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Some Magnavox 175 help

janikphoto

janikphoto

Lunatic Member
I'm about to rebuild my pair of 175's very soon. Where do I find some info on bypassing/removing the switch that was used for the external speaker? I know how to recap, add the power switch, put in a fuse holder, and put some speaker binding posts on the wires coming out of the transformers. But, after that, it might get a bit dicey for me!
 
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Sorry, I do not have pictures. If you can trace the wires, it should not be too difficult. As I remember it:

1. Connect the secondary of the output transformers to the Left and Right speaker terminals. Each secondary should have one wire connected to ground speaker jack and the other wire connected to the "hot" speaker jack.

2. Run the feedback resistor + capacitor combination between the "hot" speaker jack and the resistor junction (220Ω / 3.3kΩ) at the first triode cathode, once for each channel.

3. You are done.

 
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There never was a speaker switch on the amp itself. It was in the cabinet. You don't even have to change or mess with the feedback circuit/resistors.
 
cademan said:
There never was a speaker switch on the amp itself. It was in the cabinet. You don't even have to change or mess with the feedback circuit/resistors.
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But, it has an umbilical connector on the amp with wires leading to the switch.
 
Simple! Just ignore the umbilical and wire straight from the speaker output transformers. I believe that Magnavox used the same color schemes with their output transformers which would be green and black. Just follow the green and black wires coming out of the speaker output transformers themselves. Green is the positive and black is the negative.

Just disconnect and/or ignore that left speaker terminal connection in front of the can capacitor.
 
Also, the AMP 175 was the amp that had the tuner/preamp which used an umbilical from the amp to power the tuner/preamp so you will have to know which wires to jump to get the amplifier to power up on its own without the tuner/preamp.

The AMP 185 was basically the same as the AMP 175 but without the tuner/preamp and 7025 phono preamp tube.
 
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cademan said:
Simple! Just ignore the umbilical and wire straight from the speaker output transformers. I believe that Magnavox used the same color schemes with their output transformers which would be green and black. Just follow the green and black wires coming out of the speaker output transformers themselves. Green is the positive and black is the negative.

Just disconnect and/or ignore that left speaker terminal connection in front of the can capacitor.
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Thanks! I just pulled one of them in from the barn and was able to trace everything. It was pretty simple to figure out, once I had it sitting in front of me!


cademan said:
Also, the AMP 175 was the amp that had the tuner/preamp which used an umbilical from the amp to power the tuner/preamp so you will have to know which wires to jump to get the amplifier to power up on its own without the tuner/preamp.

The AMP 185 was basically the same as the AMP 175 but without the tuner/preamp and 7025 phono preamp tube.
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Yeah, that's what I'm about to trace. I think I have it figured out, but I'm going to grab the pre-amp and see if that helps me verify my hunch that I jump the black to the white... though, there's a possibility it's black to gray. I'm going to play it safe and trace the preamp all the way through first!
 
There ya go! :thmbsp:

Also, without having to trace the wires, you can simply wire a plug directly to the two black wires coming out of the power transformer, plus some power transformers have a black wire and a black with red stripe wire as the 120 volts AC in.
 
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cademan said:
There ya go! :thmbsp:

Also, without having to trace the wires, you can simply wire a plug directly to the two black wires coming out of the power transformer, plus some power transformers have a black wire and a black with red stripe wire as the 120 volts AC in.
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Thanks! My first hunch was wrong. I was having quite a time tracing wires through the preamp, as they go into the turntable switch and then into the main power/input switch... it gets pretty confusing trying to follow it all the way around!

Right now, I'm testing the amps by leaving the preamp plugged in. One amp works fine. The other has one channel OK and the other is super quiet. It's not the balance knob. I tried flipping the 12ax7's and the 6v6's from one channel to the other, but the quiet channel didn't move with them. I tried flipping the rca's from my test music, but the problem stayed the same. I'm guessing it has the potential to be a bad output transformer?
 
OK, you tip for identifying the correct power wires to jump was perfect. I now have the preamp removed and am testing the good 175 with great success. It's super quiet in between tracks and loud during the songs. It has no hum, though I'll still do a full recap, since these will be running the mids and highs on my main tri-amped system.
 
Its hard to kill output transformers so I would first look for bad connections, cold solder joints, out-of-spec or open resistors.
 
cademan said:
Its hard to kill output transformers so I would first look for bad connections, cold solder joints, out-of-spec or open resistors.
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Sure thing. I'll recap the bad one first, and maybe I'll spot a bad solder joint or something along the way... I'll replace all the resistors on the output end of both channels, too. Then we'll see what happens from there.
 
These are great amplifiers along with those 9300 series amplifiers which use the 6BQ5 tubes. You don't have to modify yours to any extreme but here is what I did to mine. I replaced the small output transformers with output transformers from a Knight KB-85 amplifier along with 6L6GC tubes for more power and better bass.

I added a voltage doubler circuit underneath for more B+ for the 6L6 tubes but I kinda got it permanent in the system so I couldn't get you an underneath shot but it does have the orange drop coupling capacitors and a recent upgrade of filter capacitors where the can is completely disconnected and is just there for looks.
 

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I don't need more bass, as these will be running the horn mids and highs in my NAS (brand) Studio Monitor (model) speakers. I'll have solid state running the lows. They are tri-amped right now with four SS amps and it's not as pleasing as when I had a single ST-70 running everything - but the bass is better. I'm thinking I need to get back to tubes on the top end. These will replace the two SS amps running the mids and highs right now.

Here's what I've got:

10431485_10152417642137817_6442378344592120474_n.jpg


They are both 175's, but the power transformers are slightly different and the 6v6's are different. One has silver tops and the other has clear. Other than that, they seem pretty much identical, though I haven't done a point to point comparison underneath.
 
I added a fuse and power switch to the first amp where the phono input holes were. I'll put the power indicator light (probably change it to an an LED) in the slug where the phono tube would be right next to the switch and fuse.

10155002_10152419786757817_418746616524346838_n.jpg


I'd like to remove the balance pot, and I've read of others deleting it. Any pointers on that one?
 
This is not your typical left & right speaker balance pot but rather the hum balance pot and it needs to stay.
 
janikphoto said:
BUt, mine has a hum balance and what appears to be a stereo balance... do they BOTH pertain to the hum?
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No, one is a hum control, the other is adjusts the gain of one channel. I recommend removing the gain control and modifying that channel to match the channel without the gain control. Pretty easy to do and it will improve the sound of your amplifier. :yes:
 
janikphoto said:
I added a fuse and power switch to the first amp where the phono input holes were. I'll put the power indicator light (probably change it to an an LED) in the slug where the phono tube would be right next to the switch and fuse.

10155002_10152419786757817_418746616524346838_n.jpg


I'd like to remove the balance pot, and I've read of others deleting it. Any pointers on that one?
Click to expand...


http://www.audiokarma.org/forums/showthread.php?t=352623


You basically have to clone one channel to the other by replacing a resistor and a capacitor...if I am not mistaken you modify the right channel to match the left. Then you have to remove a cap from the underside of one of the RCA inputs and install a new resistor there...in other words bypass the balance pot completely. If you are confused find Bobabode or Battradio.

Robbin
 
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