Purpose of tubes in hybrid preamps?

Discussion in 'Amps and Cabs [BG]' started by alexclaber, Jun 19, 2003.

  1. alexclaber

    alexclaber Commercial User

    Jun 19, 2001
    Brighton, UK
    Director - Barefaced Ltd
    I'm asking this because I've noticed that changing the single tube in SWR preamps can radically change the tone, whilst in Eden preamps it makes little difference. Why is this, how does the Eden use the tube?

    How about the other hybrid preamps out there like Ashdown, Ampeg, Carvin, Mesa Boogie, etc?

  2. Paul A

    Paul A

    Dec 13, 1999
    Hertfordshire U.K!
    Dunno, it SHOULD make a difference.
    The only thing I can think is that on something like Trace Elliot you can "blend" the tube stage with the solid state pre amp.Turning the pot fully one way or the other you get full on tube or full on solid state.
    Maybe the Eden has something like this that was set right over to SS?
    Or maybe it's just broke........
  3. alexclaber

    alexclaber Commercial User

    Jun 19, 2001
    Brighton, UK
    Director - Barefaced Ltd
    I think it's something to do with the gain structure of the preamp, how many volts the tube is running at, and how much of the tube flavour is changed by the S/S circuitry before or after it. However, Eden and SWR's designs look fairly similar from the outside in terms of knob layout, where the tube lives in the signal chain and their overall ethos - hence my confusion...

  4. saqle


    Jun 18, 2003
    How much $ does it usally cost changing the tube in a preamp like ashdown.
  5. The non-linear gain attributes of a tube preamp add harmonics to the signal chain, "colouring" the sound with these haemonics. The most common description of these is "Warmth". How much gain the pramp tube adds depends on the passive components in the circuit that set the tube's grid, plate and cathode voltages. Depending on where these are set, the amount of non-linearity (i.e. distortion) is affected.

    Thus the same tube can colour sounds differently depending on the gain level it is operating at.

    Properly designed solid state preamp circuits tend not to add any harmonics because of the linear nature of transistors through their designed operating range

    It's been a few years (i.e. 25+) since my circuit design classes at university, so hopefully I am not adding too much inaccuracy to the discussion. I'm sure one of our resident engineers might add more to this, or correct any unintended inaccuracies. :D
  6. brianrost

    brianrost Gold Supporting Member

    Apr 26, 2000
    Boston, Taxachusetts
    Actually one of the big lies in the audio biz is that properly designed TUBE circuits should also not add any harmonics. Why? Because adding harmonics is a form of distortion! If you go listen to really pricey tube stereo gear you'll be amazed how much it sounds like...solid state!!!

    However, most guitar and bass gear using tubes is NOT "properly designed". We have gotten used to this coloration as "normal" over the years.

    Warmth = no treble :D
    Fat = poor damping at low frequencies :cool:
  7. I'll agree with you to a point on this one. A lot of what musicians are drooling about is nothing more than tube distortion. There are a few bass amps out there that operate at low distortion. My Eden VT300A actually has a distortion rating of .2%. It also has a "Overdrive" setting just to the get the distortion back up to a tube type sound. It's really a musical thing. If you are looking for low distortion just get a decent power-amp and use a studio quality EQ for a pre-amp. Trouble is, when you add a dash of tube distortion it will always sound better. jmho
  8. jobu3

    jobu3 Artist formerly known as Big Joe

    Feb 17, 2002
    Mountain Top, PA
    damn straight! :D

  9. D.I.Y. - tube can cost from $8 to $100.

    Plan on spending $12 for a preamp tube.
  10. I agree with you here Brian. The difference, of course, is that high end tube gear is extremely expensive to "design properly" because of the need to eliminate the non-linear nature of the cathode / grid /anode physical behaviour with relatively complex compensation circuits; or by using only a small amount of the gain range of the tubes, and stack the stages.

    Transistor behaviour over standard operating ranges is inherently more linear, therefore less THD.

    And so we like the right amount of distortion as long as it is pleasing to us - yes, we do!!:eek:
  11. KB


    Jan 13, 2000
    Chapel Hill, NC
    My understanding of the Eden vs SWR tube usage is that Eden buffers the tube in some way that SWR doesn't, thus different tubes in Eden heads don't have as great of impact to the tone as they do in SWR. It also may have to do with the clarity of SWR vs. Eden. SWR has a slightly clearer tone which also may show a more dramatic change with tube differences.