Which Ampeg B15 for standalone preamp?

[andrew_k]

I want to build an external preamp to accompany a 100W KT88 guitar amp I recently finished, and have decided upon the Ampeg B15.

My question is, which schematic should I take the preamp from, and which of the two channels is best suited to bass?

Thanks

[loudthud]

The two main variations among B15 preamps is the location of the volume control and tone network in relation to the gain stages. The later designs moved the volume control between the two gain stages and the tone network to the output. This probably gives the best headroom. Look at some of the other Ampeg designs and they are all pretty similar except each has its own variation of the Ultra-Hi and Ultra-low switches. Look at the B18, B25, B25B, V4 and SVT. I would start with the basic B15N and tweek on those switch networks to find what works best with your bass, amp, speaker and playing style. If you feel good about using the Octal tubes with somewhat limited availability, you can go that way or use the 12A?7 route. Build a single channel but plan for two tubes so you can use different types on each gain stage and possibly add a voltage follower on the output to drive your power amp.