Ampeg SVT 3 PRO tube upgrade?

Discussion in 'Amps and Cabs [BG]' started by kgmitchell, Sep 9, 2013.

  1. I'm looking for a new set of tubes for my Ampeg SVT 3 PRO. Right now it has the factory tubes and it's three different brand of tube, groove tube, electro, and sovtek. It's been eight years since I pulled it out of the box new. The heads not as loud as it used to be and it's a bit distorted. I have heard stories about the bias voltage drifting on these things. I just ordered a set of JJ Gold Pins but after looking at a few different sites I see they don't have the highest reviews. So I'm thinking of myabe shelving them for another time and order another set of tubes. Does anyone have anything good to contribute in favor of JJ's or are they really that bad? Who has suggestions on a nice smooth sound out of one of these heads.
  2. rickdog

    rickdog Gold Supporting Member

    Mar 27, 2010
    "Not as loud as it used to be and it's a bit distorted" is very suggestive of MOSFET bias problems. A good place to start reading on this topic is That particular post has valuable, specific information, but the whole thread is useful. There are plenty of other threads on the topic. Note: this adjustment really is as touchy as people say, and it is possible to smoke the entire output stage if you're not very careful. So unless you are confident in your tech skills, it's worth taking it to a professional for this.

    I haven't tried the gold pin JJs myself, but I've read a lot of positive comments about them. Once you get the bias sorted, try swapping them in one by one and listen to what they do. At the very worst, you have a complete set of spares!
  3. Absolutely agree... I had one that showed the same symptoms and it was the bias. I also managed to smoke the output section the first time I did it. As long as you pay close attention, it's not that tough a job. I also retubed it with JJs after the repair and thought that they brought a good bit of dimension back into the sound. They seemed to have a bit more bite at higher settings. This was the JJ ecc83s and not the ecc803s
  4. JimmyM


    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    Hey, it's the dude from the Ampeg FB page! Cool...yeah, the more I think about it, the more it sounds like you need a rebias. And honestly, don't worry about the tubes so much. Use the JJ's and replace them as they go bad.
  5. JBNeedsBeer

    JBNeedsBeer Supporting Member

    May 13, 2011
    New Brunswick, NJ
    Honestly, I had the exact same problem on mine until I replaced the 12AU7 (just used another GT as I was in a hurry). I replaced all of the 12AX7's prior to that with new Tung Sol's and enjoyed the results in the head.
  6. Foz


    Jul 26, 2008
    Jax FL USA
    +1 - if you have one of these amps making sure the bias is right is essential - this is an easy amp to despise if it aint. Rolling tubes without having the bias correct is a waste of time and money IMO.

    Then just make sure the clip light is blinking and Bob's your ankle.
  7. Thats another thing I forgot to mention. Even at full volume the clip light won't blink at all, nothing, pushed it pretty hard the other day and couldn't get it to light up at all...
  8. rickdog

    rickdog Gold Supporting Member

    Mar 27, 2010
    You mean the light labeled "Peak" next to the Gain knob? That's not a power amp clip indicator; it comes on when the signal level in the preamp is high enough to start getting some tube warmth.

    Try cranking the gain knob all the way (and make sure the 15 dB pad button isn't pressed in). Start with the master volume low! The peak light should come on solidly when you play. Bring the master up to a comfortable listening level and back off the gain until you get a sound you like.

    I set mine so it flashes when I dig in on my fretless... a bit higher for more hair when I'm using my Rick.

    If you try this and still can't get it to light up, maybe it's time to try those new JJ tubes, starting with V1 (closest to the front, I think).
  9. sincity

    sincity Supporting Member

    Oct 16, 2002

    I have an SVT 3 Pro from the late 90s. I have gobs of power from the amp, but I am beginning to hear a fuzz effect on higher registered notes on the D & G strings, especially above the 12th fret. I want to check the mosfet bias on the amp. Can someone tell me where my meter probes should be touching to get the readings? I am also assuming that the trim pot does not bias each individual mosfet, but all the mosfets at once? Is this true?

    My preamp tubes are a few years old, the correct type and are in the correct preamp locations. I play with passive basses and no effects.

