Magnavox Amp 175 project

[AaronP.]

Hi everyone! My name is Aaron and this is my first post here. I'm a relative noob to electronics work. I found my way here by first being interested in building and modifying guitar pedals, and then moving on to being interested in tube amp work.
I recently picked up a magnavox 175 amp for free. Here is all of the info for it. It's not the console. It's just the amp w/ 2 12ax7s and 4 6v6s.
http://www.chibardun.net/~kjkuen/Mag175.pdf

I've been looking over schematics for small tube amp for a few days and I finally tracked down the schematic for this Mag175. So, by looking at this schematic, what do you guys think I can convert it into for guitar use? I'd like to get a nice sound w/out having to totally gut this thing, but if I have to rebuild a lot of the circuit to guitar standards that's Ok too.

Keep in mind this will be my first tube project/conversion. Go easy on me.

regards,
Aaron

[Tubenit]

two output trannies

Did you notice that the amp schematic has two output transformers? Most guitar amps have one OT. I am not saying you can't make what you have work, but note that it will be different than most guitar amps.

When I get old Bogen amps that I convert to guitar amps, I gut them and install (inexpensive) paralleled terminal strips which is easy to do and then rebuild them into an amp that I am familiar with and know that I will like.

You could change the two OT's out for one (that would be suitable with 6L6's) and possibly use 6L6's like in a 5E5 Pro or if you didn't need that much power ....... try using one of your OT's and maybe build a 5E3 Deluxe. Obviously, you'd need to gut it to do that.

I think you can do something with the amp but trying to work with the circuitry in there would be tough for a beginning amp project.

With respect, Tubenit

[Steve Conner]

Well, it's a stereo amp: two identical channels. So why change the two OTs out? You could make a stereo guitar amp with two speakers and a stereo FX loop, put a nice chorus pedal in the loop, and enjoy.

Or you could just connect the two OTs and power tube sets in parallel, like the mythical dual OT plexi Marshall, and drive it all off one of the existing driver tubes. Use the spare tube for your preamp.
http://www.webalice.it/giacomo.pelagatti/jtm45-100/

I think the original console paralleled the two amps to drive its internal speakers, from looking at the schematic. You could also bridge them, if you have speakers with an impedance that's not suitable for paralleling. Or just connect one OT to each speaker. Either way, you'd have a 30-40 watt amp that could well blow your head off.

It might be worth rewiring the driver tubes to the LTPI circuit used in guitar amps. But I'd still try playing it stock first.

[Tubenit]

To gut or not gut?

Steve is correct and you can probably make the amp work as is with minor changes. That is worth a try.

Here is my thinking ......... you posted that you are somewhat new to amp electronics. Because of that, my response was to offer you something that has already been done & sounds great that you can convert your amp to. I was trying to direct you to a schematic that you could reference to rewire the amp.

There are different type of phase invertors. It appears your amp is a paraphase type phase invertor. There are other ones such as the concertina and the LTPI (long tailed phase invertor that Steve mentioned). I don't know how knowledgeable you are about those things or other amp design concepts?

Most guitar amps have one or two or more preamp stages PRIOR to going into the phase invertor. Your amp doesn't that I can tell. That's why I stated you can make it work but it may sound quite different than most guitar amps. My guess is the original consol had a preamp section also.

The gain stages are usually needed to add gain and overdrive to the amp. I would suspect that this amp would have very lower overdrive and perhaps a very clean sound to it if you just plug into the phase invertor. That may be what you are looking for? But you haven't stated what tone you're seeking.

My thinking was to use one of the 12AX7's for several gain stages then going into ONE phase invertor which drives the power tubes. This is also what Steve is suggesting If you did that then you may have to use one OT to get the correct speaker impedence (or you may prefer that to match a known schematic)? Steve is suggesting to use one phase invertor and parallel the OT's and power tubes. That may be simple to do, but I've never done that & don't know if there would be an issue with that?

As it stands now, you have two phase invertors but no preamp gain stages in your amp.

Steve perhaps is more knowledgeable and can help you simply parallel the OT's and could draw you a schematic for that. If that is the case, then you can use one 12AX7 for gain stages and use the other one for a paraphase phase invertor.

Would you know how to do that if a schematic was provided??

OR ......... you can pick a known amp design like a 5E3 (using one of your existing OT's) or
5E5 and copy that. I'd be happy to draw up a layout for you for either of those using paralleled terminal strips if you go that route. You can email me at js2x at carolina.rr.com & I'll get one to you. (replace at with @).

It won't hurt anything to take Steve's approach and try the amp as is with minimal changes. If you don't like it, maybe try a known amp design.

With respect, Tubenit

[AaronP.]

Hey guys,
Thanks for all of the great info and ideas.

Tubenit, my email is being funny right now. I DID get all of the documents that you sent. Thank you for both. The CBS looks really cool.

