what do you think

[mark enger]

i have a magnavox amp out of a hi fi it has 4 6v6 2 12ax7 2 5y3gt tubes 1 large trans 3 smaller it worked fine im worried about shock im new to amp building (very new) is it a decent start for a amp where can i get plans or schmatics for aproject like this any sites or books i would like to do this in a realistic way

[Chuck H]

That would probably be a very good platform for a guitar amp. If you have the model # of the amp you should try to find a schematic for it. It will help your process. Do you hope to build the amp in the existing chassis or gut it for parts? And what kind of tone are you after?

Having said that, Let me say this.

Getting started in amp building by building an amp from basically scratch is a big leap. Most people that end up building amps started with mods, built a couple of kits or have an electronics background in some other area. The problem is that if someone here told you to 'check the primary impedance on the output transformer' you'd be lost. You don't know what it is or what to do at all. That level of inexperience will make it nearly impossible for anyone here to assist you. But if you built a kit the process would aquaint you with the parts and componants. A kit will most likely come with some safety basics (or at least warnings that will clue you in). You would also have simpler questions along the way that would help aquaint you with the jargon and terms. Plus you know your going to get a good sounding amp out of the work your doing. In the end you would have a great amp and be much more prepared to build another amp from salvaged parts.

Chuck

[stokes]

Converting the amp from a hi-fi to a guitar amp would be a good way to start learning about amp building.As Chuck pointed out getting a schem for what you have is necessary as a first step.Getting some books on basic amp circuitry is also a must. http://www.pacificrecone.com/JackDarrBook.html this is a good book to get you started.Modifying a hi-fi amp is pretty basic.Hi-fi amps are intended to be very clean,where a guitar amp is not.So your basic mods would be to increase the gain and distortion levels in the pre and power amp sections.Compare what you have there to a 5E3 and you will likely see some similarities,if you can get hold of one of the 5E3X2 schems from one of the kit builders it would probably be very close to what you have.The 5E3X2's are basically a 5E3 with 4 6V6's instead of 2.

[Chuck H]

Well...This is the best I could come up with. It's a cut and paste from another forums post. It's not complete. I created an account with the site and when my registration goes through I'll try to contact the poster and see if I can get a complete schem.

The magnavox 175 is a stereo amp good for about 15 watts per channel. Could be fun. You could run it into two 12s, one from each output. Then use a switch to choose one or both outputs for 15 or 30 watts depending on the venue.

Lost of extra console/ multi componant/ switchable input hi fi crap in there. It might be best to rape it for parts and build on a new chassis or just gut it and start from scratch.

Cheers

Chuck

[loudthud]

IMHO the cool thing to do would be get the amp working as a stereo amp and listen to some CD's. It probably needs a good cap job and doing that will help get you familiar with the amp and it's inner workings. Once that's done you can mod it to a guitar amp.

[guitronics]

Old Hi-Fi Amp to Guitar Amp Conversion

IMHO the cool thing to do would be get the amp working as a stereo amp and listen to some CD's. It probably needs a good cap job and doing that will help get you familiar with the amp and it's inner workings. Once that's done you can mod it to a guitar amp.

I agree,use a voltmeter to identify the High Voltage wiring! Get the thing working first. I'd use a 40 watt light bulb(Standard incandescent) in series with the Hot (Black) side of the amp power input(this will limit the current, and if you're lucky,the cap's will reform)...make sure you have speakers wired in(both channels) for a load.

Put the whole chassis on a piece of wood,and don't touch ANYTHING with your fingers/body.The Voltmeter should be set at 500v (or more).

Leave the power switch in the "On" position before you use the Plug to power it up....so you're not touching anything.

Any problems...just unplug it. Do not touch the chassis.

Once the light goes down in brightness, let it stay connected for an hour.

Only then switch it over directly to 120v. AC.(Direct to outlet,no light bulb).

If you don't feel comfortable working around high voltage, DON'T.

Maybe you can get someone who knows a bit about tube circuits to assist you.

[Chuck H]

I'll go you one better. I found the partial schem on a hi fi geek page (not at all like our guitar amp geek page ) and there was some opinion that these old Magnavox units were really very good sounding. In that case you may just want to use it as a stereo.

Chuck

[mark enger]

thanks for all the replys the amp works fine i was thinking just put a 1/4 jack on it and safe from shock and run a hughes kettner cream machine as a pre see how it sounds i been reading about tube amps and i cant make out a schematic or make sence of it id like too get a kit just to see what all is in a tube amp.maybe find a simple plan for the simpliest of circuts anyone know of one let me know with a parts list i mean i can just buy a kit but almost every thing i need is in magnavox already so its alot of thought (the amp needs a grounded cord for sure its just a two wire thanks for the sch# it is a good start i now need to do little homework theres a cool looking mag bassmen on frizzles amp site check it out well thank ive got a good start ill get busy on it anymore info would be great

[cubby01]

Mark coincidently I posted a couple days ago in another thread about using an old maggie amp. The thread is http://music-electronics-forum.com/s...ead.php?t=9170

My amp chassis is an AMP 128 (128-D I think). Mine has two 5y3, four 6v6, and a 12a?7a on it. There is a yahoo group called Magnavox Friends that has some schematic files and people that can help understand these amp chassis. I think I'm going to rewire mine as a 5E3x2. I think I'll only need to drill one socket hole in the chassis and of course all the holes for controls and jacks but those are fairly easy.

