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12bh7 for Ampeg SVT 2 pro?

Discussion in 'Amps and Cabs [BG]' started by cellkirk74, Mar 28, 2018.

  1. Did someone try swapping the 12AU7s in the svt2 pro with the
    BTB Online Shop

    Can 12bh7s be used or would that require more bias adjustment than the amp provides with the bias controls on the back? I am looking for a little more input gain since I feeel I have to push the gain a lot (3/4) to get a decent sound from the amp. It is less if I engage the drive.

    I currently run nos jan 6189 in there.
  2. coreyfyfe

    coreyfyfe Supporting Member

    Nov 19, 2007
    boston, ma
    The 12AU7 are your power amp drivers. If you want more input gain you’re looking in the wrong spot. The difference in current capacity doesn’t make much difference in the amp, se comment here from Jerrold Tiers
    Driver Tubes (SVT)

    Edit to add - I had forgotten there’s a 12AU7 in the 2 pro preamp as well but from the schematics it doesn’t appear associated with the “drive” circuit or input gain. The 12AU7 is V4 here:

    http://www.tangible-technology.com/schematics/Ampeg/SVT/2pro/SVT-2PRO 51903H3_Preamp.pdf
    Last edited: Mar 28, 2018
  3. jastacey

    jastacey Supporting Member

    Feb 8, 2009
    Last edited: Mar 28, 2018
  4. Funny that you mention these threads because I came to the idea by the second article. Sad that there was no result and the OP obviosly did not test if he got improved volume with other 12au7 or 12bh7 tubes.

    My main goal is to get more output, but peoples opinions on the question vary a lot. I already exchanges v1 to a jj 83s beeing a "highgain" tube. This made no big difference.

    I am aware that the reason for the rather weak preamp input signal might also be mechanical or a dirty input jack.

    But replacing the 2 jan 6189s with new balanced 12au7s or (if possible, J.Tierrs said no if I am correct) seems a lot cheaper and quicker than a visit with the amp tech.

    I hope to get some quilified opinions by someone who has made personal expieriences with the amp.
    Last edited: Mar 28, 2018
  5. Unless your bass is a very low output (have you tried it with a regular p bass or yours into another amp? ) I'd take it to a tech.
    Into a decent cab, these are loud amps and will overdrive heaps- especially if you crank the Drive and gain. Might be easier and cheaper to take it to a good tech who could identify a problem quickly.
    coreyfyfe likes this.
  6. Wasnex


    Dec 25, 2011
    IHMO, you shouldn't worry about where your controls are. If you can get a good sound is all that matters.

    I subbed a 12AU7 into the reverb driver of a Fender Showman Reverb. The original tube is a 12AT7. This is a common mod intended to reduce gain so the reverb control works in a more effective range. The reverb driver is really a small power amp, with two triodes tide together pushing a transformer. I tried a 12BH7 in the circuit and it sounded great but the extra current drawn by the 12BH7 produced way too much hiss. There really wasn't much difference in gain. IHMO, the circuit would need to be rebiased to work with the 12BH7.

    I would recommend against changing tube types in the driver circuit (V2,3 in the power amp) because this is where bias for the output tubes is set. Using tubes with different operating characteristics will shift the range of the bias.

    If you want more drive I would suggest putting a higher gain tube in V1. This is where most of your gain occurs. Feel free to replace all of the 12AX7s in the front of the amp with high gain tubes (I believe there are 4). A good tube seller can test and select tubes for this purpose. The Chinese 12AX7B is the highest gain tube I have tried. In my experience JJ ECC83S are relatively low gain, but they break up easily in some circuits.

    There is one 12AX7 in the back of the amp. You don't need high gain here, but it might be beneficial to get a tube with balanced sections. If you go to this extreme, you will want strong, matched 12AU7s in V2, 3 (drivers) as well. The idea here is to make sure each side of the power amp gets an equal level of signal. If this can be achieved, it will maximize the amps ability to produce clean power. Vintage SVTs have a balance control for this purpose, but I don't see that feature on the SVT 2 Pro schematic.
    Last edited: Mar 29, 2018
  7. anderbass


    Dec 20, 2005
    Phoenix. Az.
    cellkirk, have you already tried patching the Effects loop? Might be worth a quick try. I'd try patching a guitar cord between the preamp-out & poweramp-in jacks too, at the same time. Poor conections in either of those return jack shunts could result in a weaker signal reaching your poweramp section.
    cellkirk74 likes this.
  8. I will try this.
  9. Update: I got it fixed by swapping some tubes. Now it dielivered plenty of gain and volume.

    Current setup is:
    v1: EHX 12ax7 (replaced JJ 83s)
    v2 +v3: Tung Sol 12ax7
    v4: EHX 12 au7
    v5: Mesa 12ax7 (replaced Sovtek 12ax7WB)

    v1: TAD 12ax7 A-C phase driver selected, balanced (had the Mesa here before)
    v2+v3: EHX 12au7 balanced (replaced Jan 6189s)
    matched JJ 6550 power tubes

    I had a very low gain JJecc83s in v1 at first that made me believe something with the input was faulty since I did not get enough gain, but it turned out to just be either v1 or v5, which was an old Sovtek 12ax7WB. I supect one of those to be responsible for the bad performance. Now I will see if the improvement lasts or if the amp "eats" tubes and the performance drops again.

    I also had a set of NOS JAN 6189 as driver tubes but have the feeling that the new balanced EHXs deliver a better performance.The TAD is advertised a a special selected phase drier. Might be marketing but at least it works for now.
    Wasnex and anderbass like this.
  10. Wasnex


    Dec 25, 2011

    No surprise about the Sovteks WBs :vomit:...especially old ones:poop:. I would expect the amp to be brighter and more articulate with the tubes you have chosen.

    In my experience TAD does a really good job selecting, grading, and matching tubes, so I think this tube is a good choice. If the 6189s aren't trashed, I would keep those.

    Did you rebias your output tubes? This should be done anytime you change a driver tube.

    Sounds like you are ready to rock brother! :bassist:
    cellkirk74 likes this.
  11. BogeyBass


    Sep 14, 2010
    Just new tubes can improve performance.

    A bogey tube is considered a tube that operates and performs as specified by its data sheet.

    After aging sometimes they aint so good no more. Pretty basic

    But a AU7 compared to a AX7 has less gain. But more bandwidth and able to drive lower impedances. Such as driving passive tone controls or in phase converter used to drive multiple tubes. So less gain and the ability to drive lower impedances is optimal.

    Good to hear the amp woke up with new tubes
    cellkirk74 likes this.

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