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dbx 386 question

Discussion in 'Recording' started by afreeid, Nov 6, 2007.

  1. afreeid

    afreeid Guest

    I want to replace the stock tubes in my dbx 386. My web search attempts have come up empty so can anyone advise? (make, model of tube)

  2. TVPostSound

    TVPostSound Guest

    I used a SIEMENS 5814A / 12AU7 when I had one a few years back.
    No much of a change, as they (DBX) are starved tube circuits.
  3. RemyRAD

    RemyRAD Guest

    That's right, not all tube circuits are there to produce gain. Many of these tube flavored preamps are just that. Tube flavored. What is being done here is that the " plate" part of the tube which receives a high voltage btween 250 to 350 volts are being supplied with a wimpy, depleted voltage much lower than the necessary voltage required to make reasonable gain. As TVpostsound indicated, this is referred to as depleted or starved plate voltage. This causes the tube to go into early nonlinear tube distortion. The amplification is actually accomplished with the transistor/integrated circuit components and not with the tube. So the type of tube utilized is really not much of a factor in this situation. You won't gain anything by replacing the tube was something else as it's not a primary amplification element but merely the distortion producing element.

    Tubular thinker
    MS Remy Ann David
  4. dementedchord

    dementedchord Well-Known Member

    sorry to be contrary but the 386 is NOT a starved plate... though realitivly low powered at 200V...
  5. 200 volts is not low, it's about what a Fender amp see on it's 12AX7's give or take a few depending on the amp model.

    But the voltage doesn't matter as much as how the tube is actually being used. To add some distortion or harmonics or as a real gain stage.

    It doesn't take much to make a 12AX7 a useful gain stage.

    The best thing is to experiment. You might be disappointed with the results. A lot of those tubes are hand picked for their lower gain because of the noise factor. If indeed it's a 12AX7, you might try a 12AY7, 12AT7, 5751 and some of it's industrial or European nomenclatures. Even a 12AU7 can be used. But they might not work well. Even preamp tubes are biased somewhat like output or commonly know as power tubes in a tube amp.

    You can also get matched tubes because a 12AX7 has 2 sides. There are also NOS tubes out there but unless you know what to look for I would stay away from them. They are expensive and have been around a long time, we don't know were they were all those years and what conditions they were stored in. I have been very disappointed with NOS tubes after spending a lot of money. Some types are abundant and you can get good ones like old Mullard CV 4024 (ECC81) which is a 12AT7. The 12AX7(ECC83) is a CV4004.

    Check out Mike at KCA tubes, he's a good guy and will share information.


    Also short plate tubes have less of a tendency to become microphonic. Longer plate tubes are some times higher gain. Tubes and their usage and types is a science by itself.

    What was the question....I forgot....Oh you want to replace the tbes...I'll be back with some suggestions. How much do you want to spend....?????
  6. Link555

    Link555 Well-Known Member

    Uhm just another thought....

    Does the way a starved tube distorts sound the same for all tubes?

    That doesn't seem right to me.
  7. dementedchord

    dementedchord Well-Known Member

    yeah a starved plate doesnt change as much with the tube type.... and 200V is about as low as you want to go... guitar amps (not all)often use 400-450V i've worked on old organs that used over 600V... got bit by one once that threw me across the room...
  8. I looked at the block diagram of the unit. It's a 12AU7 and it seems to be used as a gain stage. Although if you check the gain factor of a 12AU7 it's 17 or so. A 12AX7 is 100, a 5751 is 70, a 12AT7 is 40 or something like that. So you can see it's really not used for a lot of gain but more of some type of tone helper.

    I know a little but not enough to really explain all the factors.

    I doubt a tube change would improve the sound or tone, in fact it will probably make it worse. Those tubes were probably picked for their low noise to begin with and the circuit designed around the parameters of that specific tube for a specific manufacturer. So even if you spend $150 on a NOS heavy duty tested and matched 12AU7 it might not match the circuit design and it might sound horrible.

