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Magnavox 175-67 Rebuild Questions

N

Noremac82

New Member
Hi everyone,
I have been lurking here for the past few days and have quickly been exposed to a wealth of knowledge and picked up on a few ideas about how i am going to go about this rebuild. Recently i found a 175-67 for a good price. It is supposed to be in working order but the power cord is in pretty rough shape so i am going to replace that before i do any type of powered testing. All the tubes are in place and the gentleman i purchased it from said he had tested them and they were not super strong but should be usable. They are all different brands but they are there. I plan to use this as my daily driver amp once completed. I have a couple speaker choices but it will probably be used mainly with my Elac Unifi B5 or Debut B6 (i liked both so kept the debut after i bought the unifi's). I have a couple of other choices but they are floor standing speakers that i would rather not move out of the living room. I would also like to add a headphone out at some point, more on that below.

So.... My plan is to replace all the wiring, caps, resistors and anything else that hasnt held up too well over the past 60 years or so. I am slightly skeptical of the output transformers because they dont look to be in the best condition. i am not measuring any shorts so i think they will be usable for rebuild and testing purposes. I will replace them with something better once i have it up and running. I have read something about ultra linear that seems interesting so i hope to look into that a little more and may go that route if it would be of benefit.

The 6v6 tubes are are all different brands so they will be replaced with a matched set of New Production TungSol's once i am satisfied i have everything in working order. Any suggestions for improvement or cautions while rebuilding the power stage would be appreciated. I try to approach everything with a mindset of cost vs reward so i am not really interested in the ultra rare, unicorn blood driven tubes out there that have tiny angels singing a chorus inside them. Honestly, i probably can afford them but probably couldnt tell the difference between them and the Tung Sol's so my mind is pretty much made up there. From what i have read on here, there arent really any bad 6v6's so that seems to be pretty much settled for now.

The Power transformer looks to be in really good condition along with the choke so i feel like i am good there for the time being. In the future i would like to buy some quality transformers that arent eye sores but for the now i just want something that is safe and will function as expected.

My questions come in on the 12AX7's. I have a friend that has a pair of 5751's that he bought years ago that he says he has never used. If he did use them it was a junk socket because the pins arent scratched at all. This amp will only be fed line level signals so is it necessary to have the extra gain of the 12AX7 or would i be OK using the 5751's? I have plans of getting my record players back out at some point but i have pre-amps for them. In the end i dont see a situation where i will ever be feeding this amp weak signals that dont first go through a preamp or mix panel. I can get the 5751's at a very good neighbor discounted price right now but dont want to waste the funds if they wont work. He is downsizing his collection so if he is letting me have first pick on everything before he puts a lot of it on ebay and sells it.

I would also like to add a headphone tap to this amp at some point in time. I plan on buying some HE-6's in the hopefully near future but for now i mainly use my trusty HD650's. I have been in love with the HE-6 since i heard them at few years ago so now that i am in a position where i can afford them without eating ramen for 3 months, why not. I dont want to end up damaging the headphones or my speakers if i do this so i have been reading up on some different types of attenuators and impedance matching in order to try to get a grasp on how to do this safely yet get good results. Any and all suggestions here will be appreciated. I have a few ideas of how to go about it but right now they are just ideas. This is not set in stone but i would like to be able to continue using this amp even after the misses goes to bed without fear of being banished to the yard. This may be a subject for an entirely different thread once i get closer to implementing it.

Anyways, this will be my first full tube amp rebuild so i am really looking forward to it. I plan to implement all of the standard stuff too, power switch, fuse, and power indicator in order to make it a little safer to operate. Over the years I have replace components, fixed friends guitar amps and all of that stuff but never took the time to build or rebuild an amp. The way i look at it is for the 60 bucks i will have in the chassis, transformers and mismatched tubes, it is worth going a little further and hopefully have something that sounds halfway decent in the end. I dont plan on throwing a whole lot of money at it but i wont be skimping on junk components or cutting corners either. If i am going to rebuild it, which i plan to, i want to do it correctly in order to have something to use until i get the inkling to upgrade again. Plus it will keep me out of trouble!

I hope to take some pictures as i go and will share them here if there is any interest.
 
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we rebuild/restore all magnavoxes here so yu've reached a wealth of knowledge in that venue.
enjoy the ride.
glad you found us. welcome
 
Upgrading OPTs is always an option....But I think the original Magnavox ones sound great.

