parallel triode question

[Rick]

Whether or not a parallel triode stage has more gain than a single triode stage depends on the rest of the circuit.

And don't worry about losing any 2nd harmonics - the signal just doesn't get big enough on an input stage to see that effect.

If you have a typical input stage, want to keep the same frequency response and voltage swing on the output, and just get the advantage of lower noise, you'd need to:

1) Cut value of grid stopper in half (since input capacitance doubles with dual triodes, halving resistance keeps same freq response).
2) Cut values of plate resistor and cathode resistor in half.
3) Double value of cathode bypass cap.

So a center-biased 12AX7 parallel input stage (in typical range of preamp B+ values) might use a 10k grid stopper, 51k plate resistor and 750R cathode resistor, and would be fully bypassed with a 44uF cathode bypass cap.

The lower noise (higher SNR) comes from using smaller resistor values in the circuit.

This also has the benefit of producing lower output impedance, which will mean less loss if a tone stack immediately follows the parallel triode stage.

Thanks!

[stretch2011]

Something I'm experimenting with right now is paralleling not the first stage, but the tone stack driver. That way even though the gain its self isn't all that much, the drive current is a lot more, therefore more gain.

[gingertube]

Search this forum using the search term Junkbox to find my Junkbox Trainwreck schematics.
I run a parallel triode input BUT with the two triodes at different operating points for some harmonic emphasis.

The use of 2 triodes paralled give you a root 2 improvement in signal to noise.

Cheers,
Ian