Psst... Ready to join TalkBass and start posting, make new friends, sell your gear, and more?  Register your free account in 30 seconds.

DB 728 help needed

Discussion in 'Amps and Cabs [BG]' started by metron, Feb 18, 2006.


  1. metron

    metron Fluffy does not agree

    Sep 12, 2003
    Lakewood Colorado
    Just got a used DB 728 delivered to me yesterday. Nice looking amp. Anyway I took it home, hooked it up, let it warm up and instantly after I hit the standby switch firecrackers started going off and then lots of smoke. Popped the lid off the bottom and my acute visual skills identify two 220 uF caps directly under the small (heater filament?) transformer are nicely cooked.

    So any idea what may have caused this? Could this be a sign of a major problem or is it possibly an easy fix? The tubes were all lit and glowing evenly. No other signs of malfunction besides the obvious ones I already mentioned. Any information and/or advice is greatly appreciated.
     
  2. metron

    metron Fluffy does not agree

    Sep 12, 2003
    Lakewood Colorado
    Anyone?
     
  3. Sorry, but this seems like the most obvious choice to me. Even though you are not the original owner (no warranty) They are still very helpfull.


    techsupport@aguilaramp.com
     
  4. Some debugging info here:
    You turned it on but the standby was still off.
    What you did here was to apply voltage to the filament of the tubes only (this is probably 6 or 12 volts). They were glowing, that's good.
    What that also should tell you is that the filament transformer (transformer from outlet to filament) was working fine.
    When you switch on standby, you apply voltage to the plates of the tubes (you turn on the transformer from outlet to plate voltage and other control screens on the tubes). This voltage is anything from 400 to 600 volts!
    The fact that the caps have fried and that the fuse has not (has it?) is kinda worrysome. It might indicate that the power transformer was defective. Caps have a voltage rating that tells at which max voltage they work. If they blow that means that they couldn't do their job as isolators... meaning too much votage was applied, they became like a wire with too much current... they fried.
    This would tell me that the power transformer was defective.
    So now, unless you can find the schematics and trace down the problem, I'd suggest and amp technician.
    One more thing: was there a cab connected and is there min/max for cab ohm-age for this amp?
    Hopefully the output transformer (transformer from the tube output to cab) isn't blown as well.
    Power and output transformer are usually between $150 and $300. Caps are probably $20 to $30 each (all USD). Add to that the bench hour... this should give you a ball park of what you should spend.

    I hope this helps and sorry you got fireworks :rolleyes:
    ;)
     
  5. metron

    metron Fluffy does not agree

    Sep 12, 2003
    Lakewood Colorado
    Thanks people! Yeah it was definitely hooked up to a speaker, my 4 ohm NV 610. There are lots of fuses under there but I dont think any of them went. Dont even want to get close!

    I got info from Aguilar that the particular caps that went are part of the screen supply and that because they fried after hitting standby it could be a shorted 6550. So at the very least its a new set of tubes, new caps, and labor assuming that was the problem.

    Could a shorted 6550 happen from shipping? Could it potentially have damaged something else more major then just two caps?
     
  6. Shorted screens? Were the tubes actually in their socket for shipping? Bad idea.

    Without the actual schematics it's hard to tell. Usually caps might be bundled together (can style), so that specific cap can might also affect other parts of the circuitry (other tube plates etc.)

    Just curious, I am also looking for a DB 728, where did you look? Pretty hard to find on used the market.
     
  7. metron

    metron Fluffy does not agree

    Sep 12, 2003
    Lakewood Colorado
    Found the amp here in the used gear section. I waited quite a while to find one, good luck with your search.

    It was shipped with the tubes in the sockets but I made sure they were seated ok before I fired it up. Could a tube possibly be damaged internally from shipping or is a shorted screen most likely to happen at the pin/socket? I guess it really doesnt matter I have to take it to a tech either way. Sigh...
     
  8. I hope you get this all sorted out without too much stress, or cash.:D

    I know that it's not fun having issues with "new to you" gear right off the get go.
     
  9. A screen short can happen either internally in the tube or at the base. If it were at the base, you would have a telltale carbon track across the socket. The last SVT I fixed had a power tube with an intermittent grid to plate short that fried the plate and screen resistors but not their respective supplies. I'll bet money your screen resistor is toast on the bad tube(s).