But then you have the 4 ohm taps to deal with....and finding some speakers that work with those.
 
Rebuilt my Maggie 175-67 and a couple others with help from the great people here.If you can work on tube guitar amps, the Maggie will be a breeze. If you run into any issues, i am sure someone here will have the answer. Welcome to AK, and that Maggie will astound you.
 
Leland47 said:
Upgrading OPTs is always an option....But I think the original Magnavox ones sound great.

But then you have the 4 ohm taps to deal with....and finding some speakers that work with those.
Click to expand...

The Elac Debut B6's are a 6ohm speaker. Someone had recommended I connect to a 4 ohm tap to use them. Now I have no idea if this is correct or not but if it is, I may be lucky to have the 4 ohm tap.

In case it won't work, is there a way to modify the output impedance without changing transformers? Could I run a resistor in parallel to drop the load to 4ohm? I have never ran into problems like this on SS amps so it's a new concept to me. Seems like if I was using a decent quality resistor with enough heat dissipation ability, I could drop even an 8 ohm speaker to match the 4 ohm output. Problem might be finding a resistor to dissipate that much heat and I am also assuming the volume would be lowered due to the loads being in parallel.

I am a long way from this even being an issue but if I get a grasp on it now I can prepare when I am ordering components
 
rudedogg said:
Rebuilt my Maggie 175-67 and a couple others with help from the great people here.If you can work on tube guitar amps, the Maggie will be a breeze. If you run into any issues, i am sure someone here will have the answer. Welcome to AK, and that Maggie will astound you.
Click to expand...

Most guitar amps I have worked on have been in a little better shape than this but I think I am up for the challenge. I'm gonna give it the good old college try at least.

I am sure I will have plenty of questions but most of the ones I had earlier in the day have already been answered with some quick searches. The memeber earlier wasn't kidding, there are a lot of people here that have rebuilt these amps so I am definitely in the right spot. Everything I need to know is right here it seems and if it isn't I feel like the combined knowledge of this forum can come up with an answer.

Thansk for the warm welcome and I can't wait to get started on this thing later in the week. Right now it's just learn learn learn and trying to absorb as much knowledge as I can on how this thing operates. That along with getting a passive component order together. Mainly focusing on electrolytic and oil caps right now. I should have enough resistors on hand to replace anything that might be NG. Its gonna a be a good time.
 
It's not really feasible to put a resistor on a speaker to drop the ohms. You would just basically be impeding the output to the speaker. A 6Ω or 8Ω speaker won't hurt an amp with a 4Ω output. You would just need to experiment which speaker(s) you have and which sounds best with your 175.

I also have a 175-67 amp in which I put different output transformers on it. It wasn't because it didn't sound great stock, it did, but I had a pair of Knight UL output transformers on hand and put them on along with upping the power supply, and using 6L6GC tubes for a little more ooomph.
 
Ive run 84 and 86db 8 ohm speakers on the stock 4 ohm OPT's and they sounded fine, you just wont shake any windows at full volume.
 
OK, first question that really shows my inexperience. I was looking to order some new wire to make my power cord and realized i have no idea how much current this this will draw. Could someone tell me the current consumption of this amp at full power?

I have some 10-3 romex here that i am going to make a completely over engineered, 'for testing purposes only' power cord from but i kinda want something other than yellow romex hanging out of the back of it if this thing is going in the my living room. Plus I am thinking 3 strands of 10g solid core wire may be a little bit excessive and unmanageable. I could buy a sleeve to go over it but i would rather have something that is a little more flexible than romex.

I found some 16g, multi strand 3 conductor made in a reproduction vintage style that isnt too bad. I feel like that will probably be sufficiently big enough but just wanted to check before i buy it because i am guessing i cant return it after its cut from the spool. I really have no idea though because i am afraid to even venture a guess at this amps power consumption due to my inexperience.
 
I use 18ga lamp cord like the factory used.
thats all U need.
the amp can poop out about 10 watts or a wee more.
none of these amps were huge power consumers.
 
