RCA Record player Amp Conversion?

[xbolanx]

Hey folks, thanks for letting me join your forums. Im a total amature at the tube amp game, but know enough that I can do some general stuff. Handy with a soldering iron. Given that I have a very old single tube amp with 1 25EH5 tube in it and thought it would make a very cool litte guitar amp, and a good starter project to enter in to the tub amp building world. First off is it possible to convert this small bit to a useable guitar amp?
And if so where do I start. Ive been unable as of yet to find schematics for it but could post a photo of the curcuitry
Thanks

[korey]

xbolanx, those little amplifiers were designed for use with a line level input(ceramic phono cartridge, CD, tuner, cassette) which is called a high level input. Guitar pickups and microphones have the exact same impedances known as low level inputs. You would need (for example) a separate pre-amp with a phono or microphone input so you could use a guitar with that amplifier. Otherwise, if you're electronic savvy, you would have to build one, There are all kinds of schematics all over the web!

[xbolanx]

Hey, thanks for the advice, I was actually planning on using a mic preamp in line with it. The Idea was to make the apm functional and then use the +/- wires that extend to the Stylus (needle) and installing an XLR jack. then using the XLR out on the mic preamp and the 1/4 inch to plug my guitar into.
Since finding this little project I have also come across an ancient GE bakelite tube radio and was going to attempt the same. if the results to either were favourable I might concider building a preamp curcuit instead of using the mic preamp. Can you recomend a schematic for that ?

Thanks alot for the responce

bolan

[6267]

you're going to have to remove the RIAA equalization circuit for proper frequency response. It'll probably be a RC network following the first gain stage...

[xbolanx]

Whoa, now you're talking a little out of my league

[6267]

Ok, sorry . What tubes are in this thing? (usually there's more than one). Second, be careful (!) and read http://geofex.com/Article_Folders/old2new.htm – this will give you a general idea of how to work a new amp into that circuit. However, some old amps (and yours sounds like it might be one of them, based on the 25v heater on the tube you mentioned) used wall power directly, without a transformer. These are dangerous and have killed musicians! It's not entirely easy, but they can be converted to much safer designs. http://www.tubedata.org will have the data sheets for damn near any tube you can find. I'd look up the ones you can find in there. You'll notice the first number of an american tube is how many volts the heater generally needs to heat up the tube. In you're amp, I expect that the heater volts of the tubes add up to something around 110-120, because that way the designers could just "plug it in" to the wall (by putting the heaters in series like christmas lights) and not need a separate transformer for the heater voltages. Sorry if I dove in a bit fast–it took me a while to figure out what in the hell people are talking about when it comes to tubes, but it eventually gets a lot clearer. Try googling "fun with tubes" for a great site that starts with basic explanations of amplifiers. It's a lot easier to work on these things than it seems at first. Just keep one hand in you're pocket when working with high voltage!

[xbolanx]

Hey man, thanks alot for the slow down LOL. The first of the amps i want to work on is a single tube amp. as near as I can find it produces 2.5 watts
the tube is a 25eh5. It happens to be a VERY basic looking curcuitwith one big capacitor and some resisters. there is also a transformer that acted as a powersupply to the turntable, and then two copper wires that lead to the speaker terminals. If you have an email address or use MSN, I could show you photos of the entire thing to give you a better idea of what it is

Thanks again, I'll browse the links you provided me with