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What would happen, a question about tubes and cabs

Discussion in 'Amps and Cabs [BG]' started by jazzmonkey, Oct 8, 2010.


  1. Selta

    Selta

    Feb 6, 2002
    Pacific Northwet
    Total fanboi of: Fractal Audio, AudiKinesis Cabs, Dingwall basses
    I can't think of a single tube bass amp that has taps for 16 ohm. Also, there is the argument of getting the power section saturated and getting drive from that, which, IMO, is where tube amps really shine. Taking a line out won't get you that at all.
    Also, you really can't just yank a pair of output tubes. Using SVTs as an example, you can run into impedance issues, and the plate voltage on the remaining tubes will go up as well. Not to mention that you won't loose that much volume by doing so (I'd venture to guess 1.5db?)
     
  2. I'm not going to yank 1/2 the tubes out of my head. Besides, I scrapped the idea of the iso box quite a while ago and decided to just keep recording direct. I still have the DIY bug and was thinking of building a fEarful 15/6 instead...but you're saying the fEarful paired with a 200w tube head would not be a good idea? If so, why are there 15/6tube plans designed for a standard tube head to sit on top?
     
  3. Bassmec

    Bassmec

    May 9, 2008
    Ipswich UK
    Proprietor Springvale Studios
    I am telling you about the impedance issues with a 4 tube output stage, remove a pair of four and half the impedance the transformer sees reflected from the speakers.
    For you edification I will list some quality tube amp manufacturers that have not only provided 16 ohm taps at least on part of their range, that have somehow slipped by your intensive wide ranging expertise and experience.
    Acoustic, Ampeg, Impact, Marshall, Sunn, Hiwatt, Reeves, Joyce, Sound City, Marshall, Selmer, Simms Watts, Traynor, Carlesbro, Roost, Maurice Placket, Burns, Trace Elliot, Laney, Kelly, Holden, Matamp, Burman, Vox, White Power, Vampower, Orange, Electric Amp Co, Park and Bird to name but a few.
    Its a little harder to power soak an svt but remove the two outer Power tubes and set the motherload elemental to special order two ohms, for a close enough for jazz match but erring on the side of caution tube voltage spike wise, use the four ohm out from the amp.
    Rick the designer at Sequis will make you one of the pro motherloads to any spec tube amp you got all the tubes in, but it will be twice the money of the elemental model that will do well anyway with just a pair pulled.
    My dual Channel pro model motherload will hang onto a dimed 4 KT88 VHT Pitbull doing downtune death metal guitar for an eight hour session and isnt even getting warm to the touch.:bassist:
     
  4. Thanks for the info, but a $500 power soak just isn't really an option....for me anyways. Just out of curiosity through, why would one use a 16ohm dummy load, yank tubes, etc rather than just using either a 4 or 8 ohm dummy load? Can they not handle 200 watts?

    edit: I wait, I see they can only handle 100w amps.
     
  5. Selta

    Selta

    Feb 6, 2002
    Pacific Northwet
    Total fanboi of: Fractal Audio, AudiKinesis Cabs, Dingwall basses
    They cannot.
    Also, I have actually owned or used to an extent most of the amp companies you listed. Not a one came with 16 ohm taps on their all tube bass amps from factory. Additionally, I know for an absolute fact that the SVT does not have a 16 ohm tap even on the transformer itself.
    If you wanna go ahead an pull pairs and play that game, go for it. Will it work? Probably. Are you risking anything in doing so? Oooooh yeah.
     
  6. Bassmec

    Bassmec

    May 9, 2008
    Ipswich UK
    Proprietor Springvale Studios
    A Fearful 12 6 is a superb cab for a high fi low sensitivity cab
    at circa 95dB I watt 1 Meter.
    Of course as there is no driver overlap there is no inherant two driver coupling but 3 db is available from the next Fearful speaker cab you get wich will get you about the same spl 1 watt one meter as a single JBL 2206 12" in a properly tuned box.
    If you had old ampeg 2 X 15 metal badge cabs with the altec 418 alnico drivers you start at 103 DB spl I watt per driver so your 2 x 15 will do about 106 db at 1 watt which means that you only need six Fearfuls to get the same level of sound at 1 watt.
    Or of course three watts will ballance two 12/6 fearfulls against one watt with an old school 2 X 15.
    Its all just maths.
     
  7. You lost me at "I watt 1 Meter"...
     
  8. Selta

    Selta

    Feb 6, 2002
    Pacific Northwet
    Total fanboi of: Fractal Audio, AudiKinesis Cabs, Dingwall basses
    I believe he meant 95db @ 1W @ 1m
     
  9. Bassmec

    Bassmec

    May 9, 2008
    Ipswich UK
    Proprietor Springvale Studios
    I said use a very common 16 ohm tapping on the output transformer of your amp to feed an 8 ohm motherload.
    But in your rare case you have no 16 ohm tapping so you would have to use 8 ohms and a four ohm sequis elemental power soak with two of your four tubes pulled. The pro version will handle 200 watts or more at 8 ohms to special order but its much larger 2U 19" and very expensive.:bassist:
     
  10. Bassmec

    Bassmec

    May 9, 2008
    Ipswich UK
    Proprietor Springvale Studios
    The facts are almost every amp but the Ampeg svt and fender bassman did offer 16 ohm taps, this is what Marshall say.
    <There are other Marshall 'quirks'. Have you ever thought about the 16-ohm speaker impedance? This is really a Celestion quirk, but one story I heard is that the British engineers had a problem with the losses in the PA systems of the day. People were using cheaper light gauge wire for the speaker leads. These thin wires, when ran over a considerable distance, offered resistance. You now have a voltage divider between the wires and the speakers. Since engineers couldn't convince people to use proper speaker wire, they adopted the use of 16-ohm speakers. How does this help? Assuming a rather high 1-ohm resistance in a very long speaker wire, in series with a 4-ohm speaker load. You now have a 20% loss of power output in the wire (1-ohm is 20% of the total resistance of 5-ohms). With an 8-ohm speaker the loss was reduced to about 11%. With a 16-ohm speaker, the loss was down to under 6%! (Of course I will get plenty of Email stating that the speaker impedance is NOT constant, or even a DC measurement. These are just mathematical numbers for the sake of argument; the same as probably used by the engineers.) We don't use long speaker leads in our guitar amplifiers today, but the thinking has stuck with the engineers.>
    It wasnt until the 1960 switchable cab that a marshall 4 X 12 was offered as anything other than 16 ohms.
    in fact it used to be a nominal 15 ohms back when hendrix was a lad.:bassist:
    PS Hiwatt 200w Bass Head
    These amps are designed to the same specification and construction as the DR201. 4 x KT88s, 3 x ECC83s and an ECC81. Original Partridge design transformers. 4, 8 and 16 ohm speaker outputs into 2 x jack and 1 x speakon outputs.

    Dimensions for Head (Width x Depth x Height) and Weight (kg)
    700 x 285 x 290 35Kg


    Hiwatt 400w Bass Head
    These amps are designed to the same specification and construction as the DR405. 6 x KT88s, 4 ECC83s and an ECC81. Original Partridge design transformers. 4, 8 and 16 ohm speaker outputs into 2 x speakon outputs.

    Please note these amplifiers are custom built orders only!
    Neither Laney or Orange offer 16 ohms anymore on the Nexus 400 or AD200 but thats only a very recent ommission.
     
  11. Primary

    Primary TB Assistant

    Here are some related products that TB members are talking about. Clicking on a product will take you to TB’s partner, Primary, where you can find links to TB discussions about these products.

     
    Apr 11, 2021

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