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Tube Amp Build, Most important Features?

Discussion in 'Amps and Cabs [BG]' started by basscooker, Mar 30, 2014.

  1. basscooker

    basscooker Commercial User

    Apr 11, 2010
    cincy ky
    Owner, Chopshopamps.com
    I'm considering another ground-up amp, and I have an old chassis and enclosure that I want to use for it.
    If there was a short list of bass amp must-haves, what would be at the top? Now, some design goals:
    It's only about 13x10x8 inside there, and the enclosure has three knobs, two connectors, and 2 switches. I'm thinking with its size, 40-50 watter might be a stretch, but I'll see what the guy says. Treble, Bass, Gain. Lo gain, Hi gain. Power on and < what for this other switch?> I was thinking half power or standby.
    My intention is as much clean power as he can get me, but to drive kindly without too much grit when it does get pushed. My questions are more, "what bones and guts will give a better chance of reaching this goal", and "Are there 'kinder' tubes that help with keeping the overdrive milder", and "Besides making sure the OT is beefier, are there other components of a bass amp that I need to make sure to mention when discussing the design?"

    Thanks for input. It will also be for sale or for keeping, whatever. The goal is enough power for a respectful trio or quartet, with the right cab; and just a simple great sounding "plug in and turn up" amp without any bells or whistles. Crossing my fingers on how much power we'll be able to put inside it! Might end up with a 20 watter.
  2. JimmyM


    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    If that's your goal, I would probably recommend overbuilt transformers a la Ampeg, as I fully believe that's why Ampegs hold together cleaner and louder than old Fender bass amps with similar wattages.
  3. JGR

    JGR The "G" is for Gustav Supporting Member Commercial User

    Jun 29, 2006
    President, CEO, CFO, CIO, Chief Engineer, Technician, Janitor - Reiner Amplification
    If you're going to all the trouble and expense of a custom build, spend another $50 and get a suitable chassis. I just built an 80 watt guitar head in a 17 x8 and that's about as small as I could go and still fit everything.
  4. will33


    May 22, 2006
    Most important features......all of them. ;)

    That said, listen to the people here who really know what they're doing. (that would not be me).
  5. will33


    May 22, 2006
    You can do a lot with simple Treble, Bass, and Volume....if your default voicing isn't overly scooped and your 2 eq shelves meet in the middle.

    I see no reason for a 1/2 power switch with 30-50 watts, even on a guitar amp, unless it is purpose-built for guitar with an eye towards more studio work or small, intimate gigs and less real live work. No need at all for bass. Leaves that hole open for something else.

    Also plenty of power saturation tone variation running high power tubes, like 6550's at lower than they're capable of compared to running smaller tubes more towards their output capabilities..
  6. Scoops

    Scoops Why do we use base 10 when we only have 8 fingers Supporting Member Commercial User

    Oct 22, 2013
    Sugar Creek, Wisc
    I am me
    I would highly recommend an Ultra linear OT, and not necessarily a beefier tranny. A beefier tanny will still leave you with some distortion on higher volumes. Ultralinears will stay clean, even when almost dimed.
  7. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    Take an early B-15 preamp, output stage two 6550 can give you up to 100W. No master, just volume, treble, bass. Keep the signal path simple and uncomplicated. Heavy iron, over spec the output transformer.
  8. basscooker

    basscooker Commercial User

    Apr 11, 2010
    cincy ky
    Owner, Chopshopamps.com
    The only question left now is space. I see how one might think limiting myself to the existing enclosure is counter intuitive, but I have fair enough reasons, (see my site).

    I had the same thought about half power for an already underpowered bass amp. Maybe use the switch for a mid boost.

    So far I have:
    Avoid a pronounced, baked-in mid scoop. Plan eq with care.
    Underpowering tubes can keep the overdrive calmer. I assume using a circuit that simply wont power the tubes into overdrive is the way to do that?

    Ultra linear transformer. I should think he'll know what that means and why it's important.

    A b-15 has a preamp setup that matches the amount of "stuff" I have slots for on mine.

