Why do you want to use a crossover with this setup? What are you trying to accomplish?

sentenza wrote:I gots a '78 Bassman 135 and would like to get some crossover action happening with a 15" cabinet and a 4x10" cab. Does anyone know if a crossover can be installed, or if there are any standalone external ones?

You could try making a simple 2nd order one from a design from the net. There are good instructions on how to make them on the net.

If you have specs for your 15" speaker or your 10" spekers then this would be handy.

The crossover board could simply be mounted on the back of the sockets on your speaker cab or in a separate box with jacks.

Some questions come to mind:

1. What is the sensitivity of each cab? A 4-10 will likely be much more efficient than a 1-15. Unless you pad down the 4-10, you might end up with a weak low end.
2. Are the impedances of each cabinet equal? You don't want to place an uneven load on a tube amp.

I am currently constructing a 4-6 cab which will crossover with a 4-10 cab. I am using a first order crossover at 500 Hz. There is also a series notch filter to tame the resonant peak on the 4-6.

This kind of application requires parts that can handle high power so don't skimp on the qulaity of the parts. I am using Solen 400v fast caps, heavy gauge air-core inductors and Mills resistors. It's not cheap, but I know it will last.

sentenza wrote:Basically I'd like some more midrange whilst playing bass and figured that adding a 4x10 cabinet would help, and since the Bassman doesn't have a crossover, I'd need to build, buy or install one.

If youre satisfied with the low end from the 15" cab you have, you could experiment with adding a guitar cabinet into the mix. A 4-12 full of Celestions would help give you that Chris Squire sorta sound, little low-end, lotsa midrange growl, even without a crossover in front of it. It would, of course, be best if you could find a shop that would let you bring your amp and bass in and play around with cabs to see if that's the sound you're looking for.