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Magnavox 175-67 questions

G

guitargrump

Active Member
Hi everyone first post here... I am getting together the parts and tools to repair a Magnavox 175-67 amp module that I bought to use as a home stereo PA for acoustic guitar... Once I get this amp done I have two consoles I am going to refurbish but because I depend on the consoles I haven't done anything until I have a backup

I'm not totally ignorant about electronics I was a radio technician for Uncle Sam 30 years ago... But I haven't used the skills much since I left the service...

So far I have ordered a oscilloscope, ESR tester, DMM, soldering station and I already have miscellaneous automotive and home electrical equipment.

First question... Do I need any other testing tools?

Second question... Is there a clear readable 175-67 schematic available all the one's I've found are fuzzy and hard to read...

Third question is there a thread here that you recommend I read / follow for pictures tips on this rebuild??? I have read so many already that I feel like I am just wasting time...
 
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Welcome aboard!! nice amp with lots of followers and respect here. I'm sure folks will be along shortly to share insight, wisdom etc
 
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There are dozens of threads here on Audiokarma discussing this amp and various updates. If you do a search here on this Tube forum you will get a lot of hits. For example: https://audiokarma.org/forums/index.php?search/62915361/&q="Magnavox+175"&o=date

Here is an example of one rebuild thread: https://audiokarma.org/forums/index.php?threads/magnavox-175-rebuild.933285/

As for tools needed, that will depend on the depth of modifications you intend to undertake. What you have will get you through many regular equipment refresh and mods. However, I would suggest getting either a variac or a dim bulb tester (do a search here for "dim bulb tester" for info, builds, etc.). Take a read through some of these threads and assess whether your equipment suffices or will need additional equipment e.g. signal generator etc.

Another point, please be aware or reminded that tube equipment has high voltages and the capacitors often hold charge for long periods of time, so familiarize with good safety practices before working on them. Several threads here on tube safety.
 
[QUOTE="Another point, please be aware or reminded that tube equipment has high voltages and the capacitors often hold charge for long periods of time, so familiarize with good safety practices before working on them. Several threads here on tube safety.[/QUOTE]

Thank you for the info/help and safety reminder

I forget to mention in my OP I also have a working antique tube tester and tube testing charts...

I know the basics of safety and reading diagrams and I know how to properly discharge a capacitor...

But I am Rusty my Service PTSD kept me away from doing electronics repair as a career...
 
guitargrump said:
Second question... Is there a clear readable 175-67 schematic available all the one's I've found are fuzzy and hard to read...
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Here's the Sams in pdf, which can be enlarged to show detail.

There are several versions of the 175 but differences should be fairly minor. They didn't publish separate schematics for them. This is the only Sams that covers the 175.
 

Attachments

In case you don't already know, this amp is a stereo amplifier made for high fidelity stereo listening and not a good candidate for a guitar amplifier.

The previous posted PDF schematic will work for you except the 175-67 model was made and sold with a tuner/preamp (and sometimes a tv), not just a preamp as the PDF posted shows.

I also have the 175-67 with the original tuner/preamp with tv input. I found the entire console with tv years ago from my local goodwill but the tv picture tube was smashed (it always seems that way). :D

Except for the tv, I still own all the original speakers, the tuner/preamp, the turntable, and the 175-67 amplifier. My amplifier has been highly modified to run 6L6GC output tubes and different output transformers for about 25 watts per channel. As it stands stock, the 175-67 is only 10 watts per channel. FYI.

You only need one channel for a guitar amp. With this amp and any stereo amp you wish to modify for guitar use, will need an extra tube or gain stage for a guitar pick-up. These amps are line level input so you would need something that will bring it up to a microphone type of input thus the extra gain stage.
 
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cademan said:
You only need one channel for a guitar amp. With this amp and any stereo amp you wish to modify for guitar use, will need an extra tube or gain stage for a guitar pick-up. These amps are line level input so you would need something that will bring it up to a microphone type of input thus the extra gain stage.
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I use a guitar preamp and regular mixer panel for bringing the audio up to line level... So it's perfect as is as long as it works... I play acoustic guitar and it's for way more than just guitar... I sing too... My magnavox astrosonic console never is off I listen to music 24-7 and plug in either my Acoustic or Electric whenever I like what is on the radio...

