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Custom Tube Amp Build Thread

Discussion in 'Amps [BG]' started by 72LML, Jan 30, 2011.


  1. 72LML

    72LML

    Joined:
    Dec 25, 2009
    Location:
    Burnsville, MN
    Well I've finally been able to get things together in order to start a build thread on my custom tube amp as requested by a few others. I first posted about it here:

    http://www.talkbass.com/forum/showthread.php?t=710734

    Scott (our guitarist, aka RoctalAmps) is building the amp so any technical details will have to be answered by him.

    Here are a few details about the build and what we're shooting for.

    -Tube compliment: 6sj7, 6sl7, 6sn7, in the pre-amp

    -Traynor style tone stack with mid-shift and parallel/serial mixing effects loop.

    -EL34 driver stage through an Fender PS-400 interstage transformer, with clipping feature to get the 'svt' cranked sound in the driver stage

    -4CX250B RF tetrode in the output stage running around 900 volts on the plates.

    -Antek toroid power transformer

    -300-watt Heyboer output transformer with home audio bandwidth of 20hz to 20khz

    -2/4/8 ohm taps

    -Shooting for 200 to 250 watts, possibly more.


    Here are a few shots of the chassis:
    [​IMG]
    [​IMG]

    And a quick shot of a partial mock up:
    [​IMG]
    The big transformer the right is the output transformer, power transformer on the left. Ceramic RF tube are in the middle with a pair of 600v capacitors flanking them. The tubes run off of 1000v supply.

    I'll try to answer any questions that I can. Otherwise I'll direct them to Scott.

    I can't wait to try this thing out! :hyper:
     
  2. babebambi

    babebambi

    Joined:
    Jan 7, 2008
    Location:
    YTZ
  3. Willicious

    Willicious Supporting Member

    Joined:
    Feb 24, 2008
    Location:
    Bend, Oregon
    very much subscribed.
     
  4. panama

    panama

    Joined:
    May 26, 2009
    Location:
    Detroit
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  6. s_mcsleazy

    s_mcsleazy

    Joined:
    Nov 23, 2009
    Location:
    glasgow (on the 16 bus)
    what does the tv control do
     
  7. MooneyDriver

    MooneyDriver

    Joined:
    Oct 6, 2010
    Location:
    Kent, OH
    You're gonna need a lot of air to cool those puppies!!!! :bag:
     
  8. Roctal-Amps

    Roctal-Amps

    Joined:
    Nov 10, 2010
    Actually, it's not as bad as one might think... The ceramic tubes cool fairly efficiently without radiating the surrounding components. We will be pressurizing the chassis as the air escapes through the ceramic tube towers through the heatsinks. I think about 5 cfm per tube is needed. I am guessing I will end up with a push/pull fan setup. They can definitely get hot when running at max plate dissipation of 250w per tube, but we will not be running them that hard.
     
  9. okcrum

    okcrum in your chest Supporting Member

    Joined:
    Oct 5, 2009
    Media:
    2
    Location:
    Verde Valley, AZ
    Disclosures:
    Endorsing Artist: Dark Horse strings
    As long as you can get good airtight sealing (save for the air intake) under the chassis, you'll get plenty of air for the two tubes up the chimneys. That's quite common in using the 3cx800a7, which needs a lot more air for a pair of them. A single small squirrelcage fan should be enough.
     
  10. greenboy

    greenboy

    Joined:
    Dec 18, 2000
    Location:
    remote mountain cabin Montana
    Disclosures:
    greenboy designs: fEARful, bassic, dually, crazy88 etc
  11. Roctal-Amps

    Roctal-Amps

    Joined:
    Nov 10, 2010
    Yes, a tone stack is equalization, but each tone network has an effect on the rest of the stack when changed. A true EQ (non-active) has the ability to attenuate the band around it's center frequency with limited affect on the neighboring band. They are different animals.
     
  12. greenboy

    greenboy

    Joined:
    Dec 18, 2000
    Location:
    remote mountain cabin Montana
    Disclosures:
    greenboy designs: fEARful, bassic, dually, crazy88 etc
    That's hilarious. Of course Fender-type tone stacks have interactive bands, much more so than Baxandall/James. But the fact remains, they are there for the EQ. Even saying "non-EQ tone stacks" in a whisper when nobody's around sounds patently ridiculous. Unless you want to take the bass mid and treble knobs off the menu and silk screen "tone stack" above the pre-gain and post-gain controls ; }
     
  13. Roctal-Amps

    Roctal-Amps

    Joined:
    Nov 10, 2010
    OK, Dude, I concede to your prowess.
     
  14. MooneyDriver

    MooneyDriver

    Joined:
    Oct 6, 2010
    Location:
    Kent, OH
    I think it's a fantastic idea! :hyper:
     
  15. MooneyDriver

    MooneyDriver

    Joined:
    Oct 6, 2010
    Location:
    Kent, OH
    It's common for RF amps, I know, but there's nothing common about a 3cx800a7 in a bass amp!:cool:
     
  16. okcrum

    okcrum in your chest Supporting Member

    Joined:
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    Media:
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    Location:
    Verde Valley, AZ
    Disclosures:
    Endorsing Artist: Dark Horse strings
    Sign, the comparison was thermal, not electronic.
     
  17. Roctal-Amps

    Roctal-Amps

    Joined:
    Nov 10, 2010
    You are correct sir, RF guys usually used the squirrell cage fan to pressurize the chassis as they can produce more pressure than the average axial fans but I have my reservations about the squirrel cage fans' noise/vibration. So I was leaning towards the standard brushless axial fans in a push/pull arrangement. I need some box/cover over the tubes anyhow to protect from the anode voltage, why not have a second set of 'pull fans' on top of it.
     
  18. fivestringgecko

    fivestringgecko Supporting Member

    Joined:
    Mar 28, 2007
    Location:
    Denver, CO
    +1!

    5sg.
     
  19. MooneyDriver

    MooneyDriver

    Joined:
    Oct 6, 2010
    Location:
    Kent, OH
    Ok, forced air cooling through chimneys is not common to bass amps....
     
  20. kosmicwizard

    kosmicwizard

    Joined:
    Nov 18, 2008
    Location:
    Victoria, B.C., Canada
    If this is a serious question, I would say gives the viewer a size reference. Cheers.
     
  21. okcrum

    okcrum in your chest Supporting Member

    Joined:
    Oct 5, 2009
    Media:
    2
    Location:
    Verde Valley, AZ
    Disclosures:
    Endorsing Artist: Dark Horse strings
    That makes sense. You'll get more CFM (running the fans in series) as long as each of them can make decent CFM into the system back pressure. If either can make the required CFM, that's even better, as you can survive a single fan failure.
     

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