Steve, I've been discussing rebuild ideas w/ Tubenit. I would like to know more about your ideas of paralleled OTs and power sections. Do you think it'd be easier (for a new guy) than a complete rebuild? How easy is it to parallel the channels and add a preamp section like you suggested I have to admit that putting something together entirely from the ground up would be a better learning process for me but whatever the case I'm sure there will be conversions and complete builds in the future.

The 5e3 looks amazing and right up my alley for sound. I would think about a different chassis (what size?) if that was to be the build. If I end up just using the existing power sections in parallel and use the 12ax7s for a nice preamp/PI I'll just leave it as is in the regular chassis and try to get a box built for it.

[Steve Conner]

I also think a 5E3 would be a pretty good conversion to do. Bearing in mind my earlier comments about paralleling things up, I'd be tempted to make it a 5E3x2, a "Double Deluxe". You could do that on the chassis you have there, since it needs two 12AX7s and four 6V6s.

This thread on the Telecaster forum discusses the 5E3x2: http://www.tdpri.com/forum/amp-centr...2-yay-nay.html and if you look carefully you can find a schematic for the Weber version:
https://taweber.powweb.com/store/5e3x2_schem.jpg

Making a 5E3x2 with your existing O.T.s basically amounts to the same as I suggested earlier: paralleling your power sections and changing the preamp. How you connected your dual O.T.s would depend on what impedance of speakers you wanted to use. You'd always have to put the three primary wires from each O.T. in parallel, but the secondaries could go either in parallel or in series, or leave them separate and connect one speaker to each.

I don't know which would work best: I guess it depends what impedance of speakers you want to use, and what the speakers in your Magnavox console were originally.

BTW, it will be pretty loud when you crank it! An ordinary 5E3 is loud to start with. But if you like to use pedals, you might actually appreciate the extra clean headroom of the 5E3x2.

There are other preamps you could try: for instance, the one from the 18 watt Marshall might do interesting things with four 6V6s bolted onto it.

[Rob Mercure]

Aaaron,

Well you've got lotsa options whirling around so I'm gonna throw another bit of debri into the whirlwind for you to chase <grin>. One of the most fun and funky amps I've ever worked on was the Ampeg "Super Echo Twin" (several models - same basis idea) which featured two output sections one of which drove a speaker and a reverb tank through dropping resistor with the second section picking up the reverb signal and amplifying it and sending it to a second speaker. The result was the non-reverberated signal in the first speaker and the totally reverbed signal in the second speaker which gave a great stage sound, especially with a remote speaker for the reverbed signal.

Your chassis is obviously the power section of a "Home Entertainment Unit" - large floor consol common in the 50's-60's (Gawd how I remember lugging these beasts - especially the one's with a TV in them - to the service shop). I've salvaged a bunch of these over the years and once even wired up my own "Super Echo Twin" bench mess just to see what it sounded like and it was fun enouh that if I'd have had a spare cabinet for two speakers I'd have probably cobbled up a professional arrangement.

Like I say, just something else to think about but if you decided that this is interesting you don't have to use an Ampeg preamp circuit - pretty much any circuit will do as long as you get enough drive for the reverb (easy) and suficient sensitivity for the second amp input without noise (doable with some planning and care) and some tailoring of the response curve (mostly high cut).

Rob

[AaronP.]

Hi Rob,
The "Super Echo Twin" sounds incredible. What fun it'd be to gig w/ something like that. I DO like the idea, but at this point my novice status is making me lean towards the easier beginner builds. I must first arm myself w/ the basic working knowledge of tube amps before getting to the really fun stuff.

Thanks for taking a little time to suggest a cool idea to a new member (saving the idea to disc for sure). Much appreciated.

Aaron

[yosemitesamiam]

Hi all! This is my first post here, and I have the same basic project idea Aaron voiced above...I own a Maggot box 175 amp, which has four 6V6 GTA tubes in it.

I plugged into the tape-ins and also an MPX Stereo in without much success with my bass guitar. That said, the bass is a passive electronics model...so I really need a direct box to boost the signal (same issue on computer speakers, have to crank all the way up to get any output).

Anyway...am I on the right track here? It seems to me the setup is monaural in nature. The amp is still in its huge floor console. The LP table is very busted, but everything else works flawlessly (tuner, speakers, tubes, etc). The turntable would be working great too if it weren't for my kids ARGH!

Anyway, I'd appreciate any input here from you all. I would love to modify this for four 6L6 tubes eventually, but if I could get it to work the way it is (for now), then major bonus!

Sam

[tboy]

The schematic link in the first post has gone dead. There's a Yahoo group called "Magnavoxfriends" that is likely to have it.

[yosemitesamiam]

The schematic link in the first post has gone dead. There's a Yahoo group called "Magnavoxfriends" that is likely to have it.

Sweet! I appreciate it!

Question though...do I need a preamp? Or should the phono preamp work in my case?