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Questions about Aguilar DB728

Discussion in 'Amps and Cabs [BG]' started by Lockout, Jun 25, 2003.


  1. BryanB

    BryanB Moderator Staff Member Supporting Member

    I got this info from the tube amp faq.
     
  2. coyoteboy

    coyoteboy easy there, Ned Supporting Member

    Mar 29, 2000
    Sactomato, CA
    #164 eh? Geez, yours is old! j/k. I've got #245 and mine has binding posts too, which I don't mind actually. I thought of actually mounting the speaker cable permanently to the binding post hole with bare wire threaded through and bent back and sealed with shrink tubing. wouldn't go anywhere then. I heard somewhere, though I'm sure Aguilar will verify if asked, that you can replace the whole rear panel for a new one with the speakon mounting hole configuration. You may not want to do this yourself, but a tech could do it.

    BTW the JJ KT88's sound very good in this amp.
     
  3. Lockout

    Lockout

    Dec 24, 2002
    Illinois
    Okay, one thing I just noticed... After having the amp on for several minutes, I start to hear a quiet buzzing hum coming out of my speakers, and a few minutes later I hear a slow pulsing sound, almost like an echo...

    I've been searching around the forums for other people who have had this problem, and from what I've read, it seems like one or more of my power tubes might be microphonic.

    Do you think that's what's causing the sounds? And is there a way to check if any tubes are microphonic without having to take the amp in to a repair shop?

    The guy I bought this amp from shipped it with the tubes still in, despite me asking him to take them out... :(
     
  4. Jerry J

    Jerry J Supporting Member

    Mar 27, 2000
    P-town, OR
    There may be a good chance that one or several of the tubes are microphonic.

    I want to caution you that tube amps will KILL YOU DEAD with the voltages that are inside. They are definitely not for the amateur as the voltages can run in the neighborhood of 300-500 volts. More than enough to make you mess your pants and ruin your day.

    I would highly recommend that you seek the help of a qualified technician on this. BUT find a guy that will let you watch what he does. He will most likely just take a chop stick (like you get at the Chinese take-out) and lightly tap on each of the tubes. The tubes that are microphonic will make a ringing sound. Also, this is important, have him show you how to bias your tubes. You should only have to do this when you replace your tubes. This is a very simple operation compared to the old 60's Fender tube amps. This will save you big bucks in the future in that you will be able to change your own tubes and bias them properly. Just search around your town for a Tech that will do this for you. Offer to buy him a 6 pack or the Chinese food. If you didn't live so far away I'd show you. But if you talk to the guitarist that have old Fender amps they usually know the best amp Techs.

    Now this may be bad news. Typically if you have to replace tubes you usually replace all of them. It's kinda like tires on your car. Some people replace one tire at a time and some do them all so that they don't have to worry about them. If you find that you can live with the noise then just run the tubes until they get unbearable. Tube amps are inherantly noisy. Well at least to he old sure were. It was the trade-off to get that magical tone.

    Oh, I've not looked at the amps specs but most likely the driver balance is to balance the output of each side of the amp. Make certain that the power cable that you substitute is of at least the same gauge as the original. It should be printed somewhere on the cable.

    Just another note, I've always come from the school that says to let the amp cool a bit before shutting it off. This way the tubes have a chance to cool off more gradually. And you should not turn the amp on and off like BFunk says. The warmup and cooling down of tubes and components in the amp is pretty hard on the this stuff. If the amp has a standby switch get use to using it. BFunk was pretty right on about all the other stuff. I use to rebuild old Fender tube amps as a hobby and still own quite a few very old tube amps.

    Good luck with your new amp.:bassist:
     
  5. BryanB

    BryanB Moderator Staff Member Supporting Member

    I got an octet of matched, tested, and burned Svetlana 6550's from Lord Valve for about $200.00. Good tubes and great service.

    As Coyoteboy said, KT88's work well too, but they will run a lot more money. (BTW, Coyoteboy, how do the KT88's sound compared to 6550's?)

    Biasing the amp is pretty easy if you have a multi-meter and a very small screwdriver. Just ask Aguilar for the instructions. If you have done any type of electronics work, you should have no problem. (I suggest a pair of alligator clips as the contacts are small.) Oh yeah, as Jerry J says, these amps have very high current that CAN KILL YOU if you are careless.
     
  6. Lockout

    Lockout

    Dec 24, 2002
    Illinois
    Yikes. After what you guys have said, maybe I oughtta just take it in and have someone look at it. :) Do you know of anyone who is qualified to fix my amp in the south-eastern Wisconsin area?
    Is it safe to do this? I mean, will I be possibly damaging anything?
     
  7. BryanB

    BryanB Moderator Staff Member Supporting Member

    Technically you could pull the offending set of tubes out of the amp and run it like that. (After rebiasing.)
     
  8. coyoteboy

    coyoteboy easy there, Ned Supporting Member

    Mar 29, 2000
    Sactomato, CA
    The KT88's are different, more different in sound than I expected from the 6550's. MORE mids. Hmm, I think they liked to be pushed a little more to get to the sweet spot in the output level, but once they do, whoa nelly! $200 is a good price, I paid about $360 for a matched 8 of the JJ's from Bob Pletka at eurotubes.com
     
  9. jokerjkny

    jokerjkny

    Jan 19, 2002
    NY / NJ / PHL
    no offense to Bfunk,

    but i always keep my tube amps in "standby" mode, by flipping off the standby switch when i'm not using it.

    and when shutting down, i always turn the standby off first, then turn off the power. sorta "eases" the amp into shutdown, rather than an abrupt "pop".
     
