by Robert Cham » Mon Jan 13, 2014 5:52 pm
Conrad Johnson has been using what they call composite triodes for decades. They do line level preamps using them in a single gain stage with no feedback. The low output impedance (500 -800 ohms) allows them to do away with the cathode follower that they would normally use to get the output impedance down. They use between one and two 6922s or 6N30s per channel with their sections paralleled.
I've wanted for some time to emulate their ACT2 preamp, without such silliness as the remote controlled volume and input selection functions. Most of their capacitors are custom $240 each, 2 microfarad teflon units, even in the power supply. The unit costs something like $16K in it's present incarnation, but I think that it should be possible to duplicate it's basic design for a whole lot less, especially if one uses good, but lesser capacitors.
I'd want to keep the separate regulators for each channel as SS experiments back in the '70s convinced me that reducing the co-channel crosstalk using separate regulators dramatically improved the stereo image. As I'l be using 2 6N30s per channel and each tube draws 830 ma of filament current, it probably makes sense to use separate filament transformers for each channel as well.
Part of my reason for doing this is my old but great Tympani 1Cs. I'd like to bi-amp them with Dyna stereo 70s, one for each channel. I want to use Nelson Pass's phase coherent passive line level crossovers to drive the Dynas. The crossovers require a preamp that can drive a 5K load, The paralleled 6N30s should do it. If they don't then I'll have something interesting to sell!
While I've installed and maintained tube FM transmitters as a part of my job for the last 30 years, I can't claim to be a tube design engineer. SS is so much easier, but I don't like to listen to it at home! So, any help in design would be most welcome.
BTW, reversing the normal path, I've moved North from Texas to central Virginia to retire, build my geeky engineer's house, raise grapes and orchards, and produce various potables.
TIA
Bob Cham