Good ol' regular hardware store lamp cord will work just fine. Probably shouldn't over think this too much. The amp is pretty much a great performer as is stock. Get it working in stock form and then think about all the available mods. :thumbsup:
 
cademan said:
Good ol' regular hardware store lamp cord will work just fine. Probably shouldn't over think this too much. The amp is pretty much a great performer as is stock. Get it working in stock form and then think about all the available mods. :thumbsup:
Click to expand...

i will be the first to admit, i do have an uncanny ability to overthink even the simplest of problems. I have spent most of the day trying to convince myself that 80uF of capacitance is enough to keep the PS from sagging. And the whole time i have known that this thing has worked for however many years in its original configuration and people have enjoyed it. I guess thats the obsessive compulsive, hyper stimulated, "bigger is always better" maniac that i try to keep locked away in the back of my brain. That works great in theory but not always in practice because he is persistent!!
 
image.jpeg
Here is mine while under construction. I used a "Mini IEC" jack and cord for power. No noise, hum, or power problems whatsoever! Also, you can see that i moved the choke below the chassis and also removed the main filter cap and replaced it with 4 seperate 47 mfd caps under chassis.
 
Hi all, I havent had much time to work on the build plan today or order any components but one question did cross my mind while I was out walking with my little girl. What is the chassis made of? Is it just steel, stainless steel or aluminum? The reason I ask is that even though I have multiple unresolved problems that need addressed, I do know one thing. I want to weld up any unnecessary holes, level it and fill any dents or dings, and put a nice automotive finish on the chassis. I want it to look like a sheet of glass when I am done. I can most likely get that done when I am visiting my parents next weekend. My dad used to do a lot of body work so he has all the tools that will make it go much quicker than I could at my place. I would be forced to go rattle can and I want a nice clear on this, might even put a clear cerakote down to keep it from getting scratched. I would powder coat it but I don't want to send it off and have to wait.

I also want also thinking about making some covers for the transformers at some point in time. I am assuming a cover would need to be something non ferrous in order to not disrupt the magnetic flux of the transformers too much. I am thinking maybe form them from fiberglass and paint. I could even 3d print them, smooth with body filler, and block sand them in order to have a good smooth surface for painting.

These are both purely cosmetic improvements and won't improve the SQ at all. However if done incorrectly they could negatively impact it. I'm looking at it like anything else though, it's the details that make the difference between being good and great. If it turns out to sound good I want to be able to show people without them needing to get a tennis shot afterwards! These transformers look awful and I would prefer to have a clean look. Plus if I have to order new OPT's, I don't want them looking new and the rest looking 60 years old.

This has to be done before any wiring is installed. I have some sheet aluminum and some sheet steel that can be used but if its aluminum I will need to order it and get here in before next friday. To add to it, I haven't Tig welded in probably 10 years so nothing like a little pressure when doing something that cant end up with more or bigger holes in it than I started with. Pressure makes diamonds though, right?!? I will also need to drill the holes for my binding posts, fuse, interconnect jacks, and power switch.

This thing will be slick when I am done with it though. I'm thinking satin black with either dark blue or dark green transformer covers. Maybe blue and white for my favorite team, who knows. I will just have to see what colors dad has. It might end up primer black with some Ed Roth-esque pinstripes for all I know. I would love to go with all brass hardware but it's too late for that because I ordered switches and binding posts before I thought of it. It's crome or stainless for this one and the next one can be brass. That's right, I'm already thinking next time because this has turned out to be more enjoyable than I ever imagined it would be. I'm already thinking about building a mixer/preamp so I don't have to switch inputs each time I want to listen to a different source. I could put multiple inputs on the amp but I want it keep it as amp only. I see why this is such a popular hobby now. If you enjoy tinkering with electronics, which is about all I do, this is definitely for you. The DIY method puts a lot less strain on the wallet and I will get more satisfaction out of it too. I'm hooked.

Until the next update, take care everyone!
 
Hello, I know this is an old thread, but I'm working on a 175-55 Magi so I thought this was as good a place to ask my question.
Yesterday I pulled the OPT's and replaced them with some Hammond 1650E's. When I was first done and plugged her back in she sounded like a police siren.
I did some trouble shooting and found that it would run fine in mono, but not with both channels without 'sounding off'. Based on that I pretty much knew it was something in the 'input' as with both AX7's out, or with one in (either channel) it was fine, but with both it was not...
I saw what looked like a previous owner mod going from OPT speaker outputs back into the input (neg feedback) and as part of trouble shooting just disconnected all of it... and bamm -- all good...
So -- do I need to research and put in a 'better' feedback circuit, or with the larger/better OPT's do I need to even bother?
 
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