Converting tube PA amps to instrument amps

[El Jefe]

I just picked up a couple amps for $100 on craigslist. The first is a Knight KN3050a. It's a 2xEL34 amp, tube rectified, with three 12AX7's in the preamp. I'm thinking of doing a 5F6a preamp and lowering the negative feedback, I don't really think it's going to need much more than that (other than changing the I/O to 1/4") but correct me if I'm wrong.

The other one is a Bogen M120. I read that this used 8417 output tubes - those are out of the question. I would really like to convert this into a bass amp. I'm pretty sure it can be modded to use 6550's or KT88's, but I'm a total novice with electronics - itching to learn though! Any particular things I should do to make it sound good for bass guitar? I'd like it to have just a volume, treble, and bass knob. I don't need any other controls. Here's a pic of the chassis:

http://i178.photobucket.com/albums/w271/Loags/IMG_5936.jpg

[Shiny McShine]

How cool. I used to use a Dyna Preamp into a solid state Dyna power amp into 4 X 12 Celestion's and it was just shimmering sounding.

[Structo]

Yes people have successfully converted old PA amps to guitar amps.

But you have to know what you are doing unless you completely convert it to a known design.

Probably best to install new filter caps.

The bypass and coupling caps also shape the tone.

But you may get real luck and just install a 1/4" jack and have it sound good.

But usually the input impedance on these amps, (since they were made for microphones) won't be correct for a guitar.

[El Jefe]

They both have mic inputs (with transformer sockets), and I think they both have RCA inputs for other sound sources. I haven't tried the knight yet but I could hook it up to my CD player to see if it actually works.

[donnyjaguar]

Guessing the Bogen could be made into a decent guitar amplifier. The output tranny doesn't look like it could serve up 100W to me though. So thinking a pair of EL34 driving it might be more practical. Unless the internal windings of the transformer are 90° offset I'd rotate one 90° to get rid of hum.

[El Jefe]

The bogen was designed to be a 120w amp, so I can only assume it really will do it.

[donnyjaguar]

It doesn't sound like you want my help. Good luck with your project.

[El Jefe]

It doesn't sound like you want my help. Good luck with your project.


It's not that, I'm just hoping it'll make a good bass amp. I agree with you about the size of the transformers, and it is strange that they're lined up close and in the same plane like that.


As for what Buck said, that sounds really promising. A quad of those tubes will cost a fortune though. Maybe I could mod just the inputs and outputs and make it run with a quad of EL=34's....