  10. BryanB

    BryanB Moderator Staff Member Supporting Member

    What? You disliked my comments so much that you had to repeat the post? :D

    From the Tube Amp Faq :

     
  11. Lockout

    Lockout

    Dec 24, 2002
    Illinois
    I thought I'd make a recording of what the noise I described sounds like, so I can know for sure if the tubes need replacing or not. You'll probably have to turn your computer speakers up quite a bit to hear this, my mic isn't very sensitive. It sounds a bit like distant...fuzzy...thunder. :)
     
  12. Jerry J

    Jerry J Supporting Member

    Mar 27, 2000
    P-town, OR
    I reread your posts and your profile. You don't list a preamp. What are you using as a preamp?

    Also when this noise occurs is your bass plugged in to the rig?

    Microphonic tubes will not hurt your amp. Weak tubes won't hurt it either. You just lose tone.

    One thing that happens is that tubes get a corrosion on the pins and the contact with the socket is not as good as it should be. Usually the trick for this is to pull the tube out, WITH THE POWER OFF AND THE AMP UNPLUGGED, and then put the tube back in. This kinda scours the contact points between the pins and the sockets.

    If your noise is kinda faint that just sounds like normal tube noise. Thermionic device technology is very old and has certain problems that have to be accepted as part of the deal.

    I was kinda hoping that Psycho Bass Guy ( a fellow TB'er) would happen along on this thread. He's a VERY knowledgeable tube guy. If you do a search for his posts here you might be able to email him directly with questions. He's always seemed more than happy to discuss tube amps at length.
     
  13. Lockout

    Lockout

    Dec 24, 2002
    Illinois
    I don't own one at the moment. I will be getting an Aggie DB680 soon. But for testing purposes, (to see if the amp worked) I plugged my bass directly into the input on the 728, and plugged the 1/4" speaker cable from my Hartke Kickback 12 speaker into one of the outputs on the back of the DB 728.

    When I get the DB680 in a few months, I'll let you know if the noise is still there.

    And yes, the noise occurs when my bass is plugged into the rig, but only when I'm not playing (at least that's what I've noticed.)
     
  14. jokerjkny

    jokerjkny

    Jan 19, 2002
    NY / NJ / PHL
    lol, sorry bro. fixed it. ;)

    but given the little quote's symantics, wouldnt it make sense then to put an amp into standby, then OFF, to fully dissapate the tubes?

    kinda like slowly stepping down the power to prolong the tubes. or is this just bad for them too?

    Lockout,

    IMHO, i dunno if you're getting the most outta your 728, if you're merely plugging directly into your power amp via your bass. might explain the weird background hiss, etc.

    somebody correct me, but outboard preamps have buffers and the like to cut down that crap, and bring up the headroom for optimal noise to tone ratios.
     
  15. Jerry J

    Jerry J Supporting Member

    Mar 27, 2000
    P-town, OR
    Joker, there will always be debate on this and many other tube amp issues. It's not really all that critical on the shutdown proceedure. The BIG ONE is when you fire it all up due to the cathode stripping. The big thing is making sure you don't get that transient "thump" going into your speakers when you shut down. I've always prefered to just idle the temperature down on bottles since they've gotten so spendy.

    Lockout, I don't think that you can really make an assessment with your rig by that method. Unplug the Hartke, with all power off and see if you still hear the noise when you power up the 728. I honestly don't think that you have a tube or amp issue. Oh, and be careful that you don't pop that speaker in your Kickback. Very easy to do with that amp.
     
  16. Lockout

    Lockout

    Dec 24, 2002
    Illinois
    Okay I've got one more quick question :)

    Since the 728 has an impedance switch with 8/4/2 ohms on it, what would happen if I used a combination of cabs that resulted in 2.67 ohms? What would I set the switch to?

    Reason I'm asking this is because I was thinking about going modular a bit with my cabs and getting a gs212 and 2 gs210s... :)
     
  17. BryanB

    BryanB Moderator Staff Member Supporting Member

    Set it at two ohms. A tube amp can easily stand driving the output transformer into twice the rated load, but aches at 1/2 the rated load. (Tube amps hate, hate, hate open shorts.) I had a bad speaker wire connected to my DB 728. It was a closed short. While I was figuring out the problem, the amp ran that way for about two minutes. Fixed the wire and restarted the amp. It was fine! It was a good thing that was not an open short.
     
    winegamd likes this.
  18. Lockout

    Lockout

    Dec 24, 2002
    Illinois
    Okay, another question. :)

    I was wondering if anyone knows of a way I could make the fans in my DB728 quieter. They're pretty loud even on the low setting. Playing with the band I don't notice it, but while using it to practice at home, it's irritating. :meh:

    Is there anything I can do? I'd even consider replacing the fans if I knew how much airflow the current fans provide.
     
  19. BryanB

    BryanB Moderator Staff Member Supporting Member

    You should never disable the fans. I think the manual mentions this.
     
  20. Lockout

    Lockout

    Dec 24, 2002
    Illinois
    Of course I wouldn't disable them... ;)

    I meant that I was wondering if anyone knew of a way I could somehow block some of the noise from the fans while practicing at home, without restricting the airflow.

    Is there any way?
     

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