay!
  4. justinc


    Dec 11, 2006
    I've have an Ampeg SVT-CL that I use in a 80s hair rock show. It's a 2004 model USA build (from memory)

    Although I loved her tone, I felt I could squeeze a bit more (pure tone) out of her... and to that end I recently went through a fair bit of tube rolling with the following findings:

    1st I changed out the Preamp tubes (originals were sovteks)... tested Mullards, Valvos and Telefunkens... and decided on a matched pair of 1960s Telefunkens. But all the tubes made a pretty big difference, AND all the NOS tubes had a different voice to each other.

    2nd was the Phase Inverter tube... here I tried various 12AT7s and 12AX7s, and decided on an Amperex Bugle Boy 12AX7. BTW: This was the most balanced 12AX7 tube I had from memory.

    3rd I changed out the 12AU7s (driver tubes), for closely matched pair of Amperex Bugle Boy 12AU7s.

    The results: And here comes the completely subjective comment about tone: It is definitely different, (and very pleasing to my ear), I won;t say it like a vintage or a VR, because I haven't got any real reference... but I'll try and describe the changes I'm hearing/feeling like this:

    The tonal "attitude" that is unmistakeably an Ampeg SVT is much more obvious to me, but it's not so much that it's been enhanced but rather that alot (if not all) of the stuff it was hiding behind has gone.

    The improvements (to my ear) are not so much in what has been added, but what has been taken away.

    So in summary,
    I WOULD DEFINITELY use matched driver tubes (SVT-CL),
    I WOULD use a balanced tube for the phase inverter.

    I hope you find this helpful.
  5. justinc


    Dec 11, 2006
    Oh 1 other thing: I seriously believe that tubes need to be "burned in". Play your amp for a couple of days, before you make your decision.

    The first time I used the Mullard 12AX7s tubes in the preamp positions, the highs sounded harsh to my ear (ie BAD). Out they came... then I put them in a Aguilar DB659, similar harshness in the high... after a week or so of playing, I noticed they had settled and the harshness I noticed before had disappeared and they were more "airy" perhaps more "articulate"... but better I think

    Back in the SVT, and they were much better. (I'm not entirely sure that I would have chosen the Telefunkens) I couldn't detect the harshness anymore.

    I don;t know why this is.

    Some of the other guys on here might be able to explain it. (Or maybe I just got used to the highs, so I couldn't discern the difference anymore??)

  6. JimmyM


    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    Driver tubes being matched aren't mission critical, but you could make a case for having them matched if you're a tech. I can't since I'm not ;) But preamp tubes don't need to be matched at all.
  7. JGR

    JGR The "G" is for Gustav Supporting Member Commercial User

    Jun 29, 2006
    President, CEO, CFO, CIO, Chief Engineer, Technician, Janitor - Reiner Amplification
    Don't buy matched/balanced tubes for the phase inverter and driver stages unless you like to waste money. Your OT isn't bifilar so things aren't balanced to begin with, and it isn't an audiophile application where you want ultra-clean, super accurate reproduction. A bit of mismatch probably sounds better for MI. Keeping the power tubes matched to some extent is certainly worthwhile, but they will drift apart over time and the initial benefit is lost unless you have individual bias for each tube - something manufacturers should really be putting in their designs. And just say no to cryo.
  8. justinc


    Dec 11, 2006
    Interesting, but I suppose you're right, changing all the tubes at once means I can;t absolutely say which change caused the change to my amps tone.

    Tomorrow I'm going to
    1. Replace the driver tubes with UNmatched 12AU7s
    and then Separately:
    2. Replace the phase inverter tube with the original Sovtek

    Maybe the entire difference is the 12AX7s in the preamp stage.

    I guess I'll find out.
  9. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    I'll chime in with the others who say that there is no advantage in using a match pre-amp tube in the SVT-CL. It won't hurt to use a matched small signal tube, you just won't gain anything. Sometimes when you buy a matched tube, it has been hand selected for low noise as well. This can be a plus.

    All tubes are slightly different. One make to the next but even with variations from one tube to the next off the manufacturing line. By trying different tubes you will notice subtle differences in tone and performance so tube rolling is worth trying. What you get out of this also depends on how you use your amp and what levels you play at.
  10. +1 unless you get a really bad valve your not gonna have any issues, the circuit is designed to keep the drive fairly even anyway with unmatched valves, so getting a perfectly matched one will make the drive worse...

    Matched preamp valves is a HiFi thing and even then a well designed circuit will rule out the need for matching!
  11. justinc


    Dec 11, 2006
    Ok so I've only done a couple of tests, and only around the phase inverter, keeping everything else the same.

    There is no doubt in my mind that Changing just this tube does effect the tone of the amp... I would say the level of change is on par with changing with preamp tubes... It doesn't change the character of the amp, but there is a discernible difference.

    I'll do a few more tests with the phase inverter and preamp tubes incase a different tube combination works better for me before i move onto a few tests with the driver section.
  12. JimmyM


    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    I always threaten to do tube swaps in my SVT but I know I'll get aggravated after a half hour. OTOH, it's got good NOS tubes in it already, so what the hey.
  13. justinc


    Dec 11, 2006
    Note to self... preamp tube rolling in an SVT is a bloody PITA! Furthermore it's not easy to be objective, because samples aren't heard close enough to each other... really need to record samples in order to hear them back. THIS IS A MUCH BIGGER PROJECT THAN FIRST THOUGHT. :(
  14. JimmyM


    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    Well you're in too deep now, so keep it going ;)