    It's possible that the tube was jarred to a short by shipping, but more likely, like microphony, it was a manufacturing defect in the tube that manifested for the first time on you.
    I also doubt you have transformer problems because a bad PT would blow the fuses and not the caps.
     
  10. metron

    metron Fluffy does not agree

    Sep 12, 2003
    Lakewood Colorado
    Thanks. Hope it goes well...
     
  11. metron

    metron Fluffy does not agree

    Sep 12, 2003
    Lakewood Colorado
    Thanks Psycho. I did not see any carbon track on the main board and didnt notice any other parts that were obviously damaged. There are a few types of fuses though and Im not sure if one went or not. I saw one that was solid white. Pardon my ignorance but is that a blown fuse?

    So... on to the tech I guess. Anyone know a good tech near Denver? Since Im probably going to have to replace the tubes anyone have recommendations?
     
  12. Maybe...The BEST way to check a fuse is to run a continuity check on it with a meter because even blown fuses can "look" OK. There are also ceramic fuses that are solid white when they're good.
     
  13. Hey metron congratulations on your 728. I'm sorry you're having troubles. Aguilar tech support is incredibly helpful, responsive, and of course knowledgeable about the products. I wouldn't hesitate to contact them.
     
  14. metron

    metron Fluffy does not agree

    Sep 12, 2003
    Lakewood Colorado
    Thanks tightbidness. You are right Aguilar is extremely helpful and I have been in contact with them. Geof has taken a good bit of time to answer my questions which I appreciate. Great customer service!
     
  15. metron

    metron Fluffy does not agree

    Sep 12, 2003
    Lakewood Colorado
    Anyone have any tube suggestions for this beast? I hear that Svetlana 6550s are good? What about replacements for the driver and phase inverter tubes? Also where is a good place to get matched pairs that are actually tested?
     
  16. tombowlus

    tombowlus If it sounds good, it is good Gold Supporting Member

    Apr 3, 2003
    North central Ohio
    Editor-in-Chief, Bass Gear Magazine
    Sorry about your problems. Man, that is a bummer. :meh:

    I bought my DB 728 new, and it shipped with eight Svetlana 6550's. I have not changed them, and to my ears, the amps sounds great. But, after researching all that I could, it seems that not everyone loves the Svetlana's, and in fact there may be better options. The proof is always in the pudding, though, I decided to order some new power tubes to try in the 728, Demeter 300M, Peavey Classic 400, and Trace Elliot V8. I have matched octets of KT-88's from JJ, EH, and Valve Art coming from Doug Preston www.dougstubes.com. Doug has always done me right, but www.tubedepot.com and www.tubestore.com also seem like excellent sources.

    Good luck, Tom.
     
  17. metron

    metron Fluffy does not agree

    Sep 12, 2003
    Lakewood Colorado
    Thanks Tom!
     
  18. jasonbraatz

    jasonbraatz

    Oct 18, 2000
    Oakland, CA
    I want a DB728 so badly, but I am scared to death of something like this happening. I'd buy it new if they still made them. :(
     
  19. metron

    metron Fluffy does not agree

    Sep 12, 2003
    Lakewood Colorado
    Actually it was Sovtek I was thinking of that I heard lots of people use. Any difference between WE and WD? I also see EH, Tung-sol, SED. Who has experience with these tubes? Are KT-88s worth getting into?
     
  20. tombowlus

    tombowlus If it sounds good, it is good Gold Supporting Member

    Apr 3, 2003
    North central Ohio
    Editor-in-Chief, Bass Gear Magazine
    The Sovtek KT-88's are normally the ones that get all the attention (very similar in layout to the new Tung-Sol 6550's [a great tube, IME], though according to New Sensor, there are definite differences - they even weigh different amounts), though I strongly considered a set of 6550WE's for some nice overdriven tones. Ultimately, I went with the EH KT-88 for my "clean/overdive combo tubes" - or so I hope, at least. The JJ's are for gobs of clean headroom (again, so I hope), and the Valve Art are supposedly very musical and well balanced. We'll see. :cool:

    Tom.