I just put the mixer between the radio and the Amplifier... and turn down the radios channel if I need to use a different source. Like my computer or mp3 player

10watts *2 is plenty for me at home... And I definitely need stereo... I thought it put out 8watts per channel

I have tube guitar Amplifiers too but they cut the high frequency end off...

I like being able to play with others but these days that's easier said than done...

This is almost as good sounding as being on stage with whoever I have on the player...

People may think I am nuts but I love the way these amplifiers sound as is... but without a humm in the pauses even better...

This project is just a backup and practice to help refresh my memory

Dswankey said:
@guitargrump

I see you already have some of AK's finest minds chiming in they are a great source of knowledge!
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Yes and I got the main thing I needed the schematics and parts locations so I was able to order most the new resistors and capacitors I think I will have to replace I can't wait for the tubes and stuff to get here I'm itching to get to work...
 
As long as it works for you. :thumbsup:

I don't play the guitar so with these kinds of amplifiers, I am always thinking in the high fidelity mode. :rockon:
 
guitargrump said:
I can't wait for the tubes and stuff to get here
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Are you getting new tubes? or is the amp your getting coming sans original tube?

One of the nice things about the Maggie amps is, and I'm sure others can attest to this is, that the original tubes are usually in great shape and have just been sitting dormant for many, many years on top of that they more then likely were never driven too hard to begin with.

Of all the Maggie amps I've come across it was very rare for a tube to be bad.
 
Dswankey said:
Are you getting new tubes? or is the amp your getting coming sans
No tubes came with it unfortunately... It was cheap though...

Seller says it doesn't hum but I don't know I hear pretty good on the low end...

I bought it to redo as a safe place to refresh my skills before I start working on the other complete consoles I have been buying them as I find them since covid started as diversions...

Since I retired I need stuff to do that exercises my mind besides just smoking herb and playing guitar :)

cademan said:
As long as it works for you. :thumbsup:

I don't play the guitar so with these kinds of amplifiers, I am always thinking in the high fidelity mode. :rockon:
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It works very well for me for both Hifi and Home electric acoustic guitar PA... Especially with a fatty rolled...:)
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6DZ7 said:
A HI-FI amp was made for reproducing music the way it sounds. Can't do better for acoustic instruments or whatever comes from a processor. My Roland synth KB and JV-800 pass through two mixers before they go to a Magnavox 9302 and speakers. With recorders and players of all types attached you can be right there in the group.
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That's the part my guitar buddies don't get... Even though They have tube guitar Amplifiers...

The sound is so alive defined it's almost like you could walk between the different musicians not, everyone hears it though...
 
Guitar G -- Also welcome to AK! Beside whatever restoration work you're doing, the one thing you might consider also doing in that process is making the two channels identical. As designed, Channel 2 uses less Negative Feedback, which then allows that channel to have more gain. To bring the gain back to normal then, they added an input level control to that channel only, so that it could be used to ultimately balance the sound of the two channels in its original stereo application. But different feedback levels will tend to make the two channels have different tonal characteristics -- particularly near the point of overload if you drive either channel to that point -- but also notably even if you don't. Guitar amplifiers typically operate with very little negative feedback versus that of reproduction amplifiers, as one of the many differences in their basic design concept. The good news for you is that Magnavox amplifiers didn't use a whole lot of negative feedback to begin with -- but enough that for your alternate application, it might make a difference. This may or may not be of concern for you, but you ought to be aware of the channel differences in any event. As an example, if one channel is strictly used for guitar and the other for a recorded source or voice, then chose the channels as they best fit your personal preferences. Or, chose the one you like most and make them both like the preferred one. Just be aware that as built, they are different.

