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Magnavox Amp 175 55 restore initial questions

PastorMartin

PastorMartin

New Member
Good afternoon,

As I move forward in my newfound addiction to tubes, I recently purchased a Magnavox Amp 175 55 chassis on ebay "as is." My goal is to restore it with minimal changes (recap, bad resistors, fuse, volume controls, etc.).

With the tubes removed, I tested the power transformer and the unloaded voltages appear good.

First question. I have to change the non-polarized plug. I plane to use a standard three wire plug. Should I connect the ground to the chassis? This seems to make sense to me, but I saw a post that says there's some disagreement. Why? Thanks.

After the plug and fuse, I plan to swap out all the electrolytic caps and check the resistances before attempting to power up. Any other suggestions?
 
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Well if you want to do that the green wire connects to the chassis .

The reasons for not doing that is it will most likely cause hum . The power transformer isolates the unit from the line voltage and prevent shock most of the time , any other audio device in the chain that is grounded will ground the input and the amp chassis and cause a ground loop if the amp is grounded .
 
I always use the ""old fashioned"" 2 wire cord. It worked ok for a hunderd yrs and will do so for the next hundered.
At least in my amps it will.
but thats just me.

some folks like the 3 wire but to me they are a pain in the arse.
 
I agree. Do not go with the three prong cord. Add a removeable cord, safety cap, socket and fuse if you wish, but use a two connector type. I am on my third Maggie rebuild and these amps are dead quiet with a two wire cord.
 
The safety ground always connects to the chassis, because its purpose is to keep the chassis at ground potential to prevent shock.

Yes, US electrical mains have an inherent ground loop in them due to the interaction between ground and neutral (which is also at ground potential), but in my experience it's not really much of a problem. Most modern gear has polarized plugs and either a non-conductive housing or is fully electrically isolated from the chassis, so they don't need a safety ground. So if you have N devices that are fully floating and only one device with a safety ground, then you're not going to end up with a ground loop so no problem.

If you do have multiple devices with a safety ground AND you end up with a noise-inducing ground loop, then you have a few of options:
  1. Electrically connect the chassis of each device with a safety ground.
  2. Ensure that each device with a safety ground is plugged into the same ganged outlet (obviously only if you have 2 safety-grounded devices).
  3. Make a cheater outlet box that connects neutral and ground at the outlet, and connect everything to that. Obviously you should only plug this into an outlet that you KNOW FOR SURE is wired correctly.
Perhaps I'm paranoid, but I'm a guitar player and many eons ago when I gigged with a terrible punk rock band, I always carried an outlet tester and NEVER plugged my gear into an outlet I hadn't tested first... it only takes one jolt (when your guitar and either a mic or a bandmate are at different potentials) to learn that lesson. For this reason, ALL of my conductive-chassis stuff gets safety-grounded. Full stop.
 
Thanks for all your helpful comments. I will eventually address the grounding issue. I'll connect the ground to chassis and see what the hum sounds like. I, like thorpej, am generally paranoid when it comes to live chassis. My church has no earth ground. One day, the floor was damp and all the sound equipment chassis and microphones started giving everyone hefty jolts. It turns out a screw in an outlet had pierced the hot side wire causing the earth ground of the metal outlet box to go to 120 to 120volts (or whatever the line voltage was). Lesson learned without grave consequences.

I recapped the entire amp and went through the appropriate power up procedures. It sounds great. Some of the resistors tested high and will be changed.

Next question: As I looked at the schematic I notices that the negative feedback is different on each channel. Channel 1 has a RC network with a R9=2200Ohm and C10=.001uF. Channel 2 is R20=5600Ohm, C17=47pF. Any idea as to why this is so? Thanks.
 

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Hi i always ground the chassis of my amp using the IEC type inlet and i have 0 noise from the AC lines. I have built at least 20 amps this way with no noise at all. Just put X1/Y2 safety caps from each AC leg to ground and across each leg which will remove almost all noise from the line. You can buy AC inlets with all of the proper safety caps and noise filters installed for an easier installation.
About your amp schematic, is this amp from a console ? Sometimes they were designed to be 2 channel amps, not stereo, where 1 channel was for highs/mids and the other channel was for the lows. The 2 output transformers would be different if that is the case.
 
Thanks. I installed the X1/Y1 caps to replace the original "death caps."

I don't know what it came out of. What primosounds says makes sense. It is stamped "Magnavox AMP 175 55" which I assume makes it some kind of console made around 1959. It has a connector for a tuner and no connector for phono or preamp. I would like to know what it came from (there descriptions of 175aa, bb, cc, dd on another website), but after searching a while, I'm stuck. I can listen to both channels and decide if I want to change the nfb. I'm not much of an audiophile. I just like to build (and rebuild) stuff.

Next question has to do with inrush thermistors. I increased the value of the first cap that the rectifier sees (40 to 47uF) on the power supply and I think I should protect the rectifier. I've heard the the CL thermistors also will lower the voltage a tad which might help. I'm guessing here that I need to know how much current the amplifier draws. I could calculate the draw by all the tubes (maybe). However, I'm wondering if I could place a 1ohm resistor (10watts) on the primary side of the PT and then measure the voltage across the resistor to get the voltage and current (I = V/R). The power, in watts should then be P=VI. This seems to make sense to me, but before I do anything foolish I thought I'd ask. Note: I don't have access to a wall outlet current monitor.
 
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