    What about preamp tubes? Is a 12au7 or at7 etc a less gritty tube? Myth?
    Thanks for replies.
  9. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    Some understanding on how the tubes will operate can come from looking at the load lines. For a given plate voltage, where the preamp and power tubes are biased determines their operating points. The excursion of the operating points with the input determines if the tubes will distort when pushed. A tube can run clean or dirty deepening on where it lies and how much excursion there is. Tubes can be operated in a conservative region. But where they operate affects how they sound. It can be very complicated.

    The larger output transformer can help reduce distortion in the low end. I like ultra-linear for hi-fi but I'm not a fan of it in musical instrument amps. I find that it constrains the rawness of the power amp. Not everyone thinks this way. It just goes to show you, what one person likes is not going to be what someone else likes.

    As for the preamp tubes, again it comes down to plate voltage and bias. The tubes in the 12A?7 family perform differently and it will depend on how much gain you require. The advantage of some of them like the 12AX7 is that there are low noise equivalents available. I find that an octal tube such as a 6SL7 is harmonically richer. I'm biased that way. :p

    My best advice is to find a design that works and have been proven over time. Make it part of your design. Perhaps find a pre and power amp that can be married together. Tweak the tone stage.
  10. Mr. Foxen

    Mr. Foxen Commercial User

    Jul 24, 2009
    Bristol, UK
    Amp tinkerer at Ampstack
    Big good quality transformers, wound with a 2 ohm tap, because modern cabs. UL is a bit questionable as they tend to sound less like a valve power section.

    How dirty the preamp is doesn't come from types of valve, it comes from what the circuit does with them, they should be designed together, the swapping one type for another in already build amps is bodging you can fully avoid with a new build. You need to use a PI valve that can push enough current for the power valves to work correctly, probably not an issue with your power requirement.

    Baxandall or James tone stack is nice for bass where you can actually wind up with a mid bump.

    Always bear in mind the full rotation of the knobs, you can have one not turned up far for clean, and retain the option of drive. There is no loss from doing that, although some people think they must be wasting money on knows that are not above half way.
  11. As others suggested, 6550 will get you where you want to be with less circuitry.
    I've been learning about tubes recently (reading for last 3 days so im definitly no expert) and there was a thread on TB about someone who wanted to build a tube amp. Someone had said you could run a tube with different voltages for different wattage and headroom.
    Ex.: Less voltage = less headroom, earlier breakup.
    That's the principle behind low voltage distortion pedals that use tubes, they starve the tube of power to create distortion.
    NOTE: I'm a tewb newb, if this is wrong please yell at me and/or correct me for saying stupid things!
    So, (assuming all 6550As are built with similar specifications) when you look at the GE datasheet from 1972 (on Dr. Tube) you can see DC plate voltage is 500 VDC. So don't feed the plate 250 VDC if you want clean headroom, take advantage of the tube's capability. I think this applies both to output and preamp tubes, not sure though, so don't take my word for it.
  12. astack


    Nov 12, 2011
    Washington, DC
    Awesome! Hope you post more info as you go forward. I'm working on a guitar amp design now, but I'm itching to do something similar to what you're doing after that's finished.

    If you aren't looking for 100W, you could do a pair of 6550's in triode mode for more like 40W. A switch to go between wouldn't be hard either. In triode mode, the break up should be much smoother than pentode mode.

    As for controls, there's also a tilt tone control that would allow you to adjust relative bass and treble in one knob. That opens up the possibility of a gain and MV. Then add a switch for mid voicing if you like or just do eq off-board.

    As mentioned above, tune the preamp tubes for the amount of headroom v. gain you need. The potential gain of each 12A_7 is more limited by the headroom anyway (output swing for a given B+ is pretty much fixed). And the output swing is usually enough to overdrive the next stage if not checked anyway.

    1/2 12A_7 at the input --> tone stack --> recovery 1/2 12A_7 --> 2x1/2 12A_7 PI (LTP probably, but a driver and cathodyne might be cleaner since the cathodyne has basically 100% NFB; I'd guess 12AU7 is best for the PI to have enough output swing to overdrive the big bottles) --> 2x6550. Then a bit of global NFB to taste.