Finally, also be aware that the Magnavox has a 4Ω output impedance, so if you're using (or will use) different speakers than the amplifier originally worked with, that's the optimum load for these amplifiers.

Good luck with your project!

Dave
 
dcgillespie Dave Thank You So Much...

I have been reading your magnavox 8800 thread It's been so informative and is so above my current skill level I'm a certified radio service repair technician but my PTSD has blocked alot of the memories of my training and military service... So I am going slow and carefully and doing alot of new relearning and studying so I don't have to disturb the past...

Plan is to make this 176 a stand alone stereo amplifier... Using what ever time energy and spare change I have...

Once I'm satisfied with it working right I am going to hook it to my 62 Astrosonic's speakers while I refurbish it's amplifier and radio... Maybe even refinish the wood and fix the phonograph...

the Astrosonic It humms too much but I use it for background noise covering and so it goes 24-7 and I play guitar though it 2 hours a day I expect it to eventually fail and so I'm building it's backup... I intend to use the Astrosonic's original speakers as long as I can...

Your work inspired me and we've never met before Thank You...

Channel 2 volume control removal is on the list of tasks once I get the thing working...

I'm not off to a good start though I blew a brand new Sovtek12ax7lps tube in the the tube tester first try. Something shorted from shipping I guess... The other 12ax7lps tube was fine...

I just ordered another tube...

No pain no gain...

You'll have that on Big Jobs

Next time I'll bring up the power slow Doh...
 
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6DZ7 said:
A HI-FI amp was made for reproducing music the way it sounds. Can't do better for acoustic instruments or whatever comes from a processor. My Roland synth KB and JV-880 pass through two mixers before they go to a Magnavox 9302 and speakers. With recorders and players of all types attached you can be right there in the group.
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I use a effects processor before the preamp as well. I might as well be playing a synth as a guitar these days... My Acoustic is Piezo with a Cherub GT6 preamp system... So there's 2 effects processors Involved, But it still depends on what you want to hear... I just enjoy the music and keep practicing for the joy of it... I'm getting as much joy out of life as I can...

They say music soothes the savage beast... I say a nice Dubie helps
 
Another thank you for your service! I've got a Maggie 185 (very similar) out getting a professional restoration. I'm looking forward to hearing it. I got it "free" because I bought two old Magnavox consoles - the consoles looked great and I wanted them as cabinets for modern speakers. Price was right and the old tube amp came along for the ride.
 
Pondoro said:
Another thank you for your service! I've got a Maggie 185 (very similar) out getting a professional restoration. I'm looking forward to hearing it. I got it "free" because I bought two old Magnavox consoles - the consoles looked great and I wanted them as cabinets for modern speakers. Price was right and the old tube amp came along for the ride.
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I have been buying Silvertone tube stereos and magnavox tube stereos as I find them for a couple of years now... I love that their repairable and reliable...

I made alot of progress last night and today..

I sorted out a bunch of possibly working tubes from a repairmans junk box... Bill the repairman traded me the tubes and tester in exchange for me repairing the local charity cemeteries lawn equipment...

I found everything I still needed for tubes yay! Now I have some spares!

I didn't realize how important the tester would become when I made the trade I just was doing a favor to a good person who has helped so many...

Bill is gone now... I still repair the cemetery's equipment and wish I had spent more time with him... He was a veteran too... We all serve the causes of freedom in the hope of making a better future...

My service was a great and horrible time but I would do it again if called on...

But I'm a hippie at heart and would rather make music and stuff to promote peace and Harmony... Now that I'm retired music is my most important passion...

So all your gratitude and respect for our Veterans is greatly appreciated and I am overwhelmed with emotions I can't express in any other way than a simple Your Welcome...
 
6DZ7 said:
So then should we hereafter consider your guitar playing, doobious? Or doobieated, or dooby-aided...?
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All of the above just depends on the situation it's doobious if I'll ever be great.. it's doobieated if I don't know the song... And dooby-aided the rest of the time...

I'm one of those that needs the peace it brings both the dooby and the guitar...:rflmao:
 
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