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Hybrid amps with power tubes

Discussion in 'Amps and Cabs [BG]' started by jcburn, Jun 12, 2018.


  1. Hi everyone. This is merely a question out of interest. I own both a Warwick Quad VI and a Warwick Tubepath 5.1 - both magical sounding amps. These are hybrid amps - the Quad has 3 preamp tubes and the Tubepath has 2, however both these amps us a single EL84 in the power stage before the Mosfet power section. I think the same design has been used in the SWR Interstellar and the Mesa M-Pulse heads.

    Just wondering what the idea around this design is in regards to tone? Both my Warwick amps sound amazing, but are different. The Quad VI stays clean no matter how hard you drive the input gain, whereas the Tubepath gets quite gritty and gnarly when the input is pushed. both amps offer a way of either engaging or bypassing the power amp tube. It's most noticeable in regards to tone with the Tubepath.

    Any thoughts or opinions?
     
  2. mesaplayer83

    mesaplayer83

    Jun 27, 2017
    I gigged an M-pulse 600 for around 6 years - it had preamp tubes only, and a mosfet power section...
     
    rodl2005 likes this.
  3. My bad mate, I was going from memory on that one and got it wrong.
    Still interested in peoples opinions.
     
  4. BogeyBass

    BogeyBass

    Sep 14, 2010
    I would assume using a small power tube to emulate power tube saturation. Opposed to high gain pre amp distortion. Slightly different.

    Or if the tube directly drives multiple output section transistors. They might have needed a higher current tube capable of driving that many transistors.

    Without being negative the last tube id wanna hear is a EL84 in saturation. Blahhh. Be easier use typical 6V6 and interstage transformer and get the full beam tetrode overdrive and transformer harmonics
     
  5. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    Tell that to the AC-30 crowd... just sayin'...
     
  6. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    Actually, not so wrong. the MOSFET driver tube in fact operates as a small power amp, with both voltage and current amplification together. You COULD connect the output of this point to a matching transformer and drive a speaker at low levels. The circuit was originally designed with a 12AT/AU7, though some players preferred the looseness that the 12AX7 provided. I prefer the 12AT7 myself.
     
    ThinCrappyTone, SactoBass and pfschim like this.
  7. BogeyBass

    BogeyBass

    Sep 14, 2010
    12At7 is somewhat similar to 12Au7 as far as impedance it can drive. Im forgetting at the moment which is capable of driving lower impedances. Under high gain the 12at7 has a narrower bandwidth. Which can be downfall or benifit. In highgain distortion circuits its benificial.

    As i mentioned above 6v6 is good driver tube and was tube of choice for early fender reverb tanks. And required a interstage transformer. And brings up cost. Later on the 12at7 was the tube of choice for reverb drives. Had enough drive capability and reduced cost with no transformer.

    Without data sheets at moment id assume the narrow bandwidth of the 12at7 was non issue cause reverb tank is already limited. Think choice was gain and low impedance cabilitys.

    12au7 better choice for bandwith. Vox had a small push pull 2 watt amp and used 12au7 as output tube. If you drove the krap out of it id assume some players would prefer 12at7 for high distortion
     
  8. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    You won't get 2 watts out of a 12AU7, even with "perfect" impedance matching. I designed a 12AU7 push-pull power amp with multiple output tubes. While it was a great R&D project that lead to some interesting (and useful) data, it wasn't as practical as I had hoped. It did work as designed, good for about 1-1.25 watts per tube (pair of sections)
     
  9. pfschim

    pfschim Just a Skeleton with a Jazz bass

    Apr 26, 2006
    SF Bay Area
    I believe that this is also how the Walkabout operates ... with one of the amps 12AX7 stages acting as the MOSFET driver. Makes sense as the Walkabout is part of the MPulse family of amps which originally included the 360, 600 and Walkabout.

    as a side note, I have always wondered what separated the Mpulse 360 from the Walkabout (other than 60 watts).
     
  10. BassmanPaul

    BassmanPaul Gold Supporting Member

    Aug 25, 2007
    Toronto Ontario Canada
    Many of the MusicMan amplifiers were solid state but had two or four 6CA7s in a grounded grid, cathode driven output circuit.
     
    rodl2005 likes this.
  11. RoadRanger

    RoadRanger Supporting Member

    Feb 18, 2004
    NE CT
    My Ashdown CTM-30 sounds great "pushed" :) .
     
    bonin in the boneyard likes this.
  12. Yep. You beat me to it. Except the MM HD130 that's at my rehearsal space has 4 6L6 power tubes. A. S.S. pre amp, 4x6L6 power amp. As far as I know, it ain't my amp, but it sure sounds cool thru the EV TL606 1x15 it's plugged into
     
    mpdd and SactoBass like this.
  13. jnewmark

    jnewmark Just wanna play the groove. Supporting Member

    Aug 31, 2006
    Stax 1966
    Play guitar.
    I had an HD 150 and it also had 6L6's.
     
    rodl2005 likes this.
  14. I think the idea was that it's the power tubes that sound good when pushed, more so than pre-amp tubes. And reducing the overall tube count lowers the cost of production. But then there's the whole marketing aspect of anything less than all-tube loses some cachet.

    Anyway, here's another example recently posted in another thread. The page is a bit cluttered, but it has some good background info on the reason for this design:

    AW99 Amplifier - Mahaffay Amplifiers Tube Amps, Tube Amplifiers, Amplifier Accessories

    I also believe the Ashdown CTM-15 (which I own, but have never opened) has an SS pre-amp driving a tube power section.
     
  15. Ahh, it might well be the HD150 in my space too, will check next time
     
  16. Hello Rod!
     
  17. BassmanPaul

    BassmanPaul Gold Supporting Member

    Aug 25, 2007
    Toronto Ontario Canada
    That AW99 seems to have pre-amp tubes but a SS output section much like many other hybrids.
     
    agedhorse likes this.
  18. Right, but it includes a small power tube driving a load, so sort of like the Warwick Tubepath or a Laney Nexus SL (the Hughes and Kettner Batt and the Freyette Valvulator do the same but without a big power amp).
     
  19. BassmanPaul

    BassmanPaul Gold Supporting Member

    Aug 25, 2007
    Toronto Ontario Canada
    Not according to the tube compliment. Pre-amp tubes only.
     
  20. seamonkey

    seamonkey

    Aug 6, 2004
    Warwick is great as they made the schematics publicly available at their site! :)

    One thing to note is the bypass for the output tube is not gained matched. With most things audio Louder sounds better. Amps/pre-amps should be gain matched when comparing them a/b. Just have to keep it in mind when comparing tube in or out and adjust the volume between switching.