    I haven't done any calcs, but I would think you could get enough gain even without bypassing cathode resistors in the preamp, too.

    Lastly, tapping a line out from the speaker side of the transformer is easy to do and seems handy to me.
  13. BassmanPaul

    BassmanPaul Gold Supporting Member

    Aug 25, 2007
    Toronto Ontario Canada
    +1 Reusing an unsuitable chassis and box are false economy at the end of the day.

    17x8 Jonathan, Hammond perhaps?? :)
  14. astack


    Nov 12, 2011
    Washington, DC
    Also similar to VVR and power scaling used for guitar amps. But if you're looking for cleaner sounds, this is a step backwards.

    While I agree it would be money well spent to me, it doesn't seem like the OP's decision to use the chassis on hand has to do with saving money. Just an arbitrary constraint. I've seen tons of builds like this with a found chassis, transformers, etc. Some people like the added challenge and reduced options.
  15. JGR

    JGR The "G" is for Gustav Supporting Member Commercial User

    Jun 29, 2006
    President, CEO, CFO, CIO, Chief Engineer, Technician, Janitor - Reiner Amplification

    Hey Paul!

    Actually I used a nice one from turretboards.com. Had the corners welded locally. It took some careful planning to get everything in there - a big space saver was the use of modern caps which can be mounted with 1" clamps.

    To the OP, just make sure everything will fit and take extra care with component location so you don't run into noise, parasitics, or heat issues. Getting a full amp on a chassis that small will be a feat unto itself!
  16. headband


    Oct 18, 2013
    I have not built guitar amps. I have repaired both guitar amps and vintage stereo gear for the last 20 years. I have built a few tube stereo preamps and power amps. So from that perspective here is my two cents.

    Don't use an ultralinear design. It works well for hifi but is not the best for bass. It can be made to work (most if not all early Sunns were based upon Dynaco's Ultralinear Design) but I think you will be more satisfied with a standard output transformer. And get the best that you can will fit. IMO good output transformers are the heart of a good tube amp.

    A lot of objectionable distortion can be avoided by making sure your preamp stages are not overloaded or saturated. Usually a higher B+ voltage results in less clipping (to a point). And designing each stage so that it does not overdrive the next is important for clean sound.

    How do you plan to deal with heat? This would be a big concern for me. Such a small box without really good ventilation will fry components over time.

    You don't have a lot of output tube choices for bass at these power levels- 6L6 family, EL34 family, or 6550/KT88. I personally like the KT88's - they have can give you the headroom you are looking for and should have long life.

    Now a totally off-topic question - have you ever built an amp using 807 transmitting tubes? They are essentially a 6L6GC with the plate connection made through a top cap. NOS can be had for cheap. I know they are making adapters today to fit them into standard 6L6 amp configurations, but this makes them too tall to fit in a lot of amps. Could be an interesting build from the ground up. I have two old IPC/Western Electric Theater amps that use them and they sound superb!
  17. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
  18. IMO, make sure it weighs at least 50 pounds, or many will feel it sounds thin and anemic :p:bag:
  19. okcrum

    okcrum in your chest

    Oct 5, 2009
    Verde Valley, AZ
    RIP Dark Horse strings
    So true. Same folks who listen with their eyes when recommending BGs. :D

    Clean and dirty preamp channels are a nice option, and if you want to go Indy, use a separate tone stack for each. A blend or master volume control then becomes a more useful tool.
  20. basscooker

    basscooker Commercial User

    Apr 11, 2010
    cincy ky
    Owner, Chopshopamps.com
    I'm not building it, I'm HAVING it built by a local shop that does this all day every day. I've already expressed most of the concerns and or rec's mentioned. At some point, though, (since I'm not the guy with the soldering iron) I just have to step back and cross my fingers. I'm on my way there in about an hour or so to drop off, so any final comments or suggestions? He did mention that it sounds like I'm going to want 6550's biased a bit cold, and 6sl7 in the pre. He said once he's got it, we'll get into the other deets.