Ampeg SVT-CL 12AU7

Discussion in 'Amps and Cabs [BG]' started by Eolson, Apr 25, 2014.


  1. Eolson

    Eolson

    Apr 4, 2011
    Sparta, WI
    Been having issues with my Ampeg, brought it up on the forums and I started with the tubes as they were about 10 years old.

    Got definite improvement once the new tubes were in (thanks guys) but when I went to set the tube bias I noticed that only Bias 1 is active with lights, Bias 2 doesn't light up, neither the red or green led light up.

    I noticed that there are some other smaller tubes above the Bias section in Tube Complement, a row of small tubes inside the cage, on the board. One of these 12AU7 tubes isn't glowing. I switched the two 12AU7 tubes to see if the same tube was in active and it did not glow in either slot. When I play through the amp, these LED's now light up as the tubes have been switched.

    Manual says if there are no LED lights - Possibly no High Voltage - Bad Bias Control - Bad Tube(s)
    If this 12AU7 tube is replaced, should I replace the other as well? Is there a direct connection between these tubes and the Bias controls?
     
  2. There will be some bona fide techs along soon to help, I'm sure. There are six 220 Ohm resistors on your output PCB. I bet you blew an output tube and one of these resistors. I don't know the history of your problem, but IMO it's often a waste of time and money to just start swapping tubes as a diagnostic tool.
     
  3. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    There are many possibilities and there could be an issue as mentioned above. But trying tubes is the simplest first step and it is something that you can try at home.

    Some tubes do not appear to glow. It has to do with how they are built and what sort of shielding is around the heater, the part that glows. If both 12AU7 tubes are the same make and only one is glowing, then you have an issue with the tube. 12AU7 tubes are relatively inexpensive, I would replace both of them. The 12AU7 tubes act as drivers for the power tubes in your amp and are part of the bias circuit, so they need to be healthy in order for you to set the bias. Change the 12AX7 as well if it is as old as the others. Keep any old good tubes as spares.

    Now if these tubes are functioning properly and you can't set the bias, then your power tubes might have a problem. A sign that you need to change them is when the bias can't be set. I assume that you installed a match set of new power tubes.
     
    Last edited: Apr 25, 2014
  4. Eolson

    Eolson

    Apr 4, 2011
    Sparta, WI
    Beans, I just put in 6 new Electro Harmonix 6550 tubes, also put in new tube retainers as the old ones were dull and grey looking. I'll try the 12AU7's and see what happens, wish I saw this earlier, could have had it all in the same order.
     
  5. B-string

    B-string Supporting Member

    Don't forget the 12AX7 with the new 12AU7 pair. These make up the drivers and phase inverter for those power tubes (as Beans was saying above).
     
  6. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    While you are at it, there are two 12AX7 tubes in the preamp. They might be fine but you are possibly changing everything else. Remove the four screws on the top of the cabinet and gently pull if forward. As you can see, there are wires connected to the preamp so you shouldn't pull it out further than you need to. Some people find it helpful to remove the screen on the back of the amp so that you can guide it out with your other hand but it isn't necessary.

    [​IMG]
     
  7. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
  8. Eolson

    Eolson

    Apr 4, 2011
    Sparta, WI
    All three 12AX7 tubes have a nice orange glow to them, should I swap them out as well anyway?
     
  9. B-string

    B-string Supporting Member

    That orange glow does not mean very much at all. A dead tube will still have that orange glow.
     
  10. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    The bias to the outputs goes THROUGH the 12AU7 driver tubes. Bad tube = maybe no bias.

    A glowing tube can be bad as anything... the glow is just the heater for the cathode. But a tube that genuinely does NOT glow is either bad, or not getting heater voltage.

    Some tubes just don't make it easy to see that they are in fact glowing, so the real check on a non-glowing tube that has voltage, is probably an ohmmeter check, to see if the heater is open (bad).
     
  11. dune2k

    dune2k

    Sep 26, 2009
    Germany
    If you order anyways just get some 12AX7 tubes to replace all the tubes. Afterwards you could always try out the old ones to see whether one is defective or sounds better. If all are fine you'd still have replacement tubes. Always good to have anyways.
     
  12. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    Just to summarize, change both 12AU7 tubes. If one isn't working, the bias can not be set properly. There are three 12AX7 tubes. It wouldn't hurt to try swapping them just because they are 10 years old. If cost is an issue, just buy one and swap out one at a time and check if there is an improvement in the sound of the amp. The 12AX7 tubes in the preamp as shown in the pic above, will have nothing to do with the bias issue. It is more a preamp/tone issue.
     
  13. Eolson

    Eolson

    Apr 4, 2011
    Sparta, WI
    Thanks for all the info guys, I probably will swap them all out as I think the amp has kind of a dead sound. While it improved slightly with the tube replacement it still has a way to go. I've seen some online videos of that amp and there are some nice sounds coming out of it. I play a Hohner and a Rickenbacker and I'm pretty disappointed in the overall sound.

    Unless people are adding on other pieces of gear to get a great bass guitar sound. I know little to nothing about gear and what makes great sound. Should I be able to have just an SVT and great sound?
     
  14. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    That dead sound, lack of responsiveness, could very well be due to the 12AX7 pre-amp tubes being worn out.
     
  15. B-string

    B-string Supporting Member

    Are you sure the all tube Ampeg sound is your "thing". Are you new to all tube SVT's?
     
  16. oldfclefer

    oldfclefer low ended

    May 5, 2005
    Southern Ohio
    what cabinet are you using? an ohms mismatch can really jinx the sound of a tube amp.
     
  17. Eolson

    Eolson

    Apr 4, 2011
    Sparta, WI
    Guys, thanks for the continued interest in helping me out on this this and all the questions.
    The cab is a SVT-810, I've had it for about 10yrs now. I got it used the amp and cab for $1600 so I didn't pass it up. We play an outdoor gig every year and it's nice to have that kind of power but its overkill for the small gigs. People kind of look at you funny when you roll that thing into a gig.
    I would be interested in knowing more about the ohms.
    As far as am I sue the Ampeg tube amp is the sound I want, I do like that heavy bottom end I get I'm just looking for more character to the sound, more bass guitar sound rather than just booming bottom end.
     
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    Primary TB Assistant

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