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Supplies of triode/pentodes, like the 7199, 6GH8, etc., have already vanished or will vanish soon. :>(( To date, New Sensor has shown no inclination to do something about the matter.
Any number of designs use this sort of tube for a voltage amplifier and a DC coupled "concertina" phase splitter. Perhaps an EF86 in combination with a ZVN0545A MOSFET, both of which are in production, can be used to keep equipment functioning well.
Constructive criticism of the idea is welcome.
Eli D.
Follow Ups:
Some info on triode pentodes.
Cheers,
Ian
The "gravity" of this situation depends highly on who you are and what you're attempting.
As a collector/hobbyist, I've had absolutely no problem finding the common NOS triode/pentodes without resorting to epay or dealers. In the past 12mos I've picked up about 100 of these types at flea mkts and elsewhere for about $1.00 ea including 6GH8, 6U8, 6AN8, 6EA8, 6AW8 & 6BL8 (no 7199).
OTOH......If I was planning to mfg something and needed reliable stock for OEM and service purposes, the situation would be very different. In this case I'd probably take the route you suggest AND stick only with tubes having guitar amp applications.
Yep, the MOSFET is amazing in the number of uses to cheaply improve how you can implement tube circuits; MOSFET source followers and pass units in regulators do for a dollar better than what a tube can do and they don't create sonic penalties in those positions. I like to build great sounding equipment, not 1920's reproductions. Tubes are very good voltage amplifiers, way better than solid state.
Possibly EF86 with a 6C4?
Your idea has possibilities.
Triode Electronics makes a replacement ST70 driver board with two EF86 and one 12AU7 replacing the pair of 7199s. The two triodes in the 12AU7 are split between the channels to provide the concertina splitters. It can take a 12BH7 or ECC99 there too, and there is a version that accepts 6922, 6CG7 etc as well as some other options.
Of course, this only helps in the specific case of the Dynaco ST70. For other 7199 amps where the original sockets must be used, but rewiring is possible, I might suggest a dissimilar dual triode such as the 12DW7 or possibly 6DR7. The high-mu section will still have less gain (by about a factor of 3, or roughly 10 dB) than the pentode in a 7199, but that can be compensated by using less global feedback (which may sound better anyway.)
In addition to what Eli posted - 6C4s are not in production either, and many of them are excessively microphonic.
Take a P2P unit, like Scott or Sherwood made, and you encounter the issue of a single Noval socket. I'm thinking in terms of retaining, more or less, the OEM circuit topology and not modifying the chassis significantly.
Ask around. You'll find that quite a few enhancement mode MOSFETs are very transparent, as long as they don't amplify voltage. The key to selecting a MOSFET for voltage follower or "concertina" phase splitter duty is low and stable reverse transfer capacitance (C RSS ). The C RSS of the ZVN0545A is tiny and wimpy tubes, like the EF86 and 12AX7, drive it with ease.
Eli D.
Yet another reason to build our own gear. :)
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Buy Chinese. Bury freedom.
Edits: 10/10/14
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