Marik
Pro Microphone Design
Matti,
I wish there was a free lunch, but all the characteristics are tightly related.
Because of tubes construction the higher gain tubes usually have larger plate impedance, so we need higher transformer ratio.
What could help however, is a transformer with a high primary inductance, so even if there is some impedance mismatch, it will be absorbed.
Did you pay attention, when I was talking about the "stock transformer" I mentioned it has low inductance, so in order to get some LF response we need to use a low gain (meaning low plate impedance) tube, so we will go overdamped. On the same manner, if we get a good and healthy pri inductance we can go underdamped and still get a good response. I'd think something like 12AT7 might go down to 6:1, but it is hard to predict things without experimenting with particular transformer.
There are some neat tricks, like using CCS or choke loading, when the tube gain almost reaches its mu.
If you are desperate for the gain, the easiest would be running the first half of the tube as a gain stage, directly coupled to the cathode follower and then use a 2:1 or 4:1 transformer, depending on the tube.
There are also some small planar tubes with crazy transconductance, resulting in a high gain and quite low impedance, but they are scarce. I have a few I accumulated throughout the years, but I did not have chance to experiment with them, yet.
.