bass amp conversion / build

[Leif H]

I'm looking to convert my Yamaha 115B to tube, preferably I'd like to do something just for jamming.

I was thinking of hunting goodwill for a PA, but not sure what exactly what I should be looking for. My thoughts are to find something in the 50w range.

is this easily done, or am I nuts?

[loudthud]

The conversion you suggest is a lot of work. Mostly in cutting the required holes in old chassis. It would be far easier to mod an old PA to be a bass amp and just set it on top of the Yamaha and use it's speaker. That would be a good first step anyway just so you get used to working on tube amps.

[Leif H]

Thanks for that loudthud, I was thinking looking at the beast, and going through the possibilities, and thinking about the spill on the controlls factor.
I'm going to start with pa, as you said, turn it into a head, and use the the cabinet for the speaker

I'd like to do this for both bass / guitar.

The more reading I do, I'd like to be able to do a High gain and a low gain input.

I read that you can cascade the two inputs for high gain and have a second input that uses only the second two pre amp tubes for the low gain which would be for "normal" or bass?

Cheers,

Leif

[loudthud]

The more reading I do, I'd like to be able to do a High gain and a low gain input.

I read that you can cascade the two inputs for high gain and have a second input that uses only the second two pre amp tubes for the low gain which would be for "normal" or bass?

Bass guitar usually doesn't require as much gain as a highly distorted guitar. Look for a Marshall 2203 schematic as an example. The problem is that if the tone is optimized for guitar, it's not very good for bass. You'll have to compromise.

[Leif H]

Bass guitar usually doesn't require as much gain as a highly distorted guitar. Look for a Marshall 2203 schematic as an example. The problem is that if the tone is optimized for guitar, it's not very good for bass. You'll have to compromise.

It looks like I'm in possession of a bell & howell film projector filmosound amp with one 5879, two 12AX7's and three 6V6's, from what I can discern, this is similar to a tweed deluxe. One of the 6V6's doesn't need to be there, and it also has an optical tube that isn't needed either.

Apparently, this is a plug & play 15W guitar amp, with a decent tone.

I'm hunting for schematics, as I kind of like the micro size head. I'm thinking of making a black screen top cover, kind of like a MacIntosh stereo amp.

If I have the sockets (not sure what the photo uses yet) but if it's octal could I use a quad of 6V6's and power the Yamaha's OT? Or is 15W of tube going to be lots running that 15" speaker?

[loudthud]

The Yamaha, being solid state, doesn't have an OT. Even if you acquired a bigger OT, the Filmosound's Power Transformer probably couldn't supply the extra juice needed. The economy gained in doing a conversion is in having a transformer set that is pretty well matched and a chassis with most of the holes already punched. You really can't do much beyond a preamp mod and filling the available space with pots. In some cases you could add Reverb, but not all. Using the Yamaha's 15 inch speaker won't be ideal for guitar but it will work until something better is available to you.

If the Yamaha is one of the ones I'm thinking of, repair should be fairly inexpensive. You can probably find the problem with a Digital Volt Meter and the parts only a couple of bucks.