I'm finishing up a design here: https://123d.circuits.io/circuits/852190-jan-6418-vaccum-tube-valve-preamp which is a preamp for an instrument.

First, let me know if anything stupid pops out at you with this circuit, I'm not an EE :(

Second, the original design I'm modifying heavily is here:http://s13.postimg.org/patw5krxj/k270.png

Here is my current progress: enter image description here

The original design calls for T1 to be a BSN254A, which is no longer available, so I need to concoct a replacement circuit. I think a mosfet was selected originally because it's voltage, not current, driven and doesn't put a load on V2.

I don't have the faintest clue how to select a proper replacement mosfet and how to calculate the values surrounding it. Where do I start?


Since this is a guitar stompbox project, supply voltage is 9v which is fed at v1 in the top right. My bad, I'll mark that on the schematic. Hoping to make this a PCB people will actually use to give themselves a nice warm tone with their guitars and mikes someday :)

  • \$\begingroup\$ What's the plate supply on that circuit? The original MOSFET (incorrectly shown as a JFET in one of the schematics) is rated at 250V. \$\endgroup\$ – Spehro Pefhany Jun 7 '15 at 0:52
  • \$\begingroup\$ 9v supply, not labeled sorry. v1 would be b+ \$\endgroup\$ – Jonathan S. Fisher Jun 7 '15 at 1:39

The biasing of the vacuum tubes looks pretty suspect- I'd expect the cathodes to be isolated from the heaters and for there to be some cathode degeneration at DC (parallel RC). Otherwise you're very much depending on the characteristics of the particular tubes at the particular moment to set the bias point.

Anyway, a suitable MOSFET for the source follower assuming your relatively low B+ would be the Toshiba SSM3K15FS. It's got similarly low input capacitance, low Vgs(th) and similar other characteristics (except it's in an SMT package and is only rated for 30V).

| improve this answer | |
  • 1
    \$\begingroup\$ The 6418 is a subminiature filament electron tube with directly heated cathode. The cathode has +1.25V at one end, so even with 0V on the grid there is some negative bias. frank.pocnet.net/sheets/127/6/6418.pdf \$\endgroup\$ – Bruce Abbott Jun 7 '15 at 4:57
  • \$\begingroup\$ Thank you both of you guys... I'm kind of interesting how the filament and cathode interact with biasing, is there some basic stuff I should know about? I just found a 1.25v supply for the filaments and thought I was good to go.... Yep my b+ pretty low (9v battery, this is a guitar stompbox). I'll take a look at that mosfet. Still wondering how you figured that out... \$\endgroup\$ – Jonathan S. Fisher Jun 7 '15 at 6:54
  • 1
    \$\begingroup\$ For the interesting "basic stuff" on how filament and cathodes interact you'd need to read back to about 1930, paying attention to "portable" radios of the era. They did bizarre stuff like varying the heater voltage as a volume control (or RF gain) \$\endgroup\$ – Brian Drummond Jun 7 '15 at 8:40
  • 1
    \$\begingroup\$ For a one - off the practical consequence is that you may have to select the plate load resistor, especially on the second stage. I guess distortion isn't a huge concern, but you may not be able to get enough volume if the bias is really off. Brian's right on the portable radio specific stuff but it wouldn't hurt to download a copy of the RCA receiving tube manual, keeping in mind the 6418 is somewhat special. \$\endgroup\$ – Spehro Pefhany Jun 7 '15 at 10:49
  • 1
    \$\begingroup\$ For a stomp box, probably a ZVNL110ASTZ will be indistinguishable. TO-92. Important characteristics are voltage rating, Vgs rating, Vgs(th), input capacitance. \$\endgroup\$ – Spehro Pefhany Jun 8 '15 at 0:52

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.