Touch Sensitivity & SE Amps

[davent]

Hello,

I've built a little SE using a 6SJ7 and a 6AQ5 and was wondering how to go about altering touch sensitivity? Both tubes are cathode biased, the the 6aq5 is scaled ~30v-275v and the 6sj7 is running off a regulated 230v. Any ideas on what components are critical to influencing the sensitivity? Thanks for any insight!

dave

[Cliff Schecht]

It has a lot to do with how you design the power supply. For maximum touch sensitivity I do two things: Add a significant amount of resistance in the B+ dropping string and drop the screen voltage on the power stage pretty low. The large dropping strings cause voltage to sag more as more current is drawn which makes the preamp easier to distort. My favorite values usually add up to about 20-40k in the preamp and at least ~5k for the screen. For the screen, the lower the voltage is the more gain you will get out of the pentode. Past a certain point the screen voltage will obviously get too low and cause the amp to sound like crap but I usually go at least 100V lower than the datasheet max value. On a custom KT88 build I did for a friend, my B+ was around 530 and I dropped my screens to around 330-350V which not only made the KT88 easier to drive but added a lot of extra even-order harmonic content (i.e. one-sided overshoot) that made the amp sound HUGE for it's size. Lots of tweaking that one can do here, the real skill is knowing how much resistor-based sag to introduce without making the bass flabby and the amp a distorted mess.