    I loved this amp to death, so I am hoping this will be a simple adjustment.
  10. Thats pretty much what I'd like to know as well. Do I ground my probe to the frame and probe each socket? Then lay out a table? What should the voltage reading be at each pin? The system has to be powered up for this so can I safely turn on the unit without the tubes installed? Where do I probe to check bias volatge? And what are safe operating voltage ranges for the 12ax7 and 12au7?
  11. okay, just read through the svt bias thread, when i gator clip my ground should i use the signal ground that grounds everything to the casing? the check the + side or the - side of the resistors? thats the voltage that needs to average out to 25mV? okay, so turning the trimpot CCW increases voltage and CW decreseases voltage? thats what I need to do, then replace the tubes and check it again. no load. can i unplug the preamp before I do this, just in case? or will it not matter?
  12. The head was built in St. Louis in 2007 so it's relatively new sat around for a long time, I have a combo that I grew attached to because this ampeg started to sound rough. I want to make it right again.
  13. rickdog

    rickdog Gold Supporting Member

    Mar 27, 2010
    You're making me nervous now :eek:

    I'd like to suggest respectfully that you take your amp to a qualified tech for this job. You don't need to take the tubes out; you do need to measure the voltage across each resistor (not from either lead of the resistor to ground); and you really can do some expensive instantaneous damage if you turn that pot the wrong way, or more than the tiniest amount you can move it at a time.

    Not to mention the damage you can do to yourself with the voltages in there....
  14. Although I read a post saying to turn the trimpot CCW, I found I needed to turn the trimpot clockwise to increase voltage.
  15. I don't get why all this information is out here and none of it is very specific. Why can't someone just explain it instead of trying to discourage me. Someone here has done this before, stop worrying about all the dangers and just explain what needs to be done. I'm just trying to bring a little life back into my sound, I know what needs to be done I just need to know how to do it. Not on person has bothered to offer up the right way to get it done. If I fry my amp so what I'll fix it. I don't care about the trimpot sensitivity, I know that thing is touchy, I know this thing might end up in meltdown, it's obviously worth the risk. Instead of talking about how much of an idiot you all think I am just tell me why adjusting the bias is such a big secret. If I knew exactly what needed to be done I would have never come here to this forum, can I get someone to work with me here step by step or should I find another source? I'm not taking it to a tech, I'm in the middle of no where and I see no point in paying for something I can do on my own with some decent advice..
  16. sincity

    sincity Supporting Member

    Oct 16, 2002
    Take a deep breath kgmitchell! No one is getting on you or, calling you an idiot! Have some patience or perhaps send a PM to those who have adjusted the mosfets before.

    Yes, the procedure is a little vague right now, but someone will help. Just don't let your frustrations boil over. That will certainly not illicit a positive response.
  17. I think you may have missed the point, I do need to take the tubes out. The whole object is to replace the tubes I have. I know that they are ready to be replaced. I'm trying to clean up the sound of my head and I know that changing the tubes alone won't fix it. I want to CHECK the bias voltage and try to rule that out before I install the new tubes, then make any adjustments if it's necessary. Then replace the tubes and check it again. Simple huh? No! Because as I clearly stated I don't know much about tube amps, which is why I am here. If I was rebuilding a transmission I'd go to automotive forum, I'm looking for the guys who know the equipment and thats it. Someone who has seen it and done it. Thats what these forums are for, I made it this far and trusted that you guys could give me the know how. I have been reading these threads for weeks, I just now decided to create an account and speak up because I don't want this to go bad for my amp... I'm a pipefitter so I have worked around high voltage equipment for quite awhile so I'm familiar with the one hand method and all the other necessary safety precations that I need to follow. I do appreciate your concern for my safety though, that says alot about you all in itself. I guess I felt a bit insulted but hey, whatever. You guys don't know me, I can't blame you for trying to talk me out of it. I promise I won't get myself killed but it's no sure thing that my Ampeg will survive... lol
  18. This would be better over on the relevant thread SVT3Pro Bias Thread as you will see examples of others findings and maybe get a better understanding.

    But ok, to check the bias voltage set your multimeter to the millivolts range and take a reading across the legs of the eight ceramic resistors that you will see adjacent to the mosfets on the heatsink.
  19. Vague? I could only find a few other places on the web that talks about the bias drifting on these heads and the way to fix it. Is it classified? It just seems like theres only one or two people who can do it but I don't know them. I have found some pretty solid information here though and through a series of what may seem like ridiculous posts I managed to put together a pretty good understanding fast. This is the most progress I have made since this whole attempt to save my amp started but theres some shakey info here so I need to stick around a little bit longer.
  20. BAM!!! There it is!!! This is the reply I was waiting for!! Thank you!