Which amp chassis do you have? Did I miss that in the thread. It should be printed on the side of the chassis.

[rathmann]

I love these old Magnavox amps. The first tube amp I ever modded was one of these 128 models. It was my experimenter's testbed for pretty much all the mods commonly discussed here - triode/pentode switching, cathode/fixed bias switching, switching to turn off two of the power tubes or one of the recto tubes. I rewired the paraphase driver to concertina splitter and a gain stage. I did a bunch of feedback loop experiments with presence and resonance circuits as well as switching different feedback resistors. Let's see, there were also number of changes to the power supply - general safety stuff like replacing the cord, adding mains and B+ fuses, beefing up voltage and capacitance capability of the PS, and various filament circuit updates.

It's nice for a beginner to have such a clear distinction of the separation of preamp and power amp - the preamp is nowhere to be found! (well, it's a completely separate unit.) Being able to focus on and learn about the power supply, heater supply, and power amp sections was made a bit easier without the daunting complexity of various preamp circuit. (Just by visual comparison.)

This is still one of my favorite amps to play through (using a halfer triple giant preamp). It's loud enough to hold it's own in a jam situation and has a very sweet breakup when pushed. The only issues I have with it is that with the 4 6v6s it wants to see 2ohms, not a huge problem since I have a couple of cabs that can be connected in parallel, but it certainly limits what I can run through.

I also have two of the stereo power consoles that Mark is describing. One is in the house as the main stereo amp, the other had been the garage CD amp but is currently on hiatus awaiting some kind of conversion to a guitar oriented thingamajig.

RG's article here: http://www.geofex.com/article_folders/old2new.htm was an extremely valuable resource. Lots of great info elsewhere at RG's site as well as at Randall Aiken's: http://www.aikenamps.com/.

You might also get a copy of Dan Torres' "Inside Tube Amps" which is a pretty good primer for a musician who wants to get into the electronics of his gear. (There are many opinions about Dan and his style/ideas, but I still think this is a great book for starters.) I also wound up with a copy of "How to Service Your Own Tube Amp" by Tom Mitchell at one point. Again, this is more about the practical aspects of amps - biasing, speaker cab wiring, troubleshooting and such, but it served as a good cross reference with Torres' book. I also had Aspen Pittman's "The Tube Amp Book" which also has some technical info, and history of popular tube amp manufacturers in the "glory days", but at the time was mostly worthwhile because of it's collection of schematics and tube spec sheets, although what enabled him to publish those at the time (early 90s) is also what makes all those schematics and tube specs available online now. Other essentials are an early copy of the ARRL Manual or the Radiotron Designer's Handbook. And although things don't get much more theoretical than the RDH, Kevin O'Connor's books (beginning with The Ultimate Tone) are definitely more advanced and theoretical but are written with a very approachable style with many practical applications in mind.

And always remember: Probe/check voltages with one hand in your pocket.

[Chuck H]

Mark, Heres the complete schem. It came in two pieces so I had to cut and paste them together. Luckily the photos were aligned enough to make it work.

It looks to me like there are two things that make turning this into a guitar amp a little problematic. First, the layout is less than ideal. The chassis is built to run as two phase inverters and two power amps. So there's no "preamp" and no place to put a preamp that won't be really close to the output. Second, each power amp wants to see a 4 ohm load. So if you paralleled them to the same speakers/s you'll need to make it a 2 ohm load, which is a little odd and cumbersome.

I thought it might be a fun idea to make it a 15 watt amp. If you pull one of the OTs (leaveing the one closest to the power tranny) then it would be easier to do a good layout with one 12ax7 as a preamp and one as a phase inverter to drive the power tubes. That would need a 4 ohm speaker load. Then if your feeling confident, you could use the other OT to build a powered cabinet that could be run from a line out on the first amp. Full stereo effects...Hmmm.

Chuck

[guitronics]

Magnavox Tube Screamer

Lookin' at the schematic....it looks like a good candidate for a tube (guitar)amp.

Two things this will cost you: Time, and Money.

If you are willing to invest both, you will learn a great deal.

If I were you, I would make up a new chassis (Bend it yourself) and add the parts to make a Fender Champ Clone.

>>>But, I would not attempt this as my very first electronic project....<<<

Like the man said, refurbishing it to play CD's while I tackled some simpler circuits (Effects Pedal?) would be cool.Maybe you already HAVE some electronics' building/repairing experience....like good, shiny solder joints.

There's a ton of Effects circuits and mods on the 'net.

[cubby01]

As someone above said it looks like the beginnings of a 5E3X2 (Fender Tweed Delux modded into a twin) but Don't know that I would tackle that as a first project, though some have.

If I were you, I would make up a new chassis (Bend it yourself) and add the parts to make a Fender Champ Clone.

>>>But, I would not attempt this as my very first electronic project....<<<

Totally agree with the first statement here but if its not a first electronic project but maybe just a first tube project then go for it. A Champ clone built upon a gutted console chassis was my first tube project. There are abundant Fender Champ clone aka 5F1 sites around with good schematics, chassis layouts, parts list, and pictures. What you need is a decent meter, soldering iron, a bit of practice and, >>>most importantly understanding of potential HV dangers and good safety practices<<<. Several good forum sites like this one can help by answering questions you'll run into.