Quieten the Express

[martin manning]

12AZ7 has the same mu, ra, Gm, and Ia at -2V Vg as 12AT7, but filament current is 450mA vs. 12AT7's 300mA.

Using 12AX7's typical 100k Ra, fully-bypassed 1k5 Rk, the stage gains are:

12AT7 33.9 dB
12AU7 23.3 dB
12AX7 35.6 dB
12AY7 29.3 dB

Using that Ra and Rk, 12AU7 has a poorly-centered and very low-current bias point. The 12AT7 is centered a little better, but still at relatively low current.

[donzoid]

Martin,

Looking at the specs, the aY7 is doing what I envisioned from the 5751. In other words it sounds like the aY7 is doing exactly what I wanted from that just a little bit less crunch and grind.

Fenderman - YES its got a fat midrange and very smooth. In fact after deciding to go with the aT7, I had changed the 500pF to about 380pF and they are polystyrenes...with this tube if I stay with it, I may go back upward a bit but I think just raising the presence and treble may do the trick. So far, a fun trial.