    Also opening the unit and changing something will probably mean you voided the warranty. Call the company, the number is on the web site. Talk to a tech and see what manufacturer DBX uses for their 12AU7's. Ask him/her if they match the triodes, if not the unit might sound better with a tube tested for noise, gain and have the triodes matched. Doing that might give you a bit more clarity and a lower noise floor. I would use the same company, if it's JJ or whatever use that company.

    Check out some 12AU7's here.

  9. Link555

    Link555 Well-Known Member

    Correct me if I am wrong but....
    Even though the current required to push the starved plate tube into saturation is much less, the curve at which it does it is unique to different tube types.

    Would that not sound different?

    different tube curves:
    (Dead Link Removed)

    (Dead Link Removed)

    both are 12AX7's but have very different curves, even at low plate voltages.

    Just a thought...
  10. I'm sure every tube even the same number from the same manufacturer will sound different in any circuit design.

    The question is will it sound better. It that particular unit I don't think so. In a lower line preamp I will say even though it is starved plate it will sound different. The only way to tell is to try it. But that's kind of expensive, for instance a Behringer MIC 200 can be had for about $40, yet a good quality 12AX7 goes for about half the price of the unit. An expensive endeavor if you want to try a few different tubes. Unless you are a guitar player and have a whole bunch of them hanging around.

    I actually do have a MIC 200 and I am going to try a 12AT7 to see if I can get it to sound cleaner with the same amount of tube effect. It's actually screwed in pretty good and I bet even though it's a Sovtek it was picked for certain qulities. It actually sounded pretty good for bass to get an almost B-15 Ampeg type sound.

    I had a Pignose 40 watt little practice amp and it only sounded right with Chinese 12AX7's, even higher quality NOS were noisy, the Chinese one's weren't. It was designed for that tube and something was making it pickup noise....go figure....engineers......geez.
  11. RemyRAD

    RemyRAD Guest

    This is the reason I stay away from tubes these days. Too much inconsistency. They can be good but they can also be very very bad. bigdaddybluesman knows from where he speaks because he designs and builds guitar amplifiers with tubes. Probably the most experienced tube builder here.

    In experienced with tubes
    Ms. Remy Ann David
  12. Thanks Remy, but I wouldn't call myself a designer or builder.

    More like a hack out of desperation.

    I'm more of a guitar slinger and figured out how to build the things out of being lied to and ripped off. Too many bogus repairmen. One guy who supposed to be a friend never did what I wanted. Once I started learning and I ripped apart the stuff he did I could see how bad he really was. He used to take months to finish some repairs when it should have taken a few hours of actual bench time.

    Tube amps are very easy to build, like a kind of jigsaw puzzle. Because of certain health issues I can no longer build them. Plus it's very frustrating when you spend countless hours putting it all together, turn it on and PUFF a nice cloud of smoke. Or better yet, it works but sounds bad and you have to trace down exactly were you went wrong....not fun and very tedious.

    I think recording can be a lot more fun than doing that.

    To me tubes are very overrated in recording and good old solid state sounds fine. Most of the recordings I love were made with SS equipment except for the actual guitar amp and maybe the bass amp. Also some tube mics but probably it was a good old SM57/58. In fact most of those recordings were probably done with a Neve board and 1176 limiters both of which are solid state upgrades of/from tube equipment.

    I guess tubes have a certain mystique which became popular because of the lack of warmth in the digital era. Tubes have their place but not as a foundation. They require a lot of maintenance beyond the average recording person's electronic ability. Plus there aren't the quality tubes there was in the good ole' days. The new one's are OK when you have hand picked the good ones out of the batch, but not like those old one's which would last 20-30 years without failure. There is a reason why by the 1970's most recording equipment was SS. Much less maintenance and the equipment would sound and work in a much more reliable and predictable way.
  13. Link555

    Link555 Well-Known Member

    Sorry I think I was unclear, I was responding to this comment....
    sorry for the tangent, but I think changing the tube will affect the sound.
  14. Yes it will, but probably not in a good way.
  15. Link555

    Link555 Well-Known Member

    Gotcha thanks!

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