Solid state tubes

Discussion in 'Amps and Cabs' started by scolfax, Apr 27, 2012.

  1. GuitarBrent

    GuitarBrent Member

    Messages:
    2,406
    Joined:
    May 17, 2004
    Location:
    Richmond, VA
    It be nice to be able to do this at least in amp modeling software right now...
     
  2. Trebor Renkluaf

    Trebor Renkluaf I was hit by a parked car, what's your excuse? Silver Supporting Member

    Messages:
    13,195
    Joined:
    Jun 9, 2006
    Location:
    InSaneDiego
    This. Came stock in Mesa Super 60s (Mark Is) in the '70s. I know 'cause mine had one.
     
  3. ekp

    ekp Member

    Messages:
    402
    Joined:
    Sep 2, 2003
    Location:
    Berkeley Springs, WV

    Actually, many years ago, there was a group who did try to get a direct replacement for a 12AX7 with a variant on a junction field effect transistor.

    Still earlier, Boogie used a device called a Fetron. This was a pair of junction field effect transistors (JFET) wired in a form of a series connection called cascode. The first JFET gate is the grid terminal and its source is the cathode terminal. Its drain is connected to the source of the second JFET. The second JFET's drain is connected to the plate terminal and its gate is connected to the cathode terminal. The first JFET is a high gain, low voltage device and the second JFET is a low gain, high voltage device.

    The FETRON's output characteristics has a resistance closer to infinite than vacuum pentodes. Consequently, they do not fully emulate vacuum tridodes.

    When I was interacting with the US Patent Office over my tube emulator circuits, this device was used as a reference by the examiner to disqualify my application. However, the FETRON was designed to replace a triode in a specific circuit used by the telephone companies and as a result, they could put off changing their systems to solid state.

    My design problem was simple, the JFETs that were available only have triode characteristics over a very small region. Otherwise they have the usual pentode style characteristics. Pentodes have completely different characteristics, which in guitar amplifier circuits can make a difference.

    As a consequence, I use an operational amplifier to provide the gain, which is then set by accurate passive components and circuits for emulating the grid conduction and plate characteristics. The plate characteristic emulators are so repeatable that over ten units the low order harmonics varied only one db and the higher order harmonics varied by about two db.

    Have a great day, Eric Pritchard
     
  4. ekp

    ekp Member

    Messages:
    402
    Joined:
    Sep 2, 2003
    Location:
    Berkeley Springs, WV

    Actually, many years ago, there was a group who did try to get a direct replacement for a 12AX7 with a variant on a junction field effect transistor.

    Still earlier, Boogie used a device called a Fetron. This was a pair of junction field effect transistors (JFET) wired in a form of a series connection called cascode. The first JFET gate is the grid terminal and its source is the cathode terminal. Its drain is connected to the source of the second JFET. The second JFET's drain is connected to the plate terminal and its gate is connected to the cathode terminal. The first JFET is a high gain, low voltage device and the second JFET is a low gain, high voltage device.

    The FETRON's output characteristics has a resistance closer to infinite than vacuum pentodes. Consequently, they do not fully emulate vacuum tridodes.

    When I was interacting with the US Patent Office over my tube emulator circuits, this device was used as a reference by the examiner to disqualify my application. However, the FETRON was designed to replace a triode in a specific circuit used by the telephone companies and as a result, they could put off changing their systems to solid state.

    My design problem was simple, the JFETs that were available only have triode characteristics over a very small region. Otherwise they have the usual pentode style characteristics. Pentodes have completely different characteristics, which in guitar amplifier circuits can make a difference.

    As a consequence, I use an operational amplifier to provide the gain, which is then set by accurate passive components and circuits for emulating the grid conduction and plate characteristics. The plate characteristic emulators are so repeatable that over ten units the low order harmonics varied only one db and the higher order harmonics varied by about two db.

    Have a great day, Eric Pritchard
     
  5. scolfax

    scolfax Member

    Messages:
    7,240
    Joined:
    Oct 15, 2008
    Interesting!

    I wonder when Jet City is going to start marketing/selling the retrovalves from the video above. Seems like they're already running later than they originally planned.
     
  6. Trebor Renkluaf

    Trebor Renkluaf I was hit by a parked car, what's your excuse? Silver Supporting Member

    Messages:
    13,195
    Joined:
    Jun 9, 2006
    Location:
    InSaneDiego
  7. scolfax

    scolfax Member

    Messages:
    7,240
    Joined:
    Oct 15